Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 47 b. Make a flow chart of the kingdom Plantae. c. What is the special feature of liver fluke? d. How do coelenterata obtain food? e. Describe the structure of a hydra. f. Write special features of the plant kingdom. g. Write the physical feature of the leaf of each monocot and dicot. h. How do porifera digest food and absorb nutrients? i. Write two similarities between snake and frog. j. Are jellyfish and cuttlefish types of fish? Why or why not? k. Give any two similarities between ants and spiders. l. All vertebrates are chordates but all chordates are not vertebrates. Justify this statement. m. Frogs have external fertilization. What does it mean? n. Tapeworm has an incomplete digestive system. What does it mean? o. What is the meaning that earthworm is hermaphrodite? p. Which class do the animal with the following traits belong to? i. oviparous, feathers, hollow bones ii. viviparous, suckle milk, fur iii. waterproof scales, oviparous, breathing through gills iv. semiaquatic, oviparous, external fertilization v rough scales, oviparous, crawl 7. Classify the organisms shown below and write their two characteristics. (i) (ii) (iii) (iv) (v) (vi)
48 cl aS Sification of living t HingS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur STEP3 8. Answer the following long questions. a. Study the given diagram and answer the following questions. (a) (b) i. Write the subdivision of the given plants with reason. ii. Write any two similarities between these two plants. iii. Write dissimilarities between these two plants. iv. Which one is more developed? Give reason. b. Mention any three characteristics of the following: i. Algae ii. Gymnosperms iii. Angiosperms iv. Chordata c. Make a table and compare monocots and dicots based on root, leaf, stem and vascular bundles. d. What are tracheophytes? Name the three sub-divisions of the Tracheophyta. Why are gymnosperms and angiosperms called tracheophytes? e. Classify the dolphin and write its three salient features. f. What are the advantages of the five-kingdom system of classification? g. Why does maize belong to monocotyledon? Write any three reasons. h. How can we tell if an animal is a reptile or amphibian? 9. Which phylum or class do the following belong to? Enlist only those characteristics due to which they belong to this phylum or class. a. Spongilla b. Sea anemones c. Planaria d. Leech e. Round worm f. Snail g. Sea urchin h. Labeo i. Toad j. Crocodile k. Ostrich Project Work Collect different parts of any five monocots and dicots and attach them in a chart paper by placing them in a plastic bag.
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 49 Key terms and terminologies of the unit 1. Life-cycle : A lifecycle is the series of stages of life of an organism, beginning with life and ending with death. 2. Nectar : Nectar is a sugary liquid produced by plants in floral or extrafloral (on leaf or stem) nectar-secreting glandular organ. 3. Royal jelly : Royal jelly is a special type of secretion produced by the hypopharyngeal glands of the worker bees which contains water, proteins, sugars, amino acids, fatty acids and vitamins. 4. Apiology : Apiology is the branch of science which deals with the study of honey bees. 5. Queen bee : A bee colony has a single queen that runs the whole hive. It is the only egg laying female in a bee colony. 6. Drones : Drones are very lazy male honey bees. Their only job is to mate with the queen. 7. Worker bees : Worker bees are sterile females that perform all the jobs in the hive. 8. Nuptial flight : After 3-5 days of maturity, the queen bee flies out of the hive to mate with the drones. This flight is called mating flight or nuptial flight. 9. Metamorphosis : The process which involves egg, larva, pupa and adult to complete life-cycle is called metamorphosis. 10. Spermathecae : After the mating, the sperms are stored in the special structure of queen bee are called spermathecae or sperm sac. 11. Moulting : During larval stage, the larvae changes its skin 4-5 times called moulting. 12. Honeycomb : A honeycomb is a mass of hexagonal cells made by the worker bees using their wax. 13. Apiculture : The commercial rearing of bees is known as apiculture. Sequence of Curriculum Issued by CDC Lifecycle of honey bee Importance of honey bee UNIT Life Cycle of Honey Bee 3 Estimated teaching periods Theory Practical 3 1 Charles Darwin is synonymous with “evolution” for his contribution to evolutionary biology. He proposed that all species of life have descended from a common ancestor. He was born in England on 12th February 1809 and died on 19th April 1882. He was a naturalist, geologist and biologist. He has contributed to the investigation of marine invertebrates and natural science. Charles Darwin About the Scientist
50 life cycle of Honey Bee Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Introduction to lifecycle From beginning to the death, an organism passes through series of stages. These stages are referred to as steps of lifecycle. A lifecycle is the series of stages of life of an organism, beginning with life and ending with death. For example, the life cycle of a bird consists of four main stages. They are: egg, hatchling, young, and adult. In many simple organisms, including bacteria and various Protista the life cycle is completed within a single generation. In these organisms lifecycle begins with the fission of an existing organism. The new organism grows to maturity and then splits into two or more new individuals and therefore lifecycle gets completed. Such type of lifecycle is called haplontic. In higher animals, the lifecycle also includes a single generation. But in these organisms, the body cell is diploid. So, the lifecycle is called diplontic. But in insects like silkworm, honeybee, etc. the lifecycle completes in four stages. They are egg, larva, pupa and adult. Honey bee Honey bee is a social insect. The members of honey bee colony are extremely disciplined and industrious. They keep themselves busy all the time. Naturally, honey bees live in a special enclosed structure known as wild hive. Whereas some are domesticated in a place called apiary. Honeybees provide a wide range of benefits to humans. They produce honey, wax and other products. To make honey and wax, honey bees feed on nectar. Nectar is a sugary liquid produced by plants in floral or extrafloral (on leaf or stem) nectar-secreting glandular organ. Honey bees are the flying insects that belong to the arthropoda phylum. Apiology is the branch of science which deals with the study of honey bees. The scientists who study about honey bees are called apiologists. Apis mellifera and Apis cerana are the two species of honey-producing bees known in Nepal. Structure of honey bee 1. The size of honey bees ranges from 9 mm to 20 mm. 2. Their bodies are divided into head, thorax and abdomen. 3. There are three segments in thorax and six segments in abdomen. 4. They have a pair of compound eyes and a pair of antennae. 5. They have three pairs of jointed legs. 6. The type of skeleton present in honey bee is exoskeleton. 7. They undergo a complete metamorphosis. 8. The queens and workers have a stinger on the rear apex of their bodies for defence. A honey bee hive Honey bees in cells MEMORY TIPS Honey bee is a social insect because it lives in a large wellorganized colony.. Systematic classification Kingdom – Animalia Phylum – Arthropoda Class – Insecta Genus – Apis Honey bee
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 51 Organization of the honey bee colony There are three types of bees in a honey bee colony. They are: i) The queen bee: A bee colony has a single queen that runs the whole hive. It is the only egg laying female in a bee colony. ii) Drones: Drones are male honey bees. Their only job is to mate with the queen. They are very lazy. There are about five hundred drones in a colony. iii) Worker bees: Worker bees are sterile females that perform all the jobs in the hive. There are 20 thousand to 80 thousand worker bees in a colony. Queen bee The queen is the largest single fertile female bee in a honey bee colony. Its body is divided into head, thorax and abdomen. Its head is smaller than other bees. It has a shorter proboscis. Queen bees have smoother stingers with smaller barbs. It can sting more than once. Generally, it stings other queen bees. A diploid queen has 32 chromosomes in its cells. During the formation of gametes, a diploid cell divides into haploid cells with 16 chromosomes in each. The queen neither collects nectar nor does any other work. Its main function is to lay eggs. The queen bee usually mates only once in her life and stores the sperms received from the drone in a sperm sack (i.e. spermathecae) in her abdomen throughout her life. A healthy queen can lay about 3000 eggs in a day. The queen bee lays eggs in the hive till she gets old, killed or lost. A healthy queen bee lives the longest in the hive i.e. 2 to 5 years. After that, a new queen is made by the workers. To make a queen bee, the worker bees feed a special liquid called royal jelly to the selected larva. The larva which is continuously fed with the royal jelly turns into a queen. The queens and the workers are only different because of the difference in feeding. The qeen bee runs the hive. It produces a number of different pheromones (i.e. characteristic scent or odour) from a gland near her jaw that attract other bees to her. Pheromones tell the other bees about the condition of the queen inside the hive. It is the scent of the queen that keeps the bees together in a swarm. The worker bees find their hives with the help of queen’s scent. Drone bees Drone bees are the fertile and haploid male bees. It means that they contain half number of chromosomes (16) in their cells. They are formed from the haploid eggs. This process is called parthenogenesis. Their number ranges from few to 500 in a hive. The body of the drones is divided into head, thorax and abdomen. They have larger head and eyes. Generally, these bees are very lazy and do not work. Their size is The venom gland is a gland containing poisonous chemicals and is located in the abdomen while the stinger is a sharp organ present at the rear end of the worker and queen bee. MEMORY TIPS MEMORY TIPS Queen bee is female, diploid, largest, fertile, elongated and smooth. Queen bee A queen bee lays both diploid and haploid eggs. The diploid egg develops into either a worker bee or another queen. Both of these are female. While the haploid develops into a drone (male). MEMORY TIPS Drone bee
52 life cycle of Honey Bee Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur also smaller than the queen bee but larger than the worker bee. Their body is black and hairy. They do not have nectar gland, pollen sac, venom gland and stinger. Drones fly out with the queen to mate outside the hive. Usually several drones mate with a single queen. They die after mating. Their life span is generally about two monts. Drone bees live temporarily in the hive either until mating with the queen or till the winter arrives. As they mate with the queen, they die because their penis and abdominal tissues are expelled along with the sperm. If winter approaches, they are expelled from the hive by the worker bees or are killed. Hence, the only function of the drones is to mate with the queen. FACTS WITH REASONS The drones die after mating. Why? The drones die after mating with the queen because their penis and abdominal tissues are expelled along with the sperm during the mating process. Worker bees The body of worker bees is divided into head, thorax and abdomen. Like other insects, they have 3 pairs of legs with hairs on them. The worker bees are structurally different from the queen bees and the drone bees. They are the smallest, non-fertile female bees in the bee hive. They live for almost 6 weeks to 6 months. A hive may have 20,000 to 80,000 workers. Colony of Apis melifera has 60,000 to 70,000 worker bees. Colony of Apis cerana has 25,000 to 30,000 worker bees. Workers gather pollen and nectar of flowers. To do so, they have mouth parts for chewing and lapping. Such mouth parts help them to collect the nectar easily. They store the nectar in their abdomen. Likewise, they have special hairs and sac like structures in their feet known as the pollen basket. They use these baskets to store pollen grains of the flowers. After collecting pollen grains and nectar, they carry them to the hive and store them in the hive to feed the larvae, queen and the drone. FACTS WITH REASONS Worker bees die after stinging, why? When a worker bee stings a person, it cannot pull the barbed stinger back out. It leaves behind not only the stinger, but also the part of its digestive tract, muscles and nerves. Due to this abdominal rupture the worker bees die after stinging. Why do worker bees collect nectar of flowers? The worker bees collect nectar because they convert nectar into honey and store it in the hive cells. Drone bee is male, haploid, medium sized, fertile, black, lazy and hairy. MEMORY TIPS Worker bee Worker bee collecting nectar 1. Worker bee is female, diploid, smallest, non-fertile with hairy legs. 2. If pollen basket gets detached, workers cannot collect pollen grains. MEMORY TIPS
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 53 FACTS WITH REASONS How are the queen and worker bees similar and different? The queen bee and the worker bees are similar because both are diploid females with similar genes. They are develop from similar kinds of eggs. The only difference between them is that the queen is fertile and is fed with royal jelly but the workers are infertile and are fed with pollen and honey in their larval stages. Function of the worker bees The worker bees are responsible for doing every work in the bee hive. Their functions are as follows: 1. They collect nectar and pollen from the flowers to feed the larvae, queen and the drone. 2. They construct the bee hive. 3. They defend the other bees and protect the hive against the predators. 4. They feed and look after the larvae. 5. They take care of the young bees (brood cells) and protect them. 6. Worker bees also maintain proper temperature of the bee hive. 7. They manufacture honey which is consumed both by bees and human beings. Comparison of the Queen, Drone and the Worker bees Basis of comparison Queen Drone Worker Size Largest Medium Smallest Stinger Present/ It can sting more than once Absent Present/ It can sting only once Life span 2 to 5 years about 2 months; dies after mating or in winter 6 weeks to 6 months. Fertility Fertile Fertile Sterile Function Laying eggs and directing the worker bees Mating with the queen and keeping hive warm Constructing the hive, feeding larvae, collecting nectar, etc. Number Usually one Up to 500 20,000 to 80,000 based on species Inside the honey bee hive The honey bee hive is a natural waxy or artificial wooden structure. It acts as a home for honey bees. The natural hive is made up of wax which is produced by young worker bees. These worker bees produce sticky substance from their glands that become wax when they come in contact with air. The hive contains numerous hexagonal and cup-like structures called cells. These cells are the places where growth and development of the broods (young bees) occur. Hence, they are also called brooding cells. There are three kinds of cells present in a hive. They are drone cells, Bees in the hive
54 life cycle of Honey Bee Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur worker cells and the queen cells. The drone cells are larger in structure and is the home for the growth of drone bees. Similarly, the worker bees grow in worker cells which are smaller. The queen cells are extra-large and hanging downwards. These cells bear queens. Life Cycle of Honey bee The time taken to complete the life cycle depends upon the type of bee. Generally, there is a single queen for each hive. When the queen becomes old or the number of worker bees increases significantly, the old queen selects another place for building hives and many worker bees follow her. The worker bees build a new hive. In the old hive, the worker bees then select another queen. The life cycle of a honey bee is also completed into four stages. They are egg, larva, pupa and adult. The process which involves egg, larva, pupa and adult to complete life-cycle is called metamorphosis. i) Egg The queen usually stays inside the bee hive until maturity. The matured queen bee releases chemicals called pheromones to attract drones towards her. After 3-5 days of maturity, the queen bee flies out of the hive to mate with the drones. This flight is called mating flight or nuptial flight. Many drones follow the queen to get the opportunity to mate with her. She chooses to fly high and away from the drones to select the best and healthy drone male and mates with about 10-20 drones. After the mating process, the sperms are stored in the special structure called spermathecae or sperm sack in the body of the queen bee. These sperms can be utilized for few years to fertilize millions of eggs. After mating, drones die. After 2-3 days of mating, the queen lays eggs in brood cells inside the hive. She lays about 3000 eggs per day depending upon the species. These eggs are 1 mm to 1.5 mm in size and are lemon yellow in colour. The eggs are of two types. i) Fertilized eggs ii) Unfertilized eggs The fertilized eggs are formed after the fusion with sperms stored inside the female. Similarly, the unfertilized eggs are formed without fusion with the sperm. After laying, the fertilized eggs turn black while the unfertilized eggs turn white. The fertilized eggs develop into worker bees and queens. Similarly, the unfertilized eggs develop into drones. On the first day, the egg lies vertical inside the brood cell. Similarly, on the second day, it lies slightly slanted and on the third day, the egg lies horizontally in the cell. The brood Lifecycle of honeybee Fertilized eggs have 32 chromosomes and unfertilized eggs contain 16 chromosomes. MEMORY TIPS The process in which a haploid egg changes into drone bee is called parthenogenesis. MEMORY TIPS
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 55 cells for workers and drones are different, so the queen lays eggs in these cells differently. She lays fertilized eggs in the brood cells of the worker bees to develop them into workers in the future. Likewise, she lays unfertilized eggs in the brood cells of the drone bees to develop them into drones. The yolk present inside the egg acts as food. ii) Larva The eggs turn into larvae after three days. The larvae are continuously fed with large amount of pollen, honey and a special type of fluid called royal jelly. Not all the larvae are fed with the same type of feeding material. All types of larvae are fed with royal jelly for the first 3 days. But, after that, the feeding pattern changes. To make a queen, the worker bees feed the larva in the queen brood cell continuously with royal jelly till the end of this stage. But, after 3 days, the drones and the workers are fed with pollen and honey but no royal jelly. Due to this difference in the feeding pattern, the queen, workers and the drones grow and develop differently. The brood cells of larvae are covered with wax by the worker bees for their safety. The larvae then spin a cocoon around themselves for protection. They turn into pupa in 5-7 days depending on the type of bee. The larva stage is a period of 5 days in queen cells and 6 days in worker cells and 7 days in drone cells. During this stage, the larvae changes its skin 4-5 times called moulting. Why is the larva for queen bee continuously fed with royal jelly? The larva for queen is continuously fed with royal jelly because it activates to produce a fertile ovary that can bear eggs and turn her into a queen. The worker bees seal the brood cells after the development of larvae, why? The workers seal the cells after the development of larva to protect the larva, pupa and the adult during the metamorphosis. FACTS WITH REASONS iii) Pupa After 5-7 days of larval stage, the larvae turn into pupae. The pupae slowly develop inside the brood cells and undergo active metamorphosis inside the cocoon. Finally, they turn into adults after about 7 to 14 days depending on the type of bee. In queen bees, the pupal stage lasts for 7 to 8 days. Similarly, in the worker bees, the pupal stage lasts for 11 to 12 days and in drones, it lasts for 14 days. The pupal stage is inactive stage because they do not move, eat and show any visible changes. iv) Adult The pupae finally turn into adults. They crawl out of the brood cells by cutting the cocoon and the cell cup with their mandible. Among the types of bees, the worker bees have very Royal jelly is a special type of secretion produced by the hypopharyngeal glands of the worker bees which contains water, proteins, sugars, amino acids, fatty acids and vitamins. MEMORY TIPS Honey bread is made from honey and pollen. The larvae for drone bee and worker bee are fed with pollen bread after 3 days. MEMORY TIPS
56 life cycle of Honey Bee Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur important function. So, they immediately join with other bees to perform various functions. Workers work inside the hive for about 3 weeks. In this period, they look after the brood cells, feed the larvae, guard the hive internally, etc. After 3 weeks, worker bees go out of the hive for doing outdoor works like collecting nectar, guarding the hive, etc. The function of the adult queen is only laying eggs after mating with the drones. If more than one queen hatches, the first queen that hatches kills the other queens and mates with the drone. Similarly, the function of drones is to mate with the queen and warm up the hive. Figure: Different stages of honey bee Time taken by bees to complete their life cycle The time taken by the queen, drones and the workers to complete their life cycle is given in the table below. Bee Eggs Larva Pupa Duration required to be an adult Average life span Queen 3 days 5 days 7 to 8 days 15 to 16 days 2 to 5 years Worker 3 days 6 days 11 to 12 days 20 to 21 days 6 weeks to 6 months Drone 3 days 7 days 14 days 24 days about 2 months The functions of various bees at their various stages S.N. Type (caste) of bees Age Interval Functions or duties 1 Drone bee Throughout their lifetime To mate with the queen and warm up the hive 2 Queen bee Throughout their lifetime To lay eggs To guide and unite the hive members by secreting a characteristic scent or odour. 3 Worker bee 1-3 days These worker bees are small and do not perform many functions. So, they are also called baby worker bees. They provide warmth to the eggs, larvae and pupae. They learn to fly, move, eat and clean the hive. 3-6 days They eat a lot and also feed other larvae. 6 - 12 days At this age, the hypopharyngeal glands are developed and produce royal jelly. The worker bees of this age feed royal jelly to the larvae and the queen.
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 57 12-18 days During this age, the hypopharyngeal glands become degenerated. Then, due to overfeeding of honey, 4 pairs of wax glands are regenerated in their abdomen. The workers of this age secrete a solid sticky substance called wax. They use wax to construct hives and seal brood cells. So, they are also called builders or constructors. 18-20 days The wax glands also get degenerated. Then, venom/ poison glands and stingers are produced. The stingers help to attack other invaders and guard the hive. Only the queen and the workers have stingers. The queen uses its stinger to attack or defend itself against other queens. Similarly, the workers use the stingers to attack or defend against other insects and protect the hive. After 21 days They start to collect nectar, pollen, water, etc. The honey making process Honey is a sweet sugary liquid prepared by honey bees. It has food and medicinal values. It has about 80-85% carbohydrates, 15-17% water, 0.3% protein and 0.2% amino acids, vitamins, etc. The bees produce the honey by collecting the nectar of the flowers. In this process, the worker bees visit many types of flowers and collect different types of nectars. The nature and colour of honey depends on the nature of the flowers from which the nectar is collected. In the honey making process, the worker bees wander many kilometres around the bee hive and visit millions of flowers. After collecting nectar, worker bees store it in their abdomen where the nectar combines with several proteins and enzymes to form honey. The honey is then deposited and stored in the honeycomb in the bee hive. A honeycomb is a mass of hexagonal cells made by the worker bees using their wax. Till all the cells in a honeycomb are full, the worker bees keep on collecting the nectar and depositing the honey. To make the honey thick, the worker bees evaporate the water present in the honey juice by flapping their wings like a fan over the comb. Uses of honey i. It is used as a tonic. ii. Honey is used as a sweetening agent in foods, juices and beverages. iii. It is useful in the manufacturing of candies, cakes and breads. iv. It removes harmful substances from the body and acts as an antioxidant. v. Honey is used in manufacturing cosmetic products. Comb honey The bees are the only insects which manufacture food that is directly consumed by human beings. Memory Tips
58 life cycle of Honey Bee Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur vi. Honey has antibacterial properties and cures cuts and bruises. vii. It cures cough and throat infection. viii. Honey lowers the risk of cancer, allergies, arthritis and heart diseases. Advantages of honey bees Honey bees are useful to both human beings and ecosystem. They are reared commercially near the crops which provide good nectar for honey. The commercial rearing of bees is known as apiculture. Apiculture provides us honey bee products. Some of the honey bee products are honey, wax, royal jelly, etc. Some advantages of honey bees are given below : i. Honey made by honey bees is consumed by human beings for food and medicinal purposes. ii. The systematic work division of honey bees is of great importance to anthropologists. iii. The wax that we get from bee hives is used for making candles, paints, shaving cream, cold creams etc. iv. Royal jelly produced from workers has several medicinal values. It is beneficial for reducing blood pressure and cholesterol levels in blood. It also helps to boost the immune system. v. Honey bees are good pollinators. They help in pollination while sucking the nectar. Thus, they increase the productivity and quality of the plants. vi. The rearing of honey bee generates income and also improves the national economy. Humans rear honey bee, why? Humans rear honey bees to get honey, royal jelly, wax and other bee products. Why does honeybee queen go out of the hive? Honeybee queen goes out of the hive to mate with the healthiest drones. Anthropologists have a keen interest in honey bees. Give reason. Anthropologists want to study about the systematic work division, dominance of certain castes and submissiveness of worker bees. The productivity of some crops increases in areas where honey bees are reared, why? Honeybees choose the best and healthy plants to collect pollen. It helps cross-pollination of the flowers easily and in a short period of time. FACTS WITH REASON ACTIVITY 1 Visit a site of honey bee hive or an apiary in apiculture site. Take papers, pencils and erasers with you. Study the behaviour of different kinds of bees, their structure and hive cells. Draw pictures of what you see by studying the site.
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 59 HOT SKILL HIGHER ORDER THINKING SKILL 1. The productivity of some crops increases if nearby farmers keep some bees in those areas. Honeybees choose the best and most healthy plants for gathering pollen. This helps cross-pollination of the flowers in a short period. As the parent crops are healthier and pollination is quicker, the productivity is increased, if nearby farmers keep some bees in those areas. 2. Differentiate between nuptial flight and swarming. The differences between nuptial flight and swarming are: SN Nuptial flight SN Swarming 1 Nuptial flight means the process in which matured queen bee flies out of the hive to mate with drones and collect sperm. 1 Swarming is a process in which a colony of bees splits into two or more separate colonies. 2 The queen bee and the drones take part in this flight. 2 Queens, drones and workers take part in swarming to make a new hive. 3. What happens if worker bees lose their pollen basket? If worker bees lose their pollen basket, they cannot help in pollination and they cannot collect pollen to make food for the larva. 4. What happens to the colony if the queen bee dies? If the queen bee dies following things can happen to the hive: i. The odour of the queen fades and the bees will go out of control. ii. No new eggs will be produced so the hive will end. iii. The worker bees will find multiple suitable female larvae and start feeding royal jelly to them. As a result, new queens are formed. If there are multiple queens, the stronger one will kill others. Then the new queen bee will spread her odour and take control of the hive. 5. Label the parts of the honey bee given in the figure alongside. A – head B – abdomen C – stinger 6. Scientists say that humans might go extinct if bees go extinct. How can we relate the extinction of bees to the extinction of humans? Besides producing honey, bees play very important role in pollination. All animals including humans directly or indirectly depend upon plants for their food. If bees go extinct, rate of pollination decreases. This results scarcity of foods for humans. So, scientists say that humans might go extinct if bees go extinct. 7. Describe the life cycle of a worker bee. The life cycle of a worker bee starts with a fertilized egg. The egg hatches in three days. In the first three days, the worker bees learn to walk and clean the hive. On the third to the sixth day, they feed the larva. From the sixth to twelfth day, they produce royal jelly. From the twelfth to the eighteenth day, they secrete wax and make honey comb. From the eighteenth to the twentieth day, they protect the beehive. And from the age of 21 days, they go outside to collect nectar and pollengrain. A B C
60 life cycle of Honey Bee Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 8. Apiculture can be a good source of income for Nepalese with low investment. Support the statement. . Apiculture is an act of rearing honey bees. It can be a good source of income for Nepalese because of the following reasons: i. Nepal has lots of forests with varieties of flowering plants so bees can find food. ii. Nepal is an agricultural country so there are lots of crops where bees can collect nectar. iii. It does not cost much to start a bee farm. 9. Consumption of honey helps to reduce cough. What might be the reason? Honey is known for its anti-microbial and anti-inflammatory properties. These properties of honey prevents the spread of bacteria and reduce inflammation resulting reducing the cough. 10. Differentiate between drone bees and worker bees. The differences between drones and worker bees are: S.N. Drones S.N. Workers 1. The drone has medium size. 1. The worker has the smallest size. 2. Stinger is absent. 2. Stinger is present. 3. Its lifespan is about 2 months. It dies after mating or in winter 3. Its lifespan is of 6 weeks to 6 months. 4. It is fertile. 4. It is sterile. 5. Its works are mating with the queen and keeping the hive warm. 5. Its works are constructing the hive, feeding larvae, collecting nectar, etc. 6. Their number is up to 500 in a natural hive. 6. Their number ranges from 20,000 to 80,000 in a natural hive based on species. 7. Drone bees are born from unfertilized eggs. 7. Worker bees are born from fertilized eggs. 8. There are 16 chromosomes in drone bees. 8. There are 32 chromosomes in worker bees. 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a. What do honey bee feed upon? i. blood ii. juice from stem of herbs iii. nectar and pollen iv. insects b. How many morphologically different bees live in a hive? i. 3000 ii. 3 iii. 20000 to 80000 iv. 100 to 500 c. How many chromosomes are present in a drone bee? i. 16 chromosomes ii. 16 pairs chromosomes ii. 32 chromosomes iv. 8 pairs
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 61 d. Where is royal jelly produced? i. pollen grain ii. hypopharyngeal of worker bee iii. sting of queen bee iv. wings of worker bee at thorax e. What are the characteristics of worker bees? i. haploid, sterile, furry, medium-sized ii. diploid, sterile, small-sized iii. diploid fertile medium-sized iv. diploid, fertile, large 2. Define the following terms with required examples. a. Nuptial flight b. Queen bee c. Drone bee d. Worker bee e. Royal jelly f. Swarming g. Hive h. Wax i. Pollen bread 3. Answer the following questions in very short. a. What is a honeybee? b. How many chromosomes are there in a fertilized and unfertilized egg of a honey bee? c. Write the names of different morphological forms of honey bees found in the honey bee colony. d. Where does the queen bee store sperm after the nuptial flight? e. Which bees in the hive are called lazy? f. What is the function of drone bees? g. In which phylum do we keep honey bees? h. What is an apiary? i. In how many days does an egg of a bee hatch into a larva? j. What do queen bees eat? k. Honeycombs are always clean. Who cleans them? STEP2 4. Give reason. a. Honey bees collect nectar of the flower. b. Honey bee is called a social insect. c. Apiculture needs bee pasture. d. The drone dies after mating with the queen. e. Nuptial flight is very important for the survival of bees. f. Larva for the queen is continuously fed with royal jelly but not the worker bees. g. Bees are useful in many ways. h. Drones come out of unfertilized eggs of bees. i. Bee is very important for the lifecycle of plants. j. Honey can last for hundreds of years.
62 life cycle of Honey Bee Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur k. Worker bees die when they sting others. l. Worker bees have a very long proboscis. 5. Differentiate between the following. i. Queen bee and worker bee ii. Drone bee and worker bee iii. Queen bee and drone bee 6. Answer the following questions in short. a. Classify honey bee. b. How does the queen bee control others? c. Write down the uses of honey. d. Write in very short about: i. Royal jelly ii. Nectar iii. Queen bee iv. Drone v. Worker bee vi. Parthenogenesis e. Which type of bee is developed, if i. the male larvae are fed only with royal jelly? ii. the female larvae are fed royal jelly in the early stage and pollen and honey later? iii. the female larva is fed on royal jelly only? f. At what age does the worker bee: i. produce wax? ii. grow a stinger? iii. collect nectar? g. How do the worker bees collect nectar from flower? h. Write the composition of royal jelly. i. What does a queen bee do in the hive? j. How do honey bees help human beings? 7. Observe the given diagram of the queen bee and answer the following questions. i. Why does it have a larger abdomen? ii. How long does it live? iii. What is the cause of its long life? iv. Why does it start swarming? STEP3 8. Answer the following long questions a. Describe the structure of the queen bee. b. Point out the functions of worker bees. c. Point out the functions of drone bees. d. How is honey prepared by the honey bee? e. "Honey bees are beneficial not just to humans but for the nation too.” Justify this statement.
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 63 f. How does a worker bee make a new queen? g. Under what conditions does a fertilized egg change into either queen or a worker bee? h. Explain the reproductive process of a queen bee. i. Write down any three advantages of the honey bee. j. Mention any four benefits of honey. k. Make a table and compare the queen, drone and worker bees based on their lifespan, function, number and fertility. l. Explain the life cycle of a honey bee in brief along with a well-labelled diagram. m. How do honey bees distribute work loads? Write down in a table. n. Many people feed honey to their kids when they have a cough. Does it work? Why? o. Archaeologists sometimes find pure and fine honey in the pyramids. Why did they last so long? p. Most drones die after nuptial flight. What could be the reason? q. What happens if there are multiple queen bees in the hive? What should we do to avoid that? r. Write what A, B, C and D represent in the figures. A B C D Project Work Visit an apiculture farm and observe the following: i. Construction of the hive ii. Structure of the different bees iii. Extraction of honey Also, make different groups of students and prepare reports along with neat and labelled diagrams.
64 H eRedity Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Heredity Unit 4 Cell Nucleus Chromosome Father XY XX Daughter Son Daughter Son XY XX XY Mother XX
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 65 Key terms and terminologies of the unit 1. Cell : The fundamental, structural, and functional unit of life is called the cell. 2. Cell division : Cell division is the biological process through which a pre-existing diploid cell divides into two or four daughter cells. 3. Mitosis : Mitosis is an equational division in which a mother cell divides into two identical daughter cells. 4. Meiosis : Meiosis is the reduction division of a diploid (2n) parent cell into four haploid (n) daughter cells. 5. DNA : DNA (deoxyribonucleic acid) is a genetic material that is responsible for the transmission of genetic information from one generation to the next. 6. RNA : Ribonucleic acid (RNA) is a single-stranded macromolecule which helps in protein synthesis. 7. Chromosomes : The thread-like microscopic structures made up of histone proteins and DNA and present in the nucleus of a cell are called chromosomes. 8. Chromatids : The two identical thread-like strands of a longitudinally split chromosome are called chromatids. 9. Centromere : The chromatids are connected together by a central spherical body called centromere. 10. Autosomes : The chromosomes which determine somatic body structure are called autosomes. 11. Sex chromosomes : The chromosomes which determine sex of the offspring are called sex chromosomes. 12. Gametes : The haploid cells which take part in sexual reproduction are called gametes. 13. Sex determination : The biological cause for an organism developing into one sex or the other due to the genes in the fused gametes is called sex determination. Sequence of Curriculum Issued by CDC Concept of mitosis and meiosis cell division Importance of mitosis and meiosis cell division Differences between mitosis and meiosis cell division (based on number of daughter cells, number of chromosomes and types of dividing cells) Chromosomes, DNA and RNA (introduction and differences) Role of sex chromosomes in sex determination of human beings UNIT Cell Division and Chromosomes 4.1 Estimated teaching period Theory Practical 4 1 Hugo Von Mohl is a botanist noted for his research on the anatomy and physiology of plant cells. He was born in Germany on 8th April 1805 and died on 1st April 1872. He is renowned for discovering protoplasm. He pursued botany and mineralogy. He discovered many facts about cell walls, vacuoles and plastids. He discovered that all metabolic activities occur in protoplasm. Most importantly he discovered role of protoplasm in 1835. Hugo Von Mohl About the Scientist
66 cell diviSion and cHRomoSomeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur The Cell Observe a building. What raw materials are used to make it? At the same time observe the wall of the building. What are used to make it? Obviously, the wall of a building is made up of many bricks. These bricks are the basic units to make the structure of the whole building. Similarly, a multicellular organism is made up of many cells. The body of living beings is made up of microscopic components called cells. Thus, the fundamental, structural, and functional unit of life is called the cell. Protoplasm, cell organelles, and inclusions are the building blocks of cells. A cell is self-contained and performs vital biological activities. Ribosome cytoplasm microtubules centrioles nucleus nucleolus golgi apparatus mitochondrion cell membrane Smooth eR endoplasmic reticulum (eR) lysosome cell wall Pinocytotic vesicle cell membrane golgi apparatus chloroplast vacuole membrane Raphide crystal druse crystal mitochondria cytoplasm large central vacuole Rough (eR) nucleus nucleolus endoplasmic reticulum (eR) Ribosome Figure: a plant cell and an animal cell Cell Division Every organism begins as a single cell. New cells are regularly formed from pre-existing ones. Cell division is the biological process through which a pre-existing diploid cell divides into two or four daughter cells. It is a significant phenomenon that occurs in all living things. Cell division is the process by which new cells are formed. Figure: a cell divides into two daughter cells Importance of cell division Cell division is necessary for the survival of a species. During cell division, all of the structures of a parent cell are copied and distributed to each of the daughter cells. Cell division is a method of reproduction in unicellular organisms because it produces two daughter organisms. In multicellular organisms, there are two types of cells: somatic cells and reproductive cells. Somatic cells, also known as body cells, make the body of living animals, whereas reproductive cells produce gametes such as eggs and sperm. Multicellular organisms begin life as a single cell (called a zygote), which divides and re-divides to produce a complete organism.
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 67 Types of cell division Depending on the method of nuclear division, cell divisions are divided into three types. They are: amitosis, mitosis, and meiosis. Mitosis (Somatic Cell Division) In 1877 AD, German biologist Walter Flemming, for the first time observed the phenomenon of the mitosis or mitotic cell division. Mitosis produces two daughter cells that are identical to the mother cell in every aspect. As a result, it is also known as equational cell division. Thus, mitosis is an equational division in which a mother cell divides into two identical daughter cells. Mitotic cell division keeps the same (i.e. diploid) number of chromosomes as the parent cell in each division of the cell. Mitosis is the biological division of a diploid parent cell into two diploid daughter cells. Mitosis occurs in either vegetative or somatic cells. As a result, it is also known as somatic cell division. It is mainly responsible for growth and development of an organism. Figure: a mitosis cell division Significance of mitosis The main importance of mitosis cell division are listed below. i. Mitosis is the process through which a multicellular organism grows and develops from a single cell called a zygote. ii. Mitosis cell division helps to replace damaged or lost body parts, heal wounds, make new cells, and replace cells that are damaged from normal use. iii. Mitosis is a method of asexual reproduction in unicellular organisms. iv. Genetic stability in living beings is maintained via mitotic cell division. This is due to the fact that mitosis produces daughter cells with the same number of chromosomes as the mother cell. v. Uncontrolled mitotic cell division can lead to tumor or cancer. Meiosis (Gametic Cell Division) In 1905 AD, JB farmer and JE Morre coined the word "meiosis. "In sexually reproducing organisms, meiosis or meiotic cell division occurs only in reproductive cells during
68 cell diviSion and cHRomoSomeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur gametogenesis (the process by which male and female gametes are formed). As a result, it is often referred to as gametogenic division. Meiotic cell division is a modified mitosis in which chromosomes split once and the nucleus divides twice, resulting in four daughter cells with half the number of chromosomes. Meiosis is hence also known as reduced division. Meiosis is the reduction division of a diploid (2n) parent cell into four haploid (n) daughter cells. Figure: a meiosis cell division Significance of meiosis The main points of importance of meiosis cell division are listed below. i. Meiosis is responsible for the formation of haploid gametes (such as sperms and ova), which after fertilization regain the original diploid number. Thus, meiotic cell division contributes to genetic stability by keeping the fixed number of chromosomes in the generation. ii. Crossing over makes a new set of chromosomes. As a result, organisms with a new characteristics and traits are formed. iii. The four chromatids of a set of homologous chromosomes are passed on to four different daughter cells. This gives the offspring with different genes. iv. It helps sexual reproduction and keeps chromosomes from being duplicated in the offspring through a process called reduction division. v. It helps in organic evolution of the organisms. Differences between mitosis and meiosis S.N. Mitosis S.N. Meiosis 1. It is the division of a diploid mother cell into two diploid daughter cells. 1. It is the division of a diploid mother cell into four haploid daughter cells. 2. Mitosis cell division occurs either in the somatic or vegetative cells. 2. Meiosis cell division occurs during gamete formation in gametic or reproductive cells. In meiosis, crossing over is a process of exchanging genetic materials between two non-sister chromatids. It brings variations in the gametes. So, offspring are not exact to the parents. MEMORY TIPS No meiosis, no gametes and no sexual reproduction. MEMORY TIPS
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 69 3. The daughter cells are identical to the mother cell in every aspect. 3. The daughter cells differ from the mother cells as they have haploid chromosomes with variation of gene. Introduction to chromosome No organism survives forever. All of them are mortal. This happens in the same way in case of their offsprings, too. Have you ever wondered why human babies resemble humans? Why do they look like their parents? Offsprings of an organism look alike, and so it is in humans. This is possible because there are some structures inside our cells. These structures act like the blueprints of the human being, same as how an engineer knows the shape of the building before it is built using a blueprint map. The structures that carry this information of our bodies are hidden inside the nucleus of all eukaryotic cells. These structures are called chromosomes. In most eukaryotic cells, chromosomes are generally present more than two in numbers. The word chromosome is derived from the Greek words ‘khroma’ meaning colour and ‘soma’ meaning body. It is because when they were being researched, they looked colourful under microscope due to staining. Why are chromosomes called the blueprint of an organism? Chromosomes store information about every characteristic of the organism like body structure, height, skin complexion, behaviour, etc. So, chromosomes are called the blueprints of an organism. FACTS WITH REASON DNA DNA stands for deoxyribonucleic acid. It is a macromolecule that is double-stranded and coiled helically. It is found in chromosomes. It is the hereditary material that is present in all organisms. DNA is a genetic material that is responsible for the transmission of genetic information from one generation to the next. Location DNA is the primary constituent of chromosomes. It can be found in the nucleus of eukaryotic cells. Eukaryotic cells have a membrane-bound nucleus. However, there is also a trace quantity of DNA in the mitochondria and plastids of an eukaryotic cell. In that of prokaryotic cells, DNA is present in cytoplasm. Prokaryotic cells do not have a membrane-bound nucleus. Structure DNA is a macromolecule. It consists of two strands of polydeoxyribonucleotides. They are twisted together in a helical form. DNA is a polymer. It is formed from a large number of individual deoxyribonucleotides. They are often Cell Nucleus Chromosome Chromosome High intensity radiation may destroy DNA. It may bring genetic variation and infertility. Memory Tips
70 cell diviSion and cHRomoSomeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur known as monomers. DNA is formed by cross linking of three chemicals. They are deoxyribose sugar, phosporic acid and nitrogen bases. A molecule of DNA is composed of two unbranched complementary strands. They are wound around a central axis in a spiral mode. It is much like the steps of a rope staircase that have been twisted into a spiral. The strands of DNA run in an orientation that is antiparallel to one another. It indicates that two strands of DNA are running in different directions, despite the fact that they are parallel to one other. Functions of DNA 1. DNA is the molecule that is responsible for the transmission of genetic information from parents to children. 2. DNA stores genetic information as codes. They are set up in nitrogen bases. 3. DNA gives rise to RNA by the process of transcription. 4. RNA controls the metabolic processes of cells under the supervision of DNA. RNA Ribonucleic acid (RNA) is a single-stranded macromolecule. It is mainly found in cytoplasm, some part in nucleolus and nucleoplasm, in association with chromosomes of a cell. It is made up phosporic acid, ribose sugar and four types of nitrogen bases (i.e. adenine, guanine, cytosine and urasil). The single strand of RNA is formed by the alternate bands of phosphoric acid and ribose sugar. RNA is formed by DNA and it cannot replicate itself. There are three types of RNA, viz. mRNA, rRNA and tRNA. mRNA stands for messenger RNA. It carries genetic information from DNA and helps in protein synthesis. rRNA stands for ribosomal RNA. It forms the basic component of ribosome. Similarly, tRNA stands for transfer RNA. It carries amino acid molecule to the ribosome for protein synthesis. Differences between DNA and RNA DNA RNA 1. DNA is a double-stranded helically coiled structure. 1. RNA is a single-stranded structure. 2. DNA is responsible for transmitting hereditary characteristics from parents to their offspring. 2. RNA is responsible for protein synthesis. Figure: Structure of DNA The monomer of DNA is nucleotide. Addition or deletion of nucleotide causes genetic problem. MEMORY TIPS Figure: RNA No transcription of DNA, no RNA and no protein synthesis. MEMORY TIPS
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 71 3. It contains deoxyribose sugar. 3. It contains ribose sugar. 4. DNA is mainly found in chromosomes in the nucleus. 4. RNA is mainly found in cytoplasm. 5. DNA can replicate itself. 5. RNA cannot replicate itself. Structure of a chromosome When we observe a nucleus of a cell, we see fine elongated thread-like structures called chromatin fibres or chromatin network. Chromatin fibres are made up of DNA, proteins and minerals. During cell division, these chromatin fibres become shorter, thicker and condensed. The thread-like microscopic structures made up of histon proteins and DNA and present in the nucleus of a cell are called chromosomes. A chromosome is made up of mainly two parts. They are chromatid and centromere. a) Chromatids An undivided chromosome (before cell division) has one chromatid containing a single molecule of DNA. During cell division, the chromosomes divide and form an exact copy of each chromatid by a process called DNA replication. Thus, after division, there will be exact two copies of DNA molecules. The two identical thread-like strands of a replicated chromosome are called chromatids. These two newly formed chromatids from a single chromosome are also called sister chromatids. A chromosome usually has two arms. They are p-arm and q-arm. The length of p-arm and q-arm may or may not be equal. There is a specific protein at the end of all chromosomes called telomere that prevents the fusion of chromosomes among each other. b) Centromere or primary constriction or kinetochore The chromatids are connected together by a central spherical body called centromere. Centromere attaches the arms of the chromatids together. Since centromere is the body which helps in the movement of chromosomes towards the opposite poles of the cells, it is also called kinetochore. During cell division, the chromatids attach to the thread-like structures called spindle fibres and help in dividing the chromosomes. Chromosomes are the vehicles of heredity, why? Chromosomes are the vehicles of heredity because they transfer genetic information from parents to the offspring during fertilization FACTS WITH REASON Number of chromosomes The number of chromosomes is generally different for different species of organisms. Even closely related organisms like humans and chimpanzees (46 and 48 respectively) or donkey and horse (62 and 64 respectively) have different Chromosome is made up of DNA and histon protein. MEMORY TIPS p-arm q-arm Chromatids Telomere Centromere Chromatids In human, there are 22 pairs (44) autosomes and one pair(2) sex chromosomes. MEMORY TIPS
72 cell diviSion and cHRomoSomeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur numbers of chromosomes. But, some organisms have the same number of chromosomes. Generally, we write the chromosome number in pairs because one set comes from the father and the other set from the mother. For example, human cells have 46 chromosomes and we write 23 pairs. Among 46 chromosomes, we get 23 chromosomes from father and the rest 23 chromosomes from mother. Some of the organisms and their chromosome numbers are given in the table below : S.N. Organism Number of chromosomes S.N. Organism Number of chromosomes 1 Mucor 2 (1 pair) 8 Rat 40 (20 pairs) 2 Housefly 12 (6 pairs) 9 Monkey 42 (21 pairs) 3 Garden pea 14 (7 pairs) 10 Human, antelope 46 (23 pairs) 4 Onion 16 (8 pairs) 11 Chimpanzee, Orangutan, Gorilla, Potato, tobacco, water buffalo 48 (24 pairs) 5 Pine 24 (12 pairs) 12 Fox, goat, bull, cow 60 (30 pairs) 6 Frog 26 (13 pairs) 13 Dog 78 (39 pairs) 7 Tiger, lion, cat 38 (19 pairs) 14 Sugarcane 80 (40 pairs) Types of chromosomes Human beings have 23 pairs of chromosomes in their cells. Out of them, 22 pairs of chromosomes determine the somatic body structure. So, they are called autosomes. The remaining one pair of chromosomes determines the sex of the offspring. So, they are called sex chromosomes. In males, the cells have 44+XY chromosomes and in females, the cells have 44+XX chromosomes. Human chromosomes The chromosomes which determine somatic body structure are called autosomes. Similarly, the chromosomes which determine sex of the offspring are called sex chromosomes. MEMORY TIPS
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 73 Gamete formation In the human body, the diploid cell undergoes meiosis cell division forming haploid cells. These haploid cells are called gametes. Gametes take part in sexual reproduction. Thus, the haploid cells which take part in sexual reproduction are called gametes. The male gametes are called sperms. Sperms are of two types. They are represented by 22+Y and 22+X. Similarly, in females, there is only one type of gametes. They are represented by 22+X. Sex determination The biological cause for an organism developing into one sex or the other due to the genes in the fused gametes is called sex determination. The sex of an offspring is determined by a pair of chromosomes called sex chromosomes. These sex chromosomes are present in all cells including the sperms and egg. In female, the egg cells have identical sex chromosomes. They are represented by X and X. But, in males, they have different sex chromosomes. They are represented as X and Y. If the sperm having an X- chromosome meets with an egg having X-chromosome, a baby girl is born. Likewise, if a sperm having Y-chromosome meets with an egg having Y-chromosome, a baby boy is born. The chart to explain sex determination is given below. From the given information, it is clear that a female has same sets of chromosomes in her egg cells (22+X) but a male has two different types of chromosomes in his sperm cells (22+X and 22+Y). Therefore, the sex determination fully depends upon the type of male sperm (X or Y) which fuses with the egg (both X). Hence, the female has no role in sex determination. Parents → Result in F1 -generation Father Mother Sperm Sperm Ovum Ovum (female) Sperms (male) Girl (daughter) Girl (daughter) Boy (son) Boy (son) Ovum Gametes → 44+XY 44+XX 44+XY 44+XX 44+XX 44+XY 22+X 22+X 22+X 22+X 22+X 22+Y 22+Y 22+X Male gametes are called sperms which are represented by 22+Y and 22+X. Similarly, female gametes are called eggs which are represented by 22+X. MEMORY TIPS The two similar sex chromosomes in a female cell are called homogametic chromosomes. Likewise, the two different sex chromosomes in a male are called heterogametic chromosomes. Males generally have heterogametic chromosomes and females generally have homogametic chromosomes. MEMORY TIPS
74 cell diviSion and cHRomoSomeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur A mother has no role in sex determination of the offspring, why? A mother has only X chromosomes in her egg cells but father has either X or Y chromosomes in his sperm cells. The sex is determined by the X or Y chromosomes of the sperm of the father that combines with the X chromosomes of the mother. Since both the sex chromosomes of the mother are the same ( XX), the offspring’s sex does not change due to the mother’s sex chromosomes. Why is there an equal probability of a boy and girl in the new born baby, though there are 3 girls already? There is equal chance of fusion of sperms having X-chromosomes and Y-chromosomes with the egg of female having X-chromosomes. So, there is an equal probability of a boy and girl in the new born baby. FACTS WITH REASON HOT SKILL HIGHER ORDER THINKING SKILL 1. A little child grows into a teen and then into an adult. Mention the type of cell division which is responsible for the growth and development of the body. A little child grows into a teen and then into an adult. The cell division responsible for such growth and development of the body is mitosis cell division. 2. Chromosomes are called vehicles of heredity. Chromosomes are called vehicles of heredity because it contains DNA that contains our hereditary information. It is transferred to children from parents. 3. Children are similar to their parents. Children are similar to their parents because they have genes from their parents. 4. Name the part of the cell which is usually damaged by high-intensity radiation. DNA of the cell is usually damaged by high-density radiation. 5. If the ovum contains X chromosome, what does sperm contain? If the ovum contains X chromosome, then the sperm contains either X or Y chromosomes. 6. How do wounds heal? Where do the new cells to fill the wounds come from? The wounds heal by generating new cells. The new cells come from mitosis cell division. 7. How does meiosis cell division help in evolution? The meiosis cell division forms the haploid gametes. Male gametes in males and female gamets in females. The male and female gamete fuses to form a zygote. The zygote has two different sets of chromosomes. So, the new offspring can select better traits from either of the chromosomes of the pair. As a result, there will be variation in off springs. After multiple generations, the offspring will be too different from their ancestors and form new species. Hence meiosis helps in evolution. 8. A couple has four girls. Now, what will be the probability of the gender of the next baby. The probability of new born child being either a girl or a boy is 50-50. This is because there is an equal chance of the fusion of sperm having an X-chromosome and a Y-chromosome with the egg of a female having an X-chromosome. 9. Differentiate between autosomes and sex chromosomes. The differences between autosomes and sex chromosomes are:
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 75 S.N. Autosomes S.N. Sex chromosomes 1. Autosomes determine somatic body structure. 1. Sex chromosomes determine the gender of the offspring. 2. There are 22 pairs of autosomes in the cells of the human body. 2. There is one pair of sex chromosomes in the cells of the human body. 10. Identify the girl and boy in A and B. Mother → Father ↓ 22+X 22+X 22+X A 44+XX 22+Y 44+XY B A has 44+XX chromosomes. So, it is a girl.Similarly B has 44+XY chromosomes. So, it is a boy. 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answers from the given alternatives. a. How do characteristics of parents transfer to the children? i. through the blood ii. through the food iii. through the genes iv. through the environment b. How does the number of cells increase in an organism? i. by adding flesh manually ii. by a bee sting iii. swelling iv. cell division c. Why do genetic engineers add a portion of DNA from one species to another? i. to kill the organism ii. to feed organism iii. to bring variation in an organism iv. for fun d. Zygote is unicellular. Which cell division changes it into the multicellular structure? i. amitosis ii. binary fission iii. mitosis iv. meiosis e. What is the basic structural and functional unit of heredity? i. chromosome ii. nucleus iii. cell iv. gene 2. Define the following terms with required examples. a. Genes b. Chromatin fibres c. Chromosomes d. Autosomes e. Sex chromosomes f. Haploid cells g. Diploid cells h. Sex determination
76 cell diviSion and cHRomoSomeS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur i. RNA j. Mitosis k. Meiosis l. DNA 3. Answer the following questions in very short. a. Mention the number of chromosomes in Humans, Housefly, Frog, Chimpanzee, Onion, Pine, Mucor, Rat, Dog and Sugarcane. b. Which gender has the role in sex determination? c. What are chromosomes made up of? d. How many chromosomes are there in a human body? How many of them are autosomes and how many of them are sex chromosomes? e. What is the primary function of autosomes? f. What is the function of sex chromosomes? g. What happens if the transcription of DNA is malfunctioning? h. Write the karyotype of man and woman. i. What is the percentage probability of a child being a girl or a boy? j. Where are chromosomes found? k. What is the process by which one mother cell produces two daughter cells? l. Which cell division helps in sexual reproduction? m. What structures transfer heredity? n. Which cell division helps in the growth of our body? STEP2 4. Give reasons. a. Chromosomes are called the blueprint of an organism. b. Chromosomes are called the vehicles of heredity. c. Males are primarily responsible for the gender of the foetus. d. Even though sperm could have either X or Y chromosomes, some couples have children of the same gender, either all boys or all girls. Why? e. Mitosis is called somatic cell division. f. Mitosis is called equational cell division. g. Meiosis brings variation but mitosis does not. h. Meiosis helps in sexual reproduction. i. Mitosis helps in the reproduction of underdeveloped unicellular organisms. j. Meiosis is called reductional cell division. k. Gametes are made haploid by meiosis.
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 77 5. Differentiate between the following. a. Autosomes and sex-chromosomes b. Gamete and somatic cells c. Diploid cells and haploid cells d. Mitosis cell division and meiosis cell division e. RNA and DNA 6. Answer the following questions in short. a. Describe the structure of chromosomes with a diagram. c. Ramesh wanted a daughter but got 4 sons in a row, does that mean the sperm contained Y chromosomes only? Discuss. d. What is the main feature of mitosis? e. What is the main feature of meiosis? f. What is the function of the chromosome? g. What happens if there is no meiosis in germ cells of gonads? 7. Answer the following questions on the basis of the figure given below. i. Which nucleic acid is shown in the diagram? ii. What are the sugar and bases present in it? iii. What is its function? STEP3 8. Answer the following long questions. a. What is the importance of cell division in the human body? b. Discuss the role of mitosis and meiosis in the reproduction of organisms. c. Discuss the roles and functions of genes in the human body. d. How is the gender of the fetus determined? Draw the diagram. Project Work With the help of colourful threads make the different types of chromosomes and display in your class.
78 H eRedity and mendeliSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Key terms and terminologies of the unit 1. Heredity : The process by which parental characteristics are transferred to their offspring is known as heredity or inheritance. 2. Gene : A gene is a segment of DNA which is responsible to transfer parental characters from one generation to the next generation. 3. Allele : An alternative form of matching genes is called allele. 4. Dominant character : The character that expresses itself by suppressing the other character is called the dominant character. 5. Recessive character : The character which fails to get expressed in the first generation and usually hides itself is known as the recessive character. 6. Monohybrid cross : The cross involving one pair of contrasting characteristics is called monohybrid cross. 7. Dihybrid cross : The cross involving two pairs of contrasting characteristics is called dihybrid cross. 8. Genetics : The branch of biology that deals with genes, heredity and variation in the organisms is known as genetics. 9. Phenotype : Phenotype is the characteristic that is seen in an organism from outside. 10. Genotype : The genetic make-up of an organism is called genotype. 11. Law of dominance : In the crossing between the pure parents having a pair of contrasting characters, only one character appears in the F1 generation as a dominant character while other character remains hidden as a recessive character. 12. Law of Segregation : The dominant and recessive characters of the hybrids separate during the gamete formation in such a way that each gamete receives only one character at a time. 13. Variation : The difference in genotype and phenotype of the organisms in each successive generation is known as variation. Sequence of Curriculum Issued by CDC Introduction to gene Reason to select pea plant in Mendel experiment Dominant and recessive characters: introduction and examples Mendel’s law: related to monohybrid cross only UNIT Heredity and Mendelism 4.2 Estimated teaching periods Theory Practical 5 1 Gregor Johann Mendel is well known as the father of genetics for discovering fundamental laws of inheritance. He was born in Austria (now the Czech Republic) on 20th July 1822 and died on 6th January 1884. He was a biologist, mathematician and meteorologist. He is well known for his pea plant experiments that established many rules of heredity. He has also experimented on bees. Gregor Johann Mendel About the Scientist
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 79 14. Hereditary variation : The type of variation which is caused due to the usual changes in the genes of the parental gametes is called hereditary variation. 15. Environmental variation: The variation in the phenotype of the organisms which occurs due to environmental factors like climate, nutrition, lifestyle, culture, etc. is known as environmental variation. 16. Continuous variation : The variation that occurs in the following generations in variety of possible limits is known as continuous variation. 17. Discontinuous variation : The variation that occurs within a limited range of options is known as the discontinuous variation. 18. Mutation : Mutation is the sudden permanent change in the DNA or genes that alters the genotype or phenotype of an organism. Introduction Living organisms have a tendency to produce offspring of the same kind. As a result, continuity of the generation takes place. In each successive generation, more or less parental characteristics such as physical appearance, nature, behaviour, etc. transfer to their offspring. This transfer takes place either through the somatic cells (as in asexual reproduction) or through gametes (as in sexual reproduction). In both sexual and asexual reproduction, the parental characteristics are transferred to their children with the help of genes which are present in the nucleus of the cell. Thus, the process by which parental characteristics are transferred to their offspring is known as heredity or inheritance. Some characteristics like appearance, structure, certain body abilities (like little finger twisting, hitchhiker’s thumb etc.) are transferred to the offspring are called hereditary characteristics. But, some characters that are gained by the people in their lifetime like education, skill, knowledge, body building, etc. not transferred to the offspring are called acquired characteristics. Genes and Inheritance The nucleus of every eukaryotic cell contains long, thin and coiled structures called chromosomes. Humans have 23 pairs of chromosomes. Chromosomes are made up of DNA molecules and histone protein. There are specific segments of DNA which act as a single unit to carry the parental characters from one generation to the next generation that are called genes. Genes can store a variety of characters like hair colour, colour of skin, height, personality, etc. There are about 20,000 known genes in the human chromosomes. Genes help to produce specific proteins that can perform a certain function in the body. Grandparents Generation 1 Generation 2 Heredity Those characteristics which are transferred from one generation to the next generation are called hereditary characteristics. MEMORY TIPS A gene is a segment of DNA which is responsible to transfer parental character from one generation to the next generation. MEMORY TIPS
80 H eRedity and mendeliSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Functions of genes i. Genes transfer parental characteristics(hereditary characters) from one generation to another. ii. Genes help in evolution by mutation and genetic recombination. iii. They code for protein synthesis. iv. Genes control protein synthesis and metabolism. Some terminologies useful in this unit a) Alleles or allelomorphs All 46 chromosomes in the cell of a human body are in pair. In a pair, one chromosome comes from father and another comes from mother. In each pair, the chromosomes are identical in shape, nature and size. The genes present in them are on the same position. Consider the gene for height. There can be two variants for height gene, i.e. dwarf and tall. These variants are called alleles or allelomorphs. That is, each of two alternative forms of a gene on the chromosomes of an individual's cell are called alleles. So, we have two alleles for height, i.e. tall allele and dwarf allele. The allele in the chromosomes can be similar (tall-tall or dwarf-dwarf) or different (tall-dwarf). Similarly, other organisms also have such alleles in their chromosomes. b) Dominant and recessive allele Out of the two alleles, one allele has the tendency to suppress the other allele and appears in the first generation. The allele that expresses itself by suppressing the other allele is called the dominant allele. Similarly, the allele which fails to get expressed in the first generation and usually hides itself is known as the recessive allele. The tall allele is the dominant allele and the dwarf allele is recessive. The dominant allele is represented by a capital letter and the recessive allele is represented by a small letter. Example: “T” for tall and “t” for dwarf. c) Phenotype and Genotype Phenotype is the characteristic that is seen in an organism from outside. It is represented by words. Examples: tall, dwarf, red flower, green pod, black hair, free earlobe, etc. The genetic make-up of an organism is called genotype. Examples: i. TT – pure tall, both tall alleles from parents. ii. Tt - hybrid tall, tall allele from one parent and dwarf from other iii. tt – pure dwarf, both dwarf alleles from parents An alternative form of matching genes is called allele. Examples, TT, Tt, RR, Rr, etc. MEMORY TIPS Alleles Violet flower White flower VV VV Genotype What the gene itself look like What the gene does Phenotype Genotype and phenotype
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 81 d) Homozygous and heterozygous alleles If the genotype contains similar alleles, it is called homozygous or pure alleles. For example, TT and tt. If the genotype contains different alleles for a gene, it is called heterozygous or hybrid alleles. For example, Tt. e) Parental and filial generations The generation obtained by crossing two parental stocks is called F1 -generation. Similarly, the generation obtained by crossing two parental stocks of F1 -generation are called F2 -generation. f) Monohybrid cross and dihybrid cross The cross involving one pair of contrasting characteristics is called monohybrid cross. Similarly, the cross involving two pairs of contrasting characteristics is called dihybrid cross. FACTS WITH REASON The offspring of the organisms look like their parents but are not exactly identical. Why? The offspring gets half of its characteristics from mother and half from father. So, they look similar to their parents. Since variation occurs in every gene that is passed to the offspring, they do not look exactly identical. Genetics and Mendelism The branch of biology that deals with heredity and variation in the organisms is known as genetics. There are various sub-branches in genetics like behavioural genetics, developmental genetics, genetic engineering, medical genetics, etc. The history of genetics started in between 1856-1863 during which research was conducted for the first time on variation and genetics by an Austrian Monk called Gregor Johann Mendel. Mendel was born on July 22, 1822 AD. He joined a monastery and began researching on pea plants. After a long time experiment, Mendel proposed several laws. But, the significance of his work was unknown to everyone including himself. Mendel’s result was recognized only when such similar results were obtained by Carl Correns (Germany), Hugo De Vries (Holland) and Erich von Tsermark (Austria). Mendel was then given credit to his theory of genetics. Therefore, he is known as the father of genetics. Mendel’s version of genetics is known as Mendelian genetics or Mendelism or Mendel’s theory of inheritance. FACTS WITH REASON Why are genes called hereditary vehicles? Genes are called hereditary vehicles because they carry and transfer the genetic characteristics from the parents to the offspring. Mendel’s material for his experiment Mendel wanted to study how characteristics are transferred from parents to offspring. So, he chose the common pea plant (Pisum sativum) for his experiment. He conducted several
82 H eRedity and mendeliSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur experiments in the Monastery garden at Brunn, Austria. The reasons for choosing pea plant for his experiment are: i. Pea plant has a short life span and produces multiple fertile offspring due to which study can be done easily. ii. They are easy to cultivate as they grow well in gardens. iii. The flowers of the pea plant are bisexual. So, both self and cross-pollination are possible. iv. In a pea plant, there are many pairs of distinct contrasting characteristics. v. Pea plants have average size and are easy to handle. vi. The hybrids obtained by crossing the pea seeds are also fertile. Recessive trait Dominant trait Height Seed shape Seed colour Seed coat Pod shape Pod colour Flower position Dwarf Wrinkled Green White Constricted (flat) Yellow Terminal Tall Round Yellow Green Inflated (full) Green Axial Mendel's material for his experiment with its traits Mendel chose seven pairs of contrasting characteristics in pea plants. They are given in the table: S.N. Characteristics Contrasting pairs Dominant Recessive 1 Height of the plant Tall Dwarf 2 Colour of the flower Purple (Red) White 3 Shape of seeds Round Wrinkle 4 Colour of pods Green Yellow 5 Shape of pods Inflated Constricted 6 Position of flower Axial Terminal 7 Colour of seeds Yellow Green FACTS WITH REASON Why did Mendel choose pea plant for his experiment? A pea plant has short life span. It is easy to cultivate. It produces fertile offspring. It is easy to do self-and cross-pollination in pea plant, it has many pairs of contrasting characters and they are easy to handle. So, Mendel chose pea plant for his experiment.
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 83 The Method of Experiment Mendel carried out his experiment on the pea plants and noted the results very carefully. First of all, he had chosen single trait in the pea plants with their contrasting characters. For example, height (tall pea plant-TT and dwarf pea plant-tt). Then, he cross-fertilized these pea plants by cross-pollination. To avoid self-pollination in one of these flowers, he removed the male part (stamens) and made the flower unisexual. After the flower gave seeds, these seeds were shown. F1 generation The pea plants that grew from these seeds represented the F1 or first filial generation. It was observed that all the plants of the F1 generation were tall. It is because the tall character in the pea plant is dominant over the dwarf one. These plants were called hybrid tall plants (Tt) as they contained the characteristics of both tall and the dwarf plants but they were phenotypically tall. After these plants were matured, their flowers were again allowed to self-pollination and then were fertilized. The seeds obtained from the plants of F1 generation were sown again. This represents F2 generation. F2 generation The plants in F2 generation were surprisingly 75% tall and 25% dwarf. The ratio of tall and dwarf plants was 3:1 (3/4 tall and ¼ dwarf). Out of these four plants, one was pure tall (TT), two were hybrid tall (Tt) and the other was pure dwarf (tt). Again, the plants in the F2 generation were self-pollinated and fertilized to obtain the result in F3 generation. F3 generation The seeds obtained from the plants of F2 generation gave rise to plants of F3 generation. In the F3 generation, the pure tall plants gave only tall offspring. P generation F1 generation F2 generation Tall Short Tall Tall Tall Tall Tall Short Mendel's experiment Parents → Genotype → Pure tall pea plant Pure dwarf pea plant Cross - pollination F1 - generation F2 - generation F3 - generation Gametes Self-pollination → Possible gametes → All hybrid tall→ Gametes → If the F1 - are bred together Tt T t TT Tt T TT t Tt T Tt t tt T TT Tt tt tt t T t Tt tt TT Tt
84 H eRedity and mendeliSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Similarly, the pure dwarf plants gave only dwarf offspring. But, the hybrid plants gave pure tall, hybrid tall and pure dwarf plants in the genotypic ratio of 1:2:1. Phenotype and genotype ratio in this experiment Phenotype is the characteristic that is seen in an organism from outside. Similarly, the genetic make-up of an organism is called genotype. In the second generation of pea plants obtained from Mendel's experiment, Phenotype ratio: 3:1(three tall and one dwarf) Genotype ratio: 1:2:1 (one pure tall, two hybrid tall and one pure dwarf). The conclusions of Mendel’s experiment i. The pure tall plants gave only tall offspring. ii. The pure dwarf plants produced only dwarf offspring. iii. The hybrid tall plants produced both tall and dwarf plants in the ratio of 3:1. Mendel tried this experiment with other pairs of contrasting characters and also found the same result. FACTS WITH REASON The cross fertilization between a tall and a dwarf pea plant produces only tall pea plants in the first generation. Give reason. The cross fertilization between a tall and a dwarf pea plant produces only tall pea plants in the first generation because in a cross between pure tall and pure dwarf, the tall character is dominant and expresses itself over the dwarf character which is recessive and remains hidden in the first generation. Mendel’s Laws Mendel formulated three laws using the conclusions of his experiment. The three laws of Mendelism are given below. i. Mendel’s first law: The Law of Dominance ii. Mendel’s second law: The Law of Segregation or The Law of purity of gametes iii. Mendel’s third law: The Law of Independent Assortment (Note: Mendel's third law, i.e. law of independent assortment is not included in the syllabus of class 10.) Mendel’s first law: The Law of Dominance Mendel’s law of dominance states that, “In the crossing between the pure parents having a pair of contrasting characters, only one character appears in the F1 generation as a dominant character while other character remains hidden as a recessive character.” Example of law of dominance In a cross between a pure tall pea plant (TT) and a pure dwarf pea plant (tt), the tall character appears as a dominant character over the dwarf character. So, all the plants in the F1 generation are tall.
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 85 T T t t Parents → Genotype → Pure tall Gametes Cross-pollination Offspring in F1 -generation Tall Tall Tall Tall Pure dwarf Tt Tt Tt Tt TT tt FACTS WITH REASON Only one character is expressed in the first filial generation, why? In a cross between contrasting characters, only one allele of the gene is expressed as a dominant character while the other remains hidden as recessive character. The dominance of one character over the other causes only one character to appear in the first filial generation. Mendel’s second law: The Law of Segregation or The Law of purity of gametes Mendel’s second law states that, “The dominant and recessive characters of the hybrids separate during the gamete formation in such a way that each gamete receives only one character at a time”. Consider a cross between pure tall and pure dwarf plants. In the F1 generation, all plants look phenotypically tall. Does it mean that the dwarf character has vanished from the gene? No. It remains hidden as a recessive character in F1 generation. When gametes are formed by the plants of F1 generation, only one of them is expressed in the gametes. A single gamete cannot have both characters at a time. It means that in normal conditions, gametes can never be hybrid. As gametes contain only one character and are never hybrid, they are considered as pure. Hence, this law is also called as the law of purity of gametes. Example of law of segregation In a cross between pure red flower of the pea plant (RR) and pure white flower of the pea plant (rr), all the flowers in F1 generation are red. There is no white flower in F1 generation. This is because red colour is a dominant character and white colour is a recessive character. If we allow self-pollination in the plants of F1 generation, we get two types of flowers in F2 generation (75% red flower pea plants and 25% white flower pea plants). The result shows that phenotypic ratio is 3:1 and genotypic ratio is 1:2:1. Hybrid black father and hybrid black mother may give pure white baby. MEMORY TIPS
86 H eRedity and mendeliSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Parents - Pure red flowered pea plant Pure white flowered pea plant RR rr RR Rr Rr rr R r R r Rr Rr Rr Rr R R r r Genotypes Gametes Cross-pollination Gametes Pure red Hybrid red Pure white F2 - generation Self - pollination F1 - generation FACTS WITH REASON We can see all characters in the F2 generation, why? The probability of separation of allele from their pair is 50-50 at the time of gamete formation. During gamete formation in the F1 generation, each gamete contains only one allele due to meiosis cell division. So, we can see all characters in the F2 generation. Genetic characteristics in human beings Humans have several dominant and recessive characters that are inherited from one generation to another. Dominant characters are common in human population while the recessive characters are rare. Some of the human characters are given below: S.N. Characteristics Contrasting pairs Dominant Recessive 1 Earlobes Free earlobes Attached earlobes 2 Rolling of tongue Tongue rolling Non-tongue rolling 3 Dimple Dimpled cheek Non-dimpled cheek 4 Thumbs Hitchhiker's thumb Straight thumb 5 Hairline Widow's peak Straight hairline 6 Chin Cleft chin Normal chin 7 Handedness Right handed Left handed 8 Hairstyle Curly hair Straight hair
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 87 Eearlobe trait Tongue-rolling trait Chin dimple trait Thick and thin lips trait Cheek dimple trait Curly and straight hair trait Hitchhiker's thumb trait Genetic characteristics in human beings Crossing in Some Organisms i) Fruit-fly (Drosophila melanogaster) Mendelian genetics was considered doubtful even in 1900s. In order to test the law of dominance, an American geneticist Thomas Hunt Morgan, experimented on fruit flies, in the Columbia University. Fruit-flies (Drosophila) have two types of wings: long wings and short wings. Out of these wings, long wing character is a dominant character while the short wing character is a recessive character. When a long wing fruit-fly (LL) is crossed with a short wing fruit-fly (ll), all fruit-flies in the F1 generation have long wings. But, in the F2 generation, 75% of the flies were long winged and the remaining 25% were short-winged. This experiment proves Mendelian law of dominance. ii) Guinea pig The black colour of a guinea pig is dominant over the white colour. If we cross a black-coloured guinea pig with genotype BB with a white-coloured guinea pig with genotype bb, all guinea pig in the F1 generation are black. But, in F2 generation, the ratio of black to white guinea pigs is 3:1( three black-colour guinea pig and one white-colour guinea pig). Guinea pigs are generally chosen for the genetic experiments because they are small, have short life cycle, easy to handle, to look after and show a wide range of pair of contrasting characters. Drosophila with short wings Drosophila with long wings Drosophila with long wings (Sibling crosses) Drosophila with long wings Drosophila with long wings Drosophila with long wings Drosophila with short wings BB B B B B B B b b b b b b BB Bb Bb Bb Bb Bb Bb bb bb
88 H eRedity and mendeliSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur HOT SKILL HIGHER ORDER THINKING SKILL 1. No dwarf pea plants were observed in the first filial generation when the pure tall pea plant and pure dwarf pea plant were crossed. No dwarf pea plants were observed in the first filial generation when the pure tall pea plant and pure dwarf pea plant were crossed because a dwarf is a recessive character. The recessive character never appears in the first filial generation when two purebreds are crossed. 2. Monohybrid cross between two hybrid parents gives birth to purebred and crossbreds. Hybrid organisms contain both the dominant genes as well as recessive genes. According to the law of purity of gamete, the genes won’t blende. So, during the monohybrid cross of hybrid parents, the dominant character, the recessive character and the hybrid characters, all of them, appear in offspring. 3. Is Mendel’s experiment applicable to animals too? Or is it applicable to plants only? Mendel’s experiment applies not only to plants but also to animals. Let’s suppose there is an animal, it has chromosomes in its cells similar to the plants. The chromosomes are in pairs as an allele. The animals also have meiosis cell division to form gametes similar to plants. When crossed, animals also inherit traits from their parents. 4. When two hybrid black pigs were crossed, 25% of the pigs were white. Which law can explain it? Describe the law with an example. When two hybrid black pigs were crossed, 25% of the pigs were white. This case can be explained by Mendel’s second law of genetics, the law of purity of gametes. It is also called the law of independent assortment. The law of independent assortment explains that during a monohybrid cross, each factor that controls each contrasting character is inherited independently. They are not affected by others and they do not blend. For example: The phylogenetic chart to show the type of offspring in the F2 generation of a cross between pure black and pure white guinea pigs is given below. Let the allele of pure black guinea pigs is BB. Let the allele of pure white guinea pigs is bb. When they are to be crossed, they form respective gamete. As a result, 25 % pure black pig, 25 % pure white pig and 50 % hybrid black pig were all seen in the second filial generation, which was hidden in the first generation. [First
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 89 5. We cannot perform Mendel’s experiment on humans. Mendel’s experiment cannot be performed on human beings because of the following reasons: i. Human subjects are difficult to choose because they have to bear babies for the experiment. ii. It is impossible to obtain the offspring of F2 and F3 generations in human beings because fertilization has to be performed on brothers and sisters (same offspring of parents). iii. The country's law and human rights do not allow direct experiments on human beings. iv. The inheritance of the characters in human beings is much more complicated because one character is controlled by multiple genes. 6. Rama Kumari has a black goat. She bred it with a white goat and all kids were born white. What could be the reason? Did the genes of the black goat got lost forever or blended? Will it ever appear again? Explain. Were the parent goats pure or hybrid? Answer with reason. Rama Kumari has a black goat which was bred with a white goat. All kids were born white. The only reason could be the black goats and white goats were purebreds. And white goat was dominant. Therefore, only white kids were born. No! the genes of the black goat are not lost for ever. It will appear again in the second filial generation because it has not blended. The probability of a black goat being born in the second filial generation is 25%. The parent goats were pure organisms because only pure organisms can produce 100% hybrid offspring. 7. Mention the kind of puppies that are produced if a hybrid black dog breeds with a pure white dog. Show with a phylogenetic chart. Let the gamete of hybrid black dog be Bb. Let the gamete of pure white dog be bb. Monohybrid cross between a hybrid black dog and pure white dog be: Hybrid black B b Pure b Bb bb white b Bb bb When a hybrid black dog was crossed with a pure white dog following puppies can be produced. Phenotype = Black : White = 2:2 Genotype = Hybrid black : Pure white = 2:2 Two puppies were black and two puppies were white. 8. List down the significance of Mendelism. The significance of Mendelism is: i. Using Mendelism, we can obtain disease-resistant and high-yielding varieties of plants and animals. ii. We can get a hybrid of the desired characteristics. iii. Dominant and recessive characteristics of the organisms can be identified. iv. We can predict the genotype and phenotype of the organisms by using Mendelism. 9. Write down the percentage of tall and dwarf pea plants in F2 generation of a cross between pure parents with genotypes TT and tt. The percentage of tall pea plants is 75% and that of dwarf plants is 25%. 10. Write down any six pairs of contrasting characteristics (hereditary characteristics) in human beings. Six pairs of contrasting characteristics in human beings: i. Earlobes- Free (dominant) and Attached (recessive) ii. Tongue rolling- Capable (dominant) and Incapable (recessive)
90 H eRedity and mendeliSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur iii. Dimple - Presence (dominant) and Absence (recessive) iv. Hitchhiker’s thumb- Incapable (dominant) and Capable (recessive) v. Cleft chin- Present (dominant) and Absent (recessive) vi. Hair style- Curly (dominant) and straight (recessive) 3 STEPS 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answers from the given alternatives a. What is the transfer of characteristics from parents to the offspring called? i. evolution ii. dominant characters iii. recessive characters iv. heredity b. What are the traits that transfer from parents to offspring called? i. contrasting characteristics ii. hereditary characteristics iii. recessive characteristics iv. dominant characteristics c. Where are chromosomes located? i. cytoplasm ii. nucleoplasm iii. ribosome iv. centrosome d. If Tt represents hybrid tall, what represents the pure dominant parent of this organism? i. Tt ii. tt ii. tT iv. TT e. Arya is similar to her sister Elle, what caused such similarities? i. variation ii. heredity iii. evolution iv. mutation 2. Define the following terms with required examples. a. Heredity b. Gene c. Genotype d. Dominant character e. Recessive character f. Phenotype g. Monohybrid cross h. Dihybrid cross i. Genetics j. Law of dominance j. Law of Segregation 3. Answer the following questions in very short. a. Which is the dominant height for a pea plant: tall or dwarf? b. Write the phenotype ratio and genotype ratio of monohybrid and dihybrid cross. c. State the phenotype and genotype ratio in F2 generation obtained from Mendel's experiment. d. On which plant did Mendel experiment about heredity? e. If a white dog is born from a black parent, what could be the genotype of the parents? f. What is the pair of alternative forms of a gene called? g. If T means tall, what means pure tall?
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 91 h. When both parents are hybrid, a recessive character will appear in the second filial generation. Which law explains this? i. Is a straight thumb either a recessive or dominant factor in human beings? j. What is an organism with different genetic makeup called? k. Write an allele to represent the dominant homozygous red pea plant. l. Who is called the father of genetics? m. No white mouse was born, when the pure white mouse was crossed with the pure black mouse. Is white colour the dominant factor or the recessive factor? n. Write down the percentage of tall and dwarf pea plants in the second filial generation of a cross between pure tall and pure dwarf parents? STEP2 4. Give reasons. a. The offspring of the organisms look like their parents but are not identical. b. Genes are called hereditary vehicles. c. Mendel chose the pea plant for his experiment. d. The cross fertilization between a tall and a dwarf pea plant produces only tall pea plants in the first generation. e. Only one character is expressed in the first filial generation. f. We can see all characters in the F2 generation. g. No two organisms look the same. h. Very few people are left-handed in society. i. Haemophilia and colour blindness are called sex-linked diseases. j. Mendel is called the father of genetics. 5. Differentiate between the following. a. Dominant characteristics and recessive characteristics b. First filial generation and second filial generation c. Monohybrid cross and dihybrid cross d. Phenotype and genotype e. Pure organisms and hybrid organisms 6. Answer the following questions in short. a. When long-winged drosophila was crossed with short-winged drosophila, all offspring had a long wing. Which law can explain it? State the law. b. Draw a chart showing the result obtained in F1 and F2 generation by crossing pure tall pea plants and pure dwarf pea plants. c. Point out any three dominant and recessive Mendelian characters in humans. d. Explain the law of dominance with the help of an example. e. Write down the significance of Mendelism.
92 H eRedity and mendeliSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur STEP3 7. Answer the following long questions. a. What are the conclusions of Mendel’s experiment on pea plants? b. A black and a white guinea pig are fertilized to produce offspring of a guinea pig. Draw a chart to show the genotype and phenotype ratio of the result obtained in the F1 and F2 generations. c. A tall pea plant is crossed with a dwarf one. Answer the following questions by studying the given table: i. What type of cross is shown in the table? ii. Which generation does the following table represent? iii. What is the percentage of tall plants? iv. What percentage of pea plants are dwarf in this generation? d. A black guinea pig was mated with a white guinea pig and all guinea pigs in the first generation were black. Answer these questions: i. Why were all guinea pigs black in the F1 generation? ii. Why were both black and white guinea pigs seen in the F2 generation? iii. Write the genotype and phenotype ratio of F2 generation. iv. What is the percentage of black and white guinea pigs in the F2 generation? e. Study the given table of a certain generation of offspring obtained by the cross between the black guinea pig and the white guinea pig. Then, answer the following questions. i. Which generation do the following chart indicate? ii. What is the ratio of dominant and recessive characters? iii. What fraction of the guinea pig is black? iv. Why is black guinea pig seen more compared to white guinea pig in this generation? f. Study the chart of a cross between hybrid black guinea pig (Bb) and pure white guinea pig (bb) and answer the following questions. i. Fill in the table alongside. ii. What is the genotype and phenotype ratio? iii. How many guinea pigs are black and how many are white? Project Work 1. Make a profile of your dominant and recessive characteristics of different traits like hair type,thumb, chin, tongue, cheek, colour of eye, hand and earlobe of your friends. 2. Visit a pea garden and observe the contrasting characteristics of the pea plants. x T t T TT Tt t Tt tt × B b B BB Bb b Bb bb × b b B ? ? b ? ?
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 93 Key terms and terminologies of the unit 1. Genes : The genes are the smallest hereditary units present in the chromosomes. 2. Genetic engineer : The person who studies about genes of living organisms is called a genetic engineer. 3. Genetic technology : The technology in which the genes are modified, cut, removed or transferred using biotechnology is called genetic technology. 4. DNA testing : The technology which helps to count the number of chromosomes and identify changes in genes, chromosomes and genomes is called DNA testing. 5. Breeding : The way in which plants and animals produce new organisms through sexual reproduction is called breeding. 6. Selective breeding : The process in which humans identify certain special traits in an organism and take steps to breed them so that a new organism will have those traits is called selective breeding. 7. Inbreeding : The act of breeding between males and females who are closely related genetically is called inbreeding. 8. Line breeding : The act of breeding between males and females who are second or third degree relatives is called line breeding. 9. Cross breeding : If a purebred male and a purebred female of two different species of same genus are crossed to produce new offspring, it is called crossbreeding. 10. Outcrossing : The act of breeding between males and females who are unrelated for at least four to six generations is called outcrossing. Sequence of Curriculum Issued by CDC Introduction to genetic technology Role of DNA tests in various investigations Methods of selective breeding Introduction, examples, merits and demerits of cross-breeding Introduction to the in vitro fertilization and artificial insemination UNIT Genetic Technology 4.3 Estimated teaching periods Theory Practical 4 1 Thomas Hunt Morgan is noted for his discovery of the role of chromosomes in heredity. He was born in America on 25th September 1866 and died on 4th December 1945. He was an evolutionary biologist, embryologist and geneticist. He won the Nobel Prize in Medicine in 1933 for discovering and elucidating the role of chromosomes in heredity. He discovered linked genes and sexlinkage. Thomas Hunt Morgan About the Scientist
94 genetic te cHnology Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 11. Purebred : If both of the chromosomes of an allele are identical, then the organism is called purebred. 12. Crossbreds : The offspring from different species of purebred parents are often called crossbreds. 13. In vitro fertilization : The process in which ovum and sperm are fertilized outside the female body in the glass vessels is called in vitro fertilization. 14. Artificial insemination : The deliberate introduction of sperms into a female’s cervix or uterus is called artificial insemination. 15. Intracervical insemination : If the raw unwashed sperms or donated sperms from a sperms bank is introduced into the vagina it is called intracervical insemination. 16. Intrauterine insemination : If the washed sperm is introduced directly into the uterus, it is called intrauterine insemination. Introduction to gene The genes are the smallest hereditary units. Genes are made up of segments of DNA. They are part of the chromosome. Hundreds to thousands of genes are found in a single chromosome. The information contained in the genes determines the physical appearance of the organisms. It also determines how a living thing is going to survive, which diseases can it resist, which diseases it is prone to and what is its mode of nutrition. Genes even determine the IQ level of a person to some degree. FACTS WITH REASON Offspring are similar to the parents. Offspring are similar to their parents because they receive genes from their parents which bring hereditary characteristics. ACTIVITY 1 Make a 3D model of DNA. Genetic technology A living thing is its genes. Our height, skin colour, eyes, nose, physical characteristics etc. are all the outcome of information contained in our genes. The person who studies about genes of living organisms is called a genetic engineer. Genetic engineers usually cut, remove geneticist or also add genes to chromosomes. They sometimes transfer genes from one organism to another. They also change the genetic makeup of the cell. As a result, the physical appearance, disease resistance, adaptational characteristics, physical abilities and intellectual abilities of the organism will be greatly modified due to changes in their genes. These organisms are called genetically modified organisms. As we discussed above genes contain every information to make up our body, physical abilities, mental abilities, survival instincts, disease resistance and many more. We also learned that genetic engineers can modify the genes and alter the physical abilities of the organisms. The In genetic technology there occurs change of one pair of nitrogen base(A-T or C-G), cut or add of gene to develop a new character. MEMORY TIPS
Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 95 technology in which the genes are modified, cut, removed or transferred using biotechnology is called genetic technology. There are many forms of genetic technology. Some common forms of genetic technology are genetic engineering, DNA molecular marker, transgenic technology, gene expression, vaccines, gene therapy, gene testing and many more. Figure: Inserting gene in a DNA Examples of genetic technology i. We have known that genes of bacteria and yeast are modified to produce insulin. Insulin is a hormone that is used to treat diabetes. This technology is called genetic engineering. ii. Certain genes of one plant can be added to other plants to produce genetically modified plants. It gives additional benefits to the plants such as increased nutritional value, and resistance to pests and drought. It increases agricultural production. iii. Vaccines are another example of gene technology. Vaccines prevent hundreds of fatal diseases such as measles, chicken pox etc. The genes of the virus are present in the vaccine, when we get vaccinated, our body produces antibodies to fight against real viruses. iv. Scientists can inject altered genes of virus in a young plant. The young plant grows and gives fruits that contain genes of the virus. When we eat these fruits, we won’t get sick. But the body prepares antibodies against the virus. It prevents us from being sick in future. v. Gene therapy is a condition where DNA is introduced into a patient to treat genetic disorders. It is also used to switch off certain genes of the cells that are causing cancer. vi. Genetic testing is a form of genetic technology to diagnose genetic disorders. It is also used to detect chromosomal disorders in the foetus such as Down’s syndrome. Genetic testing is also called DNA testing. FACTS WITH REASON Scientists mix DNA of a virus into the DNA of banana plants. Scientists mix DNA of a virus into the DNA of banana plants so that the banana fruits contain some genes of virus. It acts like a vaccine and prevents us from diseases. Genetic engineers can create entirely synthetic chromosomes in the lab. MEMORY TIPS
96 genetic te cHnology Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur ACTIVITY 2 How do gene technology help to develop a vaccine? Learn more about it from the Internet. Potential issues of genetic technology Genetic technology exists to benefit humans by preventing diseases, curing diseases, and ensuring food security. However, there are potential issues related to any form of genetic technology. i. Genetically modified characteristics of a crop plant can transfer to wild plants. It can hamper the ecosystem. ii. Farmers will be forced to buy expensive genetically modified seeds. iii. It is unethical to move genes from one species to another. iv. Using genetic modification to treat diseases is actually not a treatment but an improvement. v. Genetic technology can be used to create superhuman soldiers. Measures against genetic technology Since genetic technology can actually be used to do unethical experiments, many countries have laws to control genetic technologies. For example: i. Modifying genes in the human embryo is banned. ii. Cloning of humans is illegal and punishable by death. DNA testing Deoxyribonucleic acid (DNA) is the genetic code found in the chromosomes of every living being. DNA is unique to every individual. It contains instructions for every cells to work properly. DNA comes to us from our parents. DNA affects our health and body. So, DNA testing will be very useful in medical fields and others. It is a form of genetic technology. In the 1950s biologists could count the number of chromosomes in the human cell. Normally there are 23 pairs (46) chromosomes in a human cell. The increase or decrease in the number of chromosomes and disarrangement of the chromosomes can lead to serious defects in the body. Such a condition is called a chromosomal disorder. DNA testing can help to identify such disorders and provide necessary treatment to the victim. The technology which helps to count the number of chromosomes and identify changes in genes, chromosomes and genomes is called DNA testing. It is also called genetic testing. DNA testing examines the chemical database of our DNA. It helps to reveal changes in genes that can cause diseases. DNA testing is growing popular among the public, medical fields and criminal investigation. FACTS WITH REASON It's wise to test DNA of foetus. It's wise to test DNA of foetus because we can abort foetus in case it has a serious illness and chromosomal disorders. The DNA in identical twins will be 100% match however the finger print will be different. MEMORY TIPS