Functions of plasma
1. to transport digested food (nutrients) to Do You Know
various body parts.
Plasma contains anticoagulant
2. to transport waste materials like urea, substance called heparin which
carbon dioxide, etc. for their removal prevents blood clotting inside the blood
from the body. vessels.
3. to regulate body temperature The proteins (albumin, globulin and
fibrinogen) help in clotting of blood.
4. to regulate the amount of water and
minerals in the body.
5. to transport hormones to different parts of the body.
6. to transport urea produced by liver to kidneys.
Blood corpuscles
The blood corpuscles are solid particles that remain immersed in plasma. They occupy
45% of the blood volume. There are three types of blood corpuscles. They are:
i. Red blood cells (Erythrocytes)
ii. White blood cells (Leucocytes) and
iii. Platelets (Thrombocytes)
i. Red blood cells (RBCs)
The red blood cells or erythrocyes are red-
coloured blood cells having biconcave
shape. They do not have nucleus. They
appear red due to the presence of an iron-
containing pigment called haemoglobin. Fig.
Oxygen is absorbed by haemoglobin and
supplied to various parts of the body.
The haemoglobin rich in oxygen is called 16.2
oxyhaemoglobin and the haemoglobin Red Blood Cells
rich in carbon dioxide is called carboxyhaemoglobin. RBCs are smaller than white blood
cells and larger than platelets.
The red blood cells are produced in bone marrow, live for about 90 to 120 days and are
destroyed in liver and spleen.
About 45 lakh to 50 lakh RBCs are found in 1 mm3 volume of blood. About 20 lakhs RBCs
are formed in the bone marrow in one second and the same number is destroyed in liver
and spleen per second. The iron present in old RBCs is reused to form new RBCs. The
formation and destruction of RBCs continues throughout the life.
GREEN Science (Biology) Book-10 301
Functions of RBCs
i. RBCs absorb oxygen from lungs and transport to various tissues.
ii. They transport carbon dioxide from various tissues to the lungs.
ii. White blood cells (WBCs)
The white blood cells (leucocytes) are colourless irregular blood cells having nucleus.
They are the largest blood cells. They are produced in bone marrow and lymph nodes.
They live for a few hours to few days. They are destroyed in liver, spleen and at the site
of infection.
Do You Know
Eosinophil Basophil A person suffers from anaemia due
to lack of RBCs or haemoglobin in
Monocyte blood.
Haemoglobin is absent in WBCs.
Fig. A person suffers from blood cancer
Lymphocytes Neutrophil leukaemia due to enormous increase
in the number of WBCs in the blood.
16.3
White Blood Cells
There are two types of white blood cells. They are granular and non-granular. Neutrophill,
Eosinophyll and Basophyll are called granular WBCs whereas Lymphocytes and
Monocytes are called non-granular leucocytes. The number of WBCs ranges from 6000 to
10,000 in 1 mm3 volume of blood.
Functions of WBCs
i. WBCs play a great role to form immune system in the body.
ii. They help in formation of antibodies to neutralize the germs that enter the body.
iii. WBCs fight against micro-organisms and protect us from various diseases.
iii. Platelets
The platelets or thrombocytes are colourless, oval or round blood cells without nucleus.
They are the smallest blood cells. They are formed in bone marrow, live for 2 to 3 days
and are destroyed in the spleen. The shape of platelets is irregular. 1mm3 volume of blood
consists of 2-4 lakhs of platelets.
Functions of Platelets
i. The platelets play an important role in clotting of blood and prevent the loss of blood
from cuts or wounds.
302 GREEN Science (Biology) Book-10
Functions of Blood
In human body, blood performs three types of functions. They are:
1. Transportation
2. Regulation
3. Protection
1. Transportation
i. Blood transports oxygen from the lungs to the different cells and tissues of the
body.
ii. It transports carbon dioxide from tissues to the lungs.
iii. Blood transports nutrients absorbed by walls of intestine to different tissues of
the body.
iv. Blood transports hormones produced by endocrine glands to different parts of
body.
v. Blood transports various waste materials towards liver, lungs, kidney and skin
for excretion.
2. Regulation
i. Blood keeps our body warm by regulating body temperature.
ii. Blood regulates the amount of water, minerals and other chemicals in the body.
3. Protection
i. Blood fights against microbes that cause various diseases and protect our body
from various diseases.
ii. Blood transports WBCs in the infected area to fight against germs.
iii. Blood helps to clot the blood in case of cuts or wound and prevents further loss
of blood and infection.
Differences between RBCs (Erythrocytes) and WBCs (Leucocytes)
RBCs WBCs
1. RBCs don't have nucleus. 1. WBCs have nucleus.
2. Haemoglobin is present. 2. Haemoglobin is absent.
3. They are biconcave in shape. 3. They are irregular in shape.
4. They transport oxygen and carbon 4. They fight against microorganisms.
dioxide.
GREEN Science (Biology) Book-10 303
Activity
Take a sterilized lancet and prick your fingertip under the supervision of your
biology teacher.
Put a drop of blood on a clean slide and spread the drop throughout the slide with
the help of another glass slide.
Put a drop of Leishman's stain on the blood film and leave it for 5 minutes.
After 5 minutes, rinse the blood film with distilled water and let it to dry in the
sunlight.
Observe the blood film under the compound microscope. Draw neat figure and
label the main parts.
Heart
The heart is a powerful Aorta
and hollow organ made Superior Pulmonary
of cardiac muscle. It is vena cava trunk
conical in shape and is
about the size of the fist Right Left
of the individual. The artium atrium
heart is located near the Pulmonary
veins
middle of the thoraic Right Left
cavity between two ventricle ventricle
Fig.
lungs. About two-third ofFig. Inferior
the heart is located on the 16.4 vena cava
left side and one-third is External morphology of human heart
located on the right side
of the thoracic cavity. The average weight of the heart is about 300 grams in adults.
Superior vena cava
Aorta Pulmonary artery
Pulmonary veins Right atriLuemft Pulmonary veins
atrium Mitral valve
Pulmonary valve Aortic valve
Tricuspid valve Left
ventricle Septum
Right
ventricle
16.5 Inferior vena cava
Internal morphology of human heart
304 GREEN Science (Biology) Book-10
The human heart is surrounded by a double-layered membrane called the pericardium.
The space between two layers of pericardium is filled with a slippery fluid called the
pericardial fluid. This fluid enables the heart contract smoothly and also protects the heart
from mechanical injury.
The human heart consists of four complete chambers, viz. two upper chambers called
auricles and two lower chambers called ventricles. The auricles are separated into right
and left auricles by a muscular septum. Similarly, the ventricles are divided into right and
left ventricle by a muscular septum. The auricles are thin-walled small chambers whereas
ventricles are thick-walled large chambers.
The wall of the right ventricle is thicker than that of the auricles as it pumps blood to a
much farther distance to the lungs for purification. Similarly, the wall of left ventricle is
the thickest of all as it pumps blood to different parts of the body.
The various blood vessels enter and leave the heart. Superior vena cava and inferior vena
cava are connected to the right ventricle whereas four pulmonary veins are connected
to the left auricle. The pulmonary artery originates from the right ventricle and aorta is
originated from the left ventricle.
Four different types of valves are present in the human heart. They are aortic valve,
pulmonary valve, tricuspid valve and bicuppid or mitral valve. The aortic valve is located
at the base of aorta inside the heart. It regulates the flow of blood from left ventricle to
different parts of the body and prevents the backflow of blood. The pulmonary valve is
located at the base of pulmonary artery. It regulates the flow of blood from right ventricle
to the lungs and prevents the back flow of blood. The tricuspid valve is located between
the right auricle and the right ventricle. It has three muscular flaps or cusps. It regulates
the flow of blood from the right auricle to the right ventricle and prevents the backflow
of blood. Similarly, bicuspid or mitral valve is located between the left auricle and the left
ventricle. It has two muscular flaps or cusps. It regulates the flow of blood from the left
auricle to the left ventricle and prevents the backflow of blood.
Blood vessels
The muscular tubes or pipes through which blood flows are called blood vessels. There
are three types of blood vessels. They are:
i. Arteries
ii. Veins
iii. Capillaries
i. Arteries Fig. External elastic membrane
Smooth muscle
The thick-walled blood vessels that carry 16.6 Internal elastic membrane
blood away from the heart are called Lumen
arteries. They are deep-seated inside the Endothelium
muscles. All arteries carry pure blood
except the pulmonary artery. Inside the Structure of an artery
arteries, blood flows with a high speed and
under high pressure. Therefore, their wall is GREEN Science (Biology) Book-10 305
thicker than that of venis. The branches of arteries are called the arterioles. An artery has a
narrow lumen and a thick wall. Valves are absent in arteries. Examples: aorta, pulmonary
artery.
ii. Veins External elastic membrane
Smooth muscle
The thin-walled blood vessels that carry
blood towards the heart are called veins. Internal elastic membrane
They are situated on the surface of muscles. Lumen
All veins carry impure blood except
pulmonary veins. Inside the veins, blood Endothelium
flows with a low speed and under low Valve
pressure. Therefore, veins have a thin wall.
The branches of veins are called venules. 16.7
A vein has a wide lumen and a thin wall. Structure of a vein
Valves are present in veins to prevent the Fig.
reverse flow of blood. Examples: vena cava, Fig.
pulmonary veins.
iii. Capillaries 16.8 Endothelium
The capillaries are extremely narrow and microscopic Structure of a capillary
blood vessels. They connect the arterioles to the
venules and can penetrate every part of the body. The
wall of the capillary consists of a single layer of cells.
The capillaries help in exchange of materials between
the blood and body cells.
Differences between Arteries and Veins
Arteries Veins
1. Arteries are thick-walled blood 1. Veins are thin-walled blood vessels.
vessels.
2. They carry blood away from the heart. 2. They carry blood towards the heart.
3. Valves are absent. 3. Valves are present.
4. They have a narrow lumen. 4. They have a wide lumen.
Process of Blood Circulation in the Human Body
In human body, blood flows twice through the heart before the pure blood is supplied to
various parts of the body. Superior vena cava and inferior vena cava collect impure blood
from different body parts to the right auricle. The right auricle passes impure blood to the
right ventricle. When the right ventricle contracts, impure blood is supplied to the lungs
through the pulmonary artery. The impure blood combines with oxygen and becomes pure
306 GREEN Science (Biology) Book-10
(oxygenated). Then the pure blood from the lungs is brought to the left auricle through the
pulmonary veins.
Capillary bed of lungs
where gas exchange occurs
Pulmonary Pulmonary circuit Pulmonary veins
arteries Systemic circuit
Aorta and branches
Vena cavae
Left atrium
Right atrium Left ventricle
Right ventricle Systemic arteries
Systemic veins
Capillary bed of all body tissuesFig.
where gas exchange occurs
16.9
Schematic figure of blood circulation in human body
The left auricle contracts and passes pure blood to the left ventricle. The left ventricle
contracts and pumps pure blood to the aorta. The aorta supplies pure blood to various
body parts through the arteries, arterioles and capillaries.
In this way, the circulation of blood takes place in a human body.
Types of Blood circulation
The blood circulation in human body is of two types. They are as follows:
1. Systemic circulation
2. Pulmonary circulation
1. Systemic circulation
The circulation of blood between the heart and various parts of body is called systemic
circulation. It takes place with the help of arteries and veins. When left ventricle of the
heart contracts, it pumps oxygenated blood to different parts of body through aorta, artery,
arterioles and capillaries. Body cells absorb oxygen and digested food from blood. The carbon
dioxide gas and waste products diffuse into blood. Then the deoxygenated blood is collected
back to the heart through capillaries, venules, veins and vena cava. In short, the circulation
of blood from the left ventricle to the right auricle of the heart is called systemic circulation.
GREEN Science (Biology) Book-10 307
2. Pulmonary Circulation
The circulation of blood between heart and lungs is called pulmonary circulation. It occurs
with the help of pulmonary artery and pulmonary veins. When right ventricle contracts,
it passes deoxygenated blood to the lungs through pulmonary artery. The deoxygenated
blood becomes oxygenated in lungs. Then the oxygenated blood is collected to the left
auricle of the heart through pulmonary circulation.
Heart beat
The heart beat is the rhythmic beat which is produced due to contraction and relaxation of
the heart muscles. The average heart beat rate of a healthy young person is about 72 times
per minute. The rate of heart beat increases with tension, physical exercise, emotion, fear,
fever, etc. The rate of the heart beat may vary with age, sex and condition of the body.
Blood Pressure
The force exerted by blood which pushes the walls of arteries is called blood pressure. It
depends on the amount of blood, speed of blood flow, shape of the blood vessel and the
force exerted by the heart while pumping blood. Blood flows through blood vessels due
to blood pressure.
The normal blood pressure of a healthy young person is 120/80 mm of Hg. Blood pressure
is measured in millimetre height of mercury column.
Types of Blood Pressure
Blood pressure is of two types, viz. systolic blood pressure and diastolic blood pressure.
Systolic blood pressure
The force exerted by blood per unit area of arterial walls when the ventricles of the heart
contract is called systolic pressure. In adult human beings, the systolic blood pressure
ranges between 90 mm of Hg to 120 mm of Hg. The average systolic blood pressure of a
healthy adult is taken as 120 mm of Hg.
Diastolic blood pressure Fig.
The force exerted by blood per unit area of arterial Sphygmomanometer
walls when ventricles of the heart relax is called
diastolic blood pressure. In adult human beings, it
ranges between 60 mm of Hg to 90 mm of Hg. The
average diastolic pressure of healthy young person is
taken as 70 mm of Hg.
Systolic blood pressure is also called upper limit
of arterial pressure and diastolic pressure is called
lower limit of arterial pressure. The instrument
which is used to measure the blood pressure is called 16.10
sphygmomanometer.
308 GREEN Science (Biology) Book-10
Now-a-days, many people suffer from high blood pressure. The major causes of high
blood pressure(Hypertension) are given below.
• Obesity
• Unhealthy lifestyle
• Over weight
• Regular smoking
• Consumption of more salt in food
• Adrenal and thyroid disorder
• Genetic cause
• Lack of physical exercise
• Drinking alcohol regularly
• Regular consumption of fatty, oily and spicy foods
• Regular consumption of cold drinks
• Kidney disorder
The symptoms of high blood pressure are as follows:
• Headache
• Dizziness
• Chest pain
• Blurred vision
Following measures should be adopted to prevent high blood pressure.
i We should adopt healthy lifestyle.
ii. We should avoid smoking and drinking alcohol.
iii. We should not consume fatty, oily and spicy foods regularly.
iv. We should consume balanced diet.
v. Physical exercise should be taken regularly.
vi. Consumption of cold drinks should be avoided.
vii. We should check the blood pressure regularly.
Diabetes
Diabetes is commonly known as sugar disease. This disease is caused due to lack of
insulin hormone secreted by pancreas. In diabetes, the amount of sugar increases in
blood. Insulin hormone regulates the amount of glucose or sugar in the blood. When
there is lack of insulin, the excess glucose in blood cannot be changed into glycogen to
GREEN Science (Biology) Book-10 309
store in the body. As a result, the amount of sugar increases in the blood and sugar passes
out through urine. This disorder is called diabetes or sugar disease.
Causes of diabetes
i. Genetic cause
ii. Insufficient insulin production
iii. Stressful life style
The major symptoms of diabetes are as follows:
i. excessive thirst and hunger ii. frequent urination
iii. blurred vision iv. weakness
v. delay in wound healing vi. muscle cramping
vii. unconsciousness, etc.
Following measures should be adopted to prevent diabetes :
• We should consume balanced diet.
• We should reduce obesity.
• We should take physical exercise regularly.
• We should consume fruits and vegetables.
• We should avoid smoking and drinking.
• We should reduce hypertension.
Uric acid
The acid produced in the body as a byproduct due to decomposition of purine is called
uric acid. Purine is an important chemical substance for our body because it provides
protein to the body. Uric acid protects the inner part of blood vessels and helps to remove
toxins from the body. In human body, uric acid is produced due to excess metabolism of
purine.
If the body produces too much uric acid and cannot be removed from the body through
urine, a person suffers from uric acid. The condition of having high level of uric acid in
blood is called hyperuricemia.
Hyperuricemia occurs either due to failure to excrete uric acid from the body or increased
production of uric acid in the body.
The major causes of hyperuricemia or uric acid are as follows:
i. Excessive consumption of meat products, pulses, beer, etc.
ii. Excessive consumption of alcohol and caffeine.
iii. Hypothyroidism
iv. Obesity
310 GREEN Science (Biology) Book-10
v. Leukaemia
The major symptoms of hyperuricemia are as follows:
i. Joint ache
ii. Deep pain in muscles
iii. Difficulty in walking
iv. Swelling and burning of skin
v. Swelling of joints of leg with pain
Following measures should be adopted to reduce uric acid in the blood:
i. We should drink sufficient water. We should drink less water but many times in a day
to wash away the excess uric acid through urine.
ii. We should reduce the consumption of Do You Know
pulses, sea foods and fatty red meat.
Cherries contain anthocyanin which
iii. We should consume baking soda. reduces burning sensation in skin and
joints.
iv. We should eat 10 - 40 cherries per day to
reduce burning sensation in the body.
Key Concepts
1. The system formed by heart, blood and blood vessels which transports various
materials from one part of the body to another is called the blood circulatory system.
2. The human blood circulatory system consists of three main parts. They are: (i)
Blood, (ii) Heart and (iii) Blood vessels.
3. The blood is a red fluid connective tissue. It consists of plasma (55%) and blood
corpuscles (45%).
4. Plasma is a straw-coloured liquid which occupies 55% of the blood volume.
5. The blood corpuscles are solid particles that remain immersed in plasma. They
occupy 45% of the blood volume.
6. The heart is a powerful and hollow organ made of cardiac muscle.
7. About two-third of the heart is located on the left side and one-third is located on
the right side of the thoracic cavity. The average weight of the heart is about 300
grams in adults.
8. Four different types of valves are present in the human heart. They are aortic valve,
pulmonary valve, tricuspid valve and bicusppid or mitral valve.
9. The muscular tubes or pipes through which blood flows are called blood vessels.
There are three types of blood vessels. They are (i) Arteries, (ii) Veins and
(iii) Capillaries.
10. The heart beat is the rhythmic beat which is produced due to contraction and
relaxation of the heart muscles.
GREEN Science (Biology) Book-10 311
11. The circulation of blood between the heart and various parts of body is called
systemic circulation. It takes place with the help of arteries and veins.
12. The circulation of blood between heart and lungs is called pulmonary circulation. It
occurs with the help of pulmonary artery and pulmonary veins.
13. The force exerted by blood which pushes the walls of arteries is called blood
pressure.
14. The normal blood pressure of a healthy young person is 120/80 mm of Hg. Blood
pressure is measured in millimetre height of mercury column.
15. The instrument which is used to measure the blood pressure is called
sphygmomanometer.
16. Diabetes is commonly known as sugar disease. This disease is caused due to lack
of insulin hormone secreted by pancreas.
17. The acid produced in the body as a byproduct due to decomposition of purine is
called uric acid.
18. Joint ache, deep pain in muscles, difficulty in walking, swelling and burning of
skin and swelling of joints of leg with pain are the major symptoms seen when the
amount of uric acid increases in the blood.
Sequential General Exercise 1
1. Choose the best answer from the given alternatives.
a. Haemoglobin is found in ...........................
RBC WBC Platelets Plasma
four
b. Human heart consists of ................................ valves. brain
one two three
c. In human body, blood is purified in ............................
heart kidney lungs
d. Which of the given diseases occurs due to lack of insulin?
high blood pressure diabetes
blood cancer anaemia
e. The enormous increase in the number of WBC in blood causes ............................
uric acid diabetes
high blood pressure blood cancer
312 GREEN Science (Biology) Book-10
2. Answer the following questions.
a. What is blood circulatory system?
b. Write down the major functions of blood circulatory system.
c. Write down the composition of blood.
d. Draw a neat and labelled figure of blood film.
e. Write down the structure and function of
• RBC
• WBC
• platelets
f. What are anaemia and leukaemia?
g. Write down the major functions of blood.
h. Where is heart located in human body?
i. What is pericardium? Write down the function of pericardial fluid.
j. Draw a neat and labelled figure of human heart and describe its structure.
k. Name the four valves present in human heart.
l. What are blood vessels? Write down the major functions of arteries, veins and
capillaries.
m. What is blood pressure? Name the instrument used for measuring blood pressure.
n. What does it mean by the fact that the blood pressure of a person is 130/90 mm
of Hg?
o. Write down the causes and preventive measures of high blood pressure.
p. Write down the cause, symptoms and preventive measures of diabetes.
q. What is uric acid? Write down the causes and measures for reducing uric acid in
blood.
3. Differentiate between:
a. WBC and platelets
b. Auricles and ventricles
c. Arteries and veins
d. Systolic pressure and diastolic pressure
e. Systemic and pulmonary circulation
f. Diabetes and uric acid
g. Pulmonary vein and Pulmonary artery
4. Give reason.
a. The wall of ventricle is thicker than that of auricles.
b. Leucocytes are called the soldiers of the body.
c. The wall of left ventricle is thicker than that of right ventricle.
GREEN Science (Biology) Book-10 313
d. Valves are present in veins but not in arteries.
e. We should not consume spicy, oily and fatty foods regularly.
5. Draw a schematic diagram showing blood circulation in human body.
6. What type of food should be consumed by the person suffering from:
a. high blood pressure?
b. uric acid?
c. diabetes?
7. Write down the advantages of blood circulation in human body.
Grid-based Exercise 2
Group’A’(Knowledge Type Questions) (1 Mark Each)
1. What is blood circulatory system ?
2. What is the life span of red blood cells and platelets?
3. Write the name of blood vessels that carry pure blood to the left auricle and impure
blood to the right auricle of the heart.
4. What is systemic circulation ?
5. Define systolic blood pressure.
6. What is haemoglobin ? Write down its function.
7. Where are aortic valve and pulmonic valve located?
8. What is pulmonary circulation ?
9. What is the blood pressure of a healthy young adult ? Write.
10. Write the name of device which is used for measuring blood pressure.
11. Write the name of four valves of human heart.
12. Write one function of left ventricle and right ventricle of human heart.
13. Define heart beat.
14. Which blood cells’ number is less than average in a person suffering from anaemia ?
15. Write the use of sphygmonamometer.
314 GREEN Science (Biology) Book-10
Group ‘B’ (Understanding Type Questions) (2 Marks Each)
16. Write any two differences between erythrocytes and leucocytes.
17. Right auricle is larger than the left auricle, why?
18. Write any two differences between auricle and ventricle.
19. The wall of the left ventricle is thicker than that of the right ventricle, why?
20. Write two differences between systemic circulation and pulmonary circulation.
21. Write any two differences between white blood cells and thrombocytes.
22. The wall of arteries is thicker than that of the veins, why?
23. A person suffering from anaemia feels tired in short walk, why?
24. The blood pressure of a healthy man is 120/80 mm Hg. What does it mean ?
25. Bleeding from artery is more dangerous than that from vein, why?
Group ’C’ (Application Type Questions) (3 Marks Each)
26. Draw a neat diagram showing the structure of blood.
27. Write any three functions of blood.
28. Name the valves present in between auricles and ventricles of human heart. Also,
write down their structure and function. A
29. A small portion of blood film is shown in the figure. Identify the parts B
A, B and C in the figure with major function of each. C
30. State one function of each of the following :
i. Fibrinogen ii. ire (Pulmonary vein iii. Capillaries
Group ‘D’ (Higher Abilities Type Questions) (4 Marks Each)
31. Describe the structure of human heart with a labelled diagram. A
B
32. What is shown in the given diagram? Name the parts A, B, C and C
D. Also, mention the function of each.
D
33. What are veins ? Describe the structure of veins in brief.
34. The blood circulating in the body does three functions. Explain
transportation and control functions of blood.
35. Write short notes on:
i. Uric acid ii. Diabetes
36. Describe the process of blood circulation in human body with a schematic diagram.
GREEN Science (Biology) Book-10 315
UNIT Chromosomes and
17 Sex Determination
Weighting Distribution Theory : 5 Practical: 1
Before You Begin
The bodies of plants and animals are made up of numerous cells. Each
cell consists of various cell organelles including nucleus. Each nucleus
consists of a network of fine threads called chromatinreticulum. During
cell division, chromatin reticulum condenses and forms thread-like
structures which are commonly known as chromosomes. The thread-
like structure present in the nucleus of a cell are called chromosomes.
The word chromosome has been derived from two Greek words
chrome-colour and soma-body. It means that chromosomes are
colourful bodies present in the nucleus. Each chromosome is made of
two strands called chromatids and a centromere. The point where two
chromatids are attached is called centromere. Chromosomes contain
DNA and genes. Chromosomes transmit parental characteristics to
the offspring with the help of genes. In this unit, we will study about
chromosomes and determination of sex in human beings. Similarly,
we will study in brief about chromosome disorders with examples.
Learning Objectives Syllabus
After completing the study of this unit, students will be able to:
i. introduce chromosomes with their types. • Introduction to chromosomes
ii. describe determination of sex in human beings. • Types of chromosomes
iii. introduce chromosome disorder with examples. • Sex determination
• Chromosome disorder
• Turner’s syndrome
• Down’s syndrome
• Klinefelter’s syndrome
Glossary: A dictionary of scientific/technical terms
chromosomes : the thread-like structures found in the nucleus
centromere : the point where two chromatids are attached together
autosomes : the chromosomes that carry genes for vegetative characters
chromatid : the strand of a chromosome
syndrome : a group of signs and symptoms that occur together and characterize a
particular abnormality or condition
316 GREEN Science (Biology) Book-10
Chromosomes
The bodies of plants and animals are made of
numerous cells. These cells act as the basic,
structural and functional units of life. A cell can
perform various metabolic activities. Nucleus is
the most important cell organelle which controls
all the activities of the cell. It also takes part in
Fig.
Fig.reproduction. A nucleus consists of nuclear
membrane, nucleus, nucleoplasm and a set of
thread-like structures made of DNA and proteins 17.1
which are commonly known as chromosomes. Chromosome
Chromosomes are the thread-like structures present in the nucleus of a cell which contain
hereditary information of the cell. Chromosomes are made up of a DNA (deoxyribonucleic
acid) and proteins. Thus, the components of chromosomes are DNA and proteins. The most
important component of a chromosome is, however, DNA, each chromosome consists of
two strands (or threads) called chromatids. The two chromatids of a chromosome are
joined together at a point called centromere. It is calso called kinetochore.
Each species has a fixed number of chromosomes in each of its cells. For example, the
human beings have 46 chromosomes in every body cell. The chromosomes of each species
occur in pair. In a pair of chromosomes, one chromosome comes from the father and the
other from the mother. Thus, human beings have 23 pair of homologous chromosomes in
each body cell. The two chromosomes in a pair of homologous chromosomes are of the
same size and shape. But the chromosomes of one homologous pair differ in size and shape
from the chromosomes of another pair. Each chromosome of a homologous pair has genes
for the same characteristics in the same place.
Secondary constriction
Pellicle
Matrix
Satellite
Secondary constriction
Spindle fibre Primary constriction
17.2 Centromere
Structure of a chromosome
Chromosomes become shorter, thicker and distinct during cell division. When the cell is
in resting stage, it consists of chromosomes in the form of long, thin and coiled structures
which is commonly known as chromatin reticulum or chromatin network.
GREEN Science (Biology) Book-10 317
Types of chromosomes Do You Know
Depending on the position of centromere, a There are two types of chromosomes
chromosome may have either equal arms or on the basis of their function. They are
unequal arms. So, on the basis of the position autosomes and sex chromosomes.
of the centromere, there are four types of
chromosomes. They are: Autosomes determine the physical
characteristics of the body whereas sex
i. Metacentric chromosomes chromosomes determine the sex of an
organism.
ii. Submetacentric chromosomes
iii. Acrocentric chromosomes
iv. Telocentric chromosomes
Satellite Chromatid
Short arm
Centromere
Stalk
Centromere
Long armFig.
17.3 Telocentric Acrocentric Submetacentric Metacentric
i. Metacentric chromosome
The chromosome in which the centromere is located near the middle point is called a
metacentric chromosome. In this chromosome, two arms are almost equal in length.
ii. Sub-metacentric chromosome
The chromosome in which the centromere is located a little away from the middle point is
called a submetacentric chromosome. In this chromosome, one of its arm is slightly short
and other arm is slightly long.
iii. Acrocentric chromosome
The chromosome in which the centromere is located near the end is called acrocentric
chromosome. In this chromosome, one of its arm is very short and the other arm is very
long.
iv. Telocentric chromosome:
The chromosome in which the centromere is located at its tip is called a telocentric
chromosome.
318 GREEN Science (Biology) Book-10
Chromosomes are responsible for transmitting hereditary characteristics from one
generation to another. So, they act as the vehicles of heredity. Sex chromosomes determine
the sex of an organism. They bring out variation and hence help in organic evolution.
They control protein synthesis and cell metabolism.
Diploid and Haploid Number of Chromosomes
Each species has a fixed number of chromosomes in its cells. For example, human beings
have 46 chromosomes in every body cell. The 46 chromosomes in an ordinary human
cells are of 23 different kinds. So, there are two chromosomes of each kind. The two
chromosomes of each kind are called homologous chromosomes. A cell which has the full
number of chromosomes, with two of each kind, is called a diploid cell. It is denoted by
‘2n’. In other words, a diploid cell has two sets of each type of chromosomes. We can now
say that the ordinary body cells of human beings have a diploid number of chromosomes.
The diploid number of chromosomes is specific for a species. The ordinary body cells of
human beings contain the full number of 46 chromosomes, so they are the diploid cells or
they contain the diploid number of chromosomes.
The gametes (or sex cells) of human beings are different from their body cells because
they contain only half the number of chromosomes. A cell which has half the number of
chromosomes, with one of each kind, is called a haploid cell. It is denoted by ‘n’. In other
words, a haploid cell has only one set of each type of chromosomes. Gametes (or sex cells)
have haploid number of chromosomes. The human gametes (or sex cells) called sperm
and egg have only 23 chromosomes each, which is half the number of chromosomes in
other body cells. So, a gamete is a haploid cell. We can also say that gametes contain
haploid number of chromosomes.
Gametes have half the usual number of chromosomes so that when male and female
gametes fuse together to form a zygote then their chromosomes will add up and the
zygote they form will have the normal number of chromosomes. For example, the human
sperm has 23 chromosomes and the human egg (or ovum) also has 23 chromosomes. So,
when a sperm and an egg fuse together during fertilization, then the zygote formed will
have 23+23=46 chromosomes, which is the normal number of chromosomes. Thus, the
gametes (or sex cells) are always haploid. But when two gametes (sperm and egg) fuse
together, they form a diploid zygote.
All other chromosomes in a cell (except the sex chromosomes) are called autosomes.
Thus, autosomes are non-sex chromosomes. Autosomes are of the same number and kind
in both, the male and female of a species. In human beings there are 22 pairs of autosomes
and one pair of sex chromosomes.
GREEN Science (Biology) Book-10 319
Number of chromosomes
The number of chromosomes present in the cells of an organism is fixed. Generally, the
somatic cells of an organism contain diploid chromosomes and sex-cells or gametes
contain haploid chromosomes.
Diploid cells contain half chromosomes from the father and remaining half-chromosomes
from the mother.
The number of chromosomes present in somatic cells and gametic cells, i.e. sex-cells is
tabulated below:
S.N. Living beings No. of chromosomes in No. of chromosomes in
somatic cells (2n) sex-cells (n)
1. Human beings
2. Housefly 23 pairs 23
3. Gorilla 12 pairs 12
4. Frog 24 pairs 24
5. Potato (Solanum) 13 pairs 13
6. Yeast 24 pairs 12
7. Pine 1 pair 1
8. Onion 12 pairs 12
9. Pea 8 pairs 8
10. Sugarcane 7 pairs 7
40 pairs 40
Importance of chromosomes
i. Chromosomes contain genes and hence carry heredity characteristics.
ii They play an important role in cell division, cell growth and cell repair.
Activity
Take a chromosome and draw a picture of different types of chromosomes.
Demonstrate the chart paper in your classroom.
Sex Determination
Chromosomes are made up of genes, i.e. tiny units of heredity. The genes present in
chromosomes of the male and female gametes determine the sex of an organism. In
human somatic cell, there are 23 pairs of chromosomes. Out of 23 pairs, 22 pairs are
called autosomes and 1 pair is called sex chromosomes. Autosomes control vegetative
characteristics whereas sex-chromosomes determine the sex of an individual.
A person can have a male sex or female sex. The process by which the sex of a person is
determined is called sex determination. Genetics is involved in the determination of the
sex of a person. This can be explained as follows:
320 GREEN Science (Biology) Book-10
Mother Father
XX XY
X Eggs X X Sperm Y
XX XY XX XY
Fig.
Fig.17.4 Girl Boy Girl Boy
Sex determination in human being
The chromosomes which determine the sex of a person are called sex chromosomes.
There are two types of sex chromosomes, one is called X chromosome and other is called
Y chromosome.
i. A male gamete (sperm) of human being has one X chromosome and one Y
chromosome. This means that half the male gametes or half the sperms will have X
chromosome and the other half will have Y chromosome .
ii. A female gamete (egg) of human being has two X chromosomes (but no Y
chromosomes).This means that all the female gametes called ova (or eggs) will have
only X chromosomes.
The sex of a child depends on what happens at fertilization: (a)If a sperm carrying
X chromosome fertilizes an
ovum (or egg) which carries
X chromosome, then the child
born will be a girl (or female).
This is because the child will
have XX combination of sex
chromosomes.
iii. If a sperm carrying Y
chromosome fertilizes an
ovum (or egg) which carries
X chromosome, then the child
born will be a boy (male).
This is because the child will
have XY combination of sex
chromosomes.
17.5
GREEN Science (Biology) Book-10 321
Please note that it is the sperm which determines the sex of the child. This is because half
of the sperms have X chromosomes and the other half have Y chromosomes. Thus, there
is a 50 percent chance of a boy and a 50 percent chance of a girl being born to the parents.
This is why the human population is roughly half males and half females.
In some of the animals, sex determination is also controlled by the environmental factors.
For example, in some reptiles, the temperature at which the fertilized egg is incubated
before hatching, plays a role in determining the sex of the offspring. It has been found
that in a turtle (Chrysema picta), high incubation temperature leads to the development of
female offspring (or female progeny). On the other hand, in the case of a lizard (Agama
agama), high incubation temperature results in male offspring (or male progency).
Chromosomal disorder Do You Know
Genes, i.e. tiny units of heredity are located in Haemophilia is a sex-linked genetic
chromosomes. They control the development disorder which leads continuous bleeding
of various characteristics in the body. There is a through cut or wound. It transmits from
fixed sequence of genes in a chromosome to parents to their offspring and is seen only
develop a certain characteristic. When the in males. Females can transmit this disease
sequence of genes or the number of but do not suffer. So, haemophilia is a sex-
chromosomes in a body changes, a variety of linked disease.
disorders are seen in the human body. These
disorders are called chromosomal disorders. Do You Know
Turner’s syndrome, Klinefelter's syndrome,
Down’s syndrome, etc. are some examples of The diseases which occur only in males or
chromosomal disorder. females are called sex-linked diseases.
Examples:
Chromosomal disorder can be defined as the 1. Haemophilia
disorder caused in human body due to decrease 2. Male pattern baldness
or increase in the number of chromosomes. 3. Uterine cancer
Chromosomal disorder occurs when the 4. Colour blindness
number of chromosomes changes during
cell division. The number of chromosomes in
cells may be more or less when centrosomes
do not divide properly. This process is called
aneuploidy. In this unit, we will discuss
about chromosomal disorders like Turner’s
syndrome, Down’s syndrome and Klinefelter’s
syndrome.
Turner’s syndrome (45 + XO) Do You Know
Turner’s syndrome was described by Dr. Henry Girls and women suffer from Turner’s
Turner. It occurs due to lack of X-chromosome. syndrome.
In this condition, the number of chromosome
becomes 45 + XO. The symptoms of this
disorder are as follows:
1. Shorter body structure
322 GREEN Science (Biology) Book-10
2. Lack of secondary sexual characteristics
3. Poorly developed ovaries
4. Lack of menstrual periods Fig. Fig.
5. Sterility in females
6. Short neck 17.6
Turner’s syndrome
Down’s syndrome (47, XY, + 21) or (47, XX, +21)
Down’s syndrome is the chromosomal Do You Know
disorder that occurs due to addition of extra
chromosome in the 21 chromosome. It is also Down’s syndrome was first described
called trisomy of chromosome 21. by John Langdon Down.
The major symptoms of Down’s syndrome are It occurs in one in every 1000 live
given below: births.
1. Flat face and small broad nose
2. Protruding tongue
3. Thick palm with creases
4. Lack of physical, mental and psychological
development
5. Furrow in tongue 17.7
6. Dwarfism or short stature Down's syndrome
7. Small and round head
8. Half-opened mouth Do You Know
9. Upward slanting eyes
Klinefelter’s syndrome (47, XXY) The person's having Down's syndrome
are at increased risk of heart diseases,
Klinefelter’s syndrome is the chromosomal leukaemia, respiratory infection, etc.
disorder occurs due to addition of sex
chromosome (X – chromosome). In this
condition, the number of Tall stature Frontal baldness
chromosome becomes 47, Slightly feminized absent
XXY karyotype. This disorder physique Poor beard
commonly occurs in males. In Mildly impaired growth
1942, Dr. Harry Klinefelter and IQ (15 points less
his co-workers described this than average) Breast
syndrome. Tendency to lose development (in
chest hairs 30% of cases)
The major symptoms of Osteoporosis
Klinefelter’s syndrome are as Female-typeFig. Small testes
follows: pubic hair
pattern Klinefelter’s syndrome
1. Growth of breasts in males
17.8
2. Infertility or sterility in males
GREEN Science (Biology) Book-10 323
3. Tall stature
4. Small genitals
5. Female type pubic hair pattern
6. Poor beard growth
7. Mildly impaired IQ
8. Wider hips, etc.
Key Concepts
1. Chromosomes are the thread-like structures present in the nucleus of a cell which
contain hereditary information of the cell.
2. The chromosomes of each species occur in pair. In a pair of chromosomes, one
chromosome comes from the father and another comes from the mother.
3. Each chromosome of a homologous pair has genes for the same characteristics in
the same place.
4. The chromosome in which the centromere is located near the middle point is called
a metacentric chromosome.
5. The chromosome in which the centromere is located a little away from the middle
point is called a submetacentric chromosome.
6. The chromosome in which the centromere is located near the end is called acrocentric
chromosomes.
7. The chromosome in which the centromere is located at its tip is called a telocentric
chromosomes.
8. A cell which has the full number of chromosomes, with two of each kind, is called
a diploid cell (2n).
9. A cell which has half the number of chromosomes, with one of each kind, is called
a haploid cell (n).
10. Gametes have half the usual number of chromosomes so that when male and female
gametes fuse together to form a zygote then their chromosomes will add up and the
zygote they form will have the normal number of chromosomes.
11. All other chromosomes in a cell (except the sex chromosomes) are called autosomes.
Thus, autosomes are non-sex chromosomes. Autosomes are of the same number
and kind in both, the male and female of a species.
12. The process by which the sex of a person is determined is called sex determination.
13. The sex of a child depends on what happens at fertilization: (a)If a sperm carrying
X chromosome fertilizes an ovum (or egg) which carries X chromosome, then the
child born will be a girl (or female).
14. If a sperm carrying Y chromosome fertilizes an ovum (or egg) which carries X
chromosome, then the child born will be a boy (male).
15. There is a 50 percent chance of a boy and a 50 percent chance of a girl being born
to the parents. This is why the human population is roughly half males and half
females.
324 GREEN Science (Biology) Book-10
16. Haemophilia is a chromosome disorder which leads continuous bleeding through
cut or wound. It transmits from parents to their offspring and is seen only in males.
17. Chromosome disorder can be defined as the disorder caused in human body due to
decrease or increase in the number of chromosomes.
18. Klinefelter’s syndrome occurs due to addition of sex chromosome (X – chromosome).
In this condition, the number of chromosome becomes 47, XXY karyotype.
Sequential General Exercise 1
1. Choose the best answer from the given alternatives.
a. ................... are the tiny units of heredity.
cell chromosomes genes chromatids
b. The strand of a chromosome is called .........................
centromere chromatid nucleus gene
c. There are ..................... types of chromosomes on the basis of position of
centromere.
1 2 3 4
d. The number of chromosomes in the somatic cells of human beings is ...................
23 46 40 12
e. The chromosome disorder that occurs due to addition of X-chromosome is called
...................
Down’s syndrome Turner’s syndrome
Haemophilia Klinefelter’s syndrome
2. Answer the following questions.
a. What is chromosome?
b. What are the components of a chromosome?
c. Describe the different types of chromosomes.
d. Define the following with the help of diagram
i. Metacentric chromosomes
ii. Submetacentric chromosomes
iii. Acrocentric chromosomes
iv. Telocentric chromosomes
GREEN Science (Biology) Book-10 325
e. Write any three functions of chromosomes.
f. Define chromosome disorder with one example.
g. What are sex-linked diseases? Give any three examples.
3. What do you understand by the following?
a. Diploid chromosomes
b. Haploid chromosomes
4. What are sex chromosomes?
5. What are autosomes?
6. How is the sex of an individual determined? Explain with a chart.
7. Differentiate between:
a. Haploid cell and diploid cell
b. Autosomes and sex-chromosomes
c. Metacentric chromosome and telocentric chromosome
d. Down’s syndrome and Klinefelter’s syndrome
8. What is Turner’s syndrome? Write its symptoms.
9. What is Down’s syndrome? Write its symptoms.
10. What is Klinefelter’s syndrome? Write its symptoms.
11. What is meant by chromosome disorder? Explain with examples.
Grid-based Exercise 2
Group ’A’ (Knowledge Type Questions) (1 Mark Each)
1. What is a chromosome?
2. What is chromosome made of?
3. What is chromatic reticulum?
4. What is kinetochore?
5. Define sub-metacentric chromosome.
6. Write down the number of chromosomes found in somatic cell and gametic cell of
given organisms. (Ans: (i) 46,23 (ii) 26,13)
i. Human ii. Frog
7. Where is chromosome found?
8. What is gene?
9. What is telocentric chromosome?
326 GREEN Science (Biology) Book-10
10. Write down the number of chromosomes found in somatic cell and gametic cell of
given organisms. (Ans: (i) 24,12 (ii) 2,1)
i. Pine ii. Yeast
11. What is sex determination?
12. Write any two characteristics of haemophilia.
13. Write any two characteristics of chromosome.
14. Write down the main cause of Down’s syndrome.
15. What is Turner’s syndrome?
Group’B’(Understanding Type Questions) (2 Marks Each)
16. What is meant by chromosomal disorder? Write with one example.
17. Differentiate between metacentric chromosome and sub-metacentric chromosome.
18. What is sex-linked disease? Write one example.
19. Differentiate between autosome and sex chromosome.
20. What is meant by aneuploidy? Write with example.
21. What do you mean by somatic cell and gametic cell?
22. Why is haemophilia called a sex-linked disease?
23. How many chromosomes are there in human sperm and zygote? Why ?
24. How many chromosomes are there in ovum and zygote and why ?
25. Differentiate between diploid and haploid chromosome.
Group ‘C’ (Application Type Questions) (3 Marks Each)
26. What is Down’s syndrome ? Write its two symptoms.
27. Define sex-linked disease with an example. Draw a figure showing metacentric
chromosome.
28. How does chromosome disorder occur ? Write with an example.
29. Name the types of chromosomes on the basis of position of centromere. Draw figure
of any one type of chromosome.
30. What is sex determination ? Write down the function of autosome and sex chromosome.
Group ’D’ (Higher Abilities Type Questions) (4 Marks Each)
31. Describe the structure of chromosome with a labelled figure.
32. How is sex determined ? Explain with figure.
33. Write short note on:
i. Turner’s syndrome ii. Klinefelter’s syndrome
34. How is sex of a boy and girl determined ? Describe.
i. Explain the role of chromosomes for living beings.
ii. Write a short note on chromosome disorder.
GREEN Science (Biology) Book-10 327
UNIT Asexual and Sexual
18 Reproduction
Weighting Distribution Theory : 10 Practical: 2
Before You Begin
Reproduction is the basic characteristic of living beings. All plants and
animal produce offspring similar to them. Reproduction is the process
of creation of new organisms by asexual or sexual method. Living
beings reproduce to continue their races on the earth. Primitive plants
and animals reproduce by asexual method whereas advanced animals
and plants reproduce by sexual method. The reproduction which
takes place without the fusion of a male gamete and a female gamete
is called reproduction. It is common in primitive plants and animals.
The reproduction which takes place by the fusion of a male gamete and
a female gamete is called sexual reproduction. It is common in higher
plants and animals. In this unit, we will study asexual and sexual
reproduction that occurs in plants and animals.
Learning Objectives Syllabus
After completing the study of this unit, students will be able to: • Introduction to reproduction
i. explain asexual and sexual reproduction that occurs
• Asexual reproduction and its
in plants and animals with examples. characteristics
ii. explain artificial vegetative propagation in animals • Sexual reproduction and its
with examples. characteristics
iii. describe the significance of asexual and sexual • Artificial vegetative propaga-
reproduction. tion in plants
Glossary: A dictionary of scientific/technical terms
reproduction : the biological process by which plants and animals produce their offspring
asexual : without involvement of gametes
sexual : by the involvement of gametes
fission : the process in which a unicellular organism splits into two or more
gamete daughter organisms
zygote : the male or female sex cell
: the cell formed by the fusion of a male gamete and a female gamete
328 GREEN Science (Biology) Book-10
A. Reproduction
Reproduction is the basic characteristic
of living beings, i.e. plants and animals.
Living organisms on the earth have a
certain life span. They cannot survive
forever and die after a certain period of
time. Therefore, living beings reproduce Fig.
their own kinds to continue their races
on the earth. This process is called
reproduction. So, reproduction is a 18.1
biological process in which living beings Tiger with cubs
produce their own kinds asexually or sexually. Some living beings reproduce from their
single body. Some animals and plants reproduce with the help of gametes. Different
animals and plants reproduce by different methods.
Types of Reproduction 2. Sexual reproduction
1. Asexual reproduction
1. Asexual reproduction
The reproduction which takes place without the fusion of a male gamete and a female
gamete is called asexual reproduction. This process is common in primitive plants and
animals. In asexual reproduction, only one organism can reproduce and the offspring
produced are exactly identical to their parent. It is an easy and fast method of reproduction.
Examples: budding in yeast, budding in hydra, fission in amoeba, sporulation in mucor,
vegetative propagation in potato, etc.
Characteristics of asexual reproduction
1. Only one organism can reproduce by this method.
2. In this type of reproduction, gametes are not produced.
3. Fertilization does not occur in this reproduction.
4. All offspring produced by this method are exactly identical to their parent.
5. It is a fast and easy method of reproduction.
6. Mitosis takes place in this reproduction.
Types of Asexual reproduction
i. Fission ii. Budding iii. Sporulation
vi. Vegetative propagation
iv. Fragmentation v. Regeneration
i. Fission
It is the most common method of reproduction in unicellular organisms. A fission is the
method of asexual reproduction in which a unicellular organism divides into two or
GREEN Science (Biology) Book-10 329
more daughter organisms. This method is common in amoeba, paramecium, bacteria,
plasmodium, euglena, etc. A fission is of two types, viz. binary fission and multiple fission.
Binary fission Daughter amoeba
Mother amoeba
The method of asexual reproduction in
which one unicellular organism divides into
two daughter organisms is called a binary
fission. Plants like bacteria, chlamydomonas,
diatoms, etc. and animals like amoeba,
paramecium, euglena , etc. reproduce by this
method.
Fig.
Fig.18.2
Fig.Multiple fission Binary fission on amoeba
The method of asexual reproduction in which one unicellular organism divides into more
than two daughter organisms is called a multiple fission. It is common in plasmodium,
chlamydomonas, amoeba, etc. It generally occurs during unfavourable condition.
Cytoplasm Nucleus Many daughter cells produced Daughter cells
by multiple fission
Cell
Cyst Cyst breaks
18.3
Multiple fission in plasmodium
During unfavourable condition, unicellular organisms like amoeba, plasmodium, etc.
from a thick cover called cyst in the outer surface of the body. Inside the cyst, nucleus
divides repeatedly and forms numerous daughter nuclei. When conditions are suitable,
the cyst breaks and each nucleus is surrounded by cell-membrane and cytoplasm forming
numerous daughter organisms.
ii. Budding 18.4
The method of asexual reproduction which Budding in yeast
takes place with the help of a bud is called
budding. Plants like yeast and animals like
hydra reproduce asexually by this method. In
this method, a small bud develops in the body
which separates as a new organism after its
growth.
Activity 1
Take a test tube and add 20 ml water and dissolve 5 gram of sugar into it.
Add some yeasts into the test tube and keep it in a warm place for a few hours.
Observe the test tube carefully. What did you observe?
330 GREEN Science (Biology) Book-10
iii. Sporulation
The method of asexual reproduction which takes place by means of spores is called
sporulation. This method is common in mucor, marchantia, moss, mushroom, fern, etc. In
sporulation, spores are formed inside sporangia. At maturity, sporangia burst and release
spores on soil. These spores germinate and form new plants on favourable condition.
Sporangium
Female gametophyte
Gemma cup
Stolon Fig. Fig. Thallus
RhizoidFig.
Rhizoids
18.5
Sporulation in mucor Sporulation in liverwort
iv. Fragmentation 18.6 Nucleus
A fragmentation is the method of asexual Spiral
reproduction in which a multicellular organism chloroplast
splits into two or more fragments and each
fragment develops into a new organism. Fragmentation in spirogyra
Spirogyra reproduces by this method. In
fragmentation a multicellular organism splits
into two or more fragments and each fragment
regains its lost body parts and develops into a
complete organism.
v. Regeneration 18.7
A regeneration is the method of asexual Regeneration in Planaria
reproduction in which each fragment of an
organism regains its lost body parts and
develops into a complete organism. This method
is common in planaria, hydra, tapeworm,
starfish, sponge, etc.
vi. Vegetative propagation
A vegetative propagation is the method of asexual reproduction in which new plants
are produced by vegetative parts like roots, stem and leaf. This process occurs in some
flowering plants which do not produce viable seeds.
Flowering plants like sweet potato, dahlia, mint, etc. reproduce asexually by means of
roots.
GREEN Science (Biology) Book-10 331
Fig. Fig.18.8 Roots of sweet potato
Roots of dahlia
Flowering plants like potato, onion, rose, sugarcane, garlic, bamboo, banana, ginger,
colocasia, etc. reproduce asexually by stem.
18.9 Buds in ginger
Buds in potato
Activity 2
Take a matured potato and observe its surface.
You will see small scars called eyes.
Cut the potato into 3-4 pieces keeping 2-3 eyes in each piece.
Plant those pieces in moist soil. Observe those pieces after 8-10 days. What do you
observe?
Flowering plants like Bryophyllum, Begonia, etc. reproduce asexually by leaf.
Fig.
18.9 Reproduction in Begonia by leaf
Reproduction in Bryophyllum by
leaf
332 GREEN Science (Biology) Book-10
Activity 3
Take a matured Bryophyllum plant and observe the plantlets developed on its leaf.
Draw neat and labelled figure.
Advantages of asexual reproduction or vegetative propagation
1. It is an easy and fast method of reproduction.
2. Only one organism can reproduce by this method.
3. The offspring produced by this method are genetically identical to their parents.
4. Some flowering plants like potato, rose, sugarcane, banana, bamboo, etc. do not
produce viable seeds. These plants can easily be propagated by this method.
5. The flowering plants produced by this method start bearing flowers and fruits earlier
than those reproduced from their seeds.
6. Farmers can produce a large number of flower and fruit trees by this method.
7. The species of rare and endangered plants can be reproduced by this method.
2. Sexual Reproduction
The method of reproduction which takes place by the fusion of a male gamete and a
female gamete is called sexual reproduction. Both male and female organisms are involved
in sexual reproduction. Flowering plants and developed animals reproduce by sexual
reproduction. Some animals produce only one types of gametes (either male or female).
These animals are called unisexual animals. Examples: human beings, birds, reptiles,
amphibians, fishes, etc. Some animals produce both male and female gametes in the same
body. These animals are called bisexual or hermaphrodite animals. Examples: hydra,
tapeworm, liverfluke, earthworm, etc. Similarly, flowering plants also have unisexual
and bisexual flowers. Pumpkin, cucumber, papaya, gourd, etc. produce unisexual flowers
whereas mustard, pea, tomato, orange, etc. produce bisexual flowers.
Sexual Reproduction in Plants Petal
Most flowering plants Stigma Anther
reproduce sexually by Style Filament
means of seeds. A flower is
the colourful, attractive and
sexually reproductive part of
flowering plants. A bisexual
flower consists of four whorls.
They are: calyx, corolla,
androecium and gynoecium.
Fig. Sepal
Ovary Receptacle
18.11
Bisexual flower
GREEN Science (Biology) Book-10 333
A brief description of each whorl of a flower is given below:
i. Calyx
It is the outermost whorl of a flower. It consists of green leaf like structures called sepals.
Calyx protects a flower in the bud stage and helps in photosynthesis.
ii. Corolla
It forms the second whorl of a flower. It consists of colourful and attractive structures
called petals. Petals protect the reproductive organs of a flower and attract insects for
pollination.
iii. Androecium
It forms the third inner whorl Anther
of a flower. It consists of a
group of male reproductive
organs called stamens.
Each stamen consists of a
filament, connective and a
Fig. Filament
Fig.
anther. Each anther consists 18.12
of four pollen sacs where
A typical androecium
microspore mother cells are
present. Each microspore mother cell undergoes meiosis cell division and forms four
haploid pollen grains. Each pollen grain contains male gametes. Anthers produce pollen
grains at maturity. Pollen grains are fine powdery structures having male gametes.
iv. Gynoecium
It forms the fourth innermost whorl of a flower. It consists of one or more female
reproductive organs called carpels or pistils. Each carpel or pistil consists of three distinct
parts. They are stigma, style and ovary.
Stigma
Stigma
Style Style
Ovary Ovary
18.13 Gynoecium of mustard
Typical carpel (gynoecium)
The uppermost spreading part of a pistil is called a stigma, the middle cylindrical part
is called a style and the lowermost swollen part is called a ovary. At maturity, an ovary
produces ovules containing female gametes or egg cells.
334 GREEN Science (Biology) Book-10
Pollination
The transfer of pollen grains from anther to the stigma of the same flower or different
flower is called pollination. Various external agents like air, water, insects, birds, animals,
etc. help in pollination.
Cross-pollination
Self-pollination
Fig.18.14
Fig. Pollination
Pollination is of two types, viz. self-pollination and cross-pollination.
i. Self-pollination
It is the transfer of pollen grains from the anther to the stigma of the same flower. This
process is common in bisexual flowers like pea, chinarose, tomato, wheat, cotton, etc.
ii. Cross-pollination
It is the transfer of pollen grains from the anther of a flower of one plant to the stigma of a
flower of another plant of the same species. This process is common in unisexual flowers
like pumpkin, cucumber, gourd, maize, coconut, oak, etc. and bisexual flowers.
In flowering plants, pollination is followed by fertilization.
Fertilization Pollination Pollen
The process of fusion of a male
gamete and a female gamete Stigma Anther
to form a zygote is called
fertilization. After pollination, Filament
pollen grains germinate and
form pollen tubes which Style
grow towards ovary through Pollen tube
style and finally reaches the
ovule in the ovary through Ovule
micropyle. After entering into Embryo sac Polar nuclei
ovule, the pollen tube enters Synergids Sperm cells
into the embryo sac. In the
Petal
embryo sac three antipodal
Sepal
cells, two synergids, a female 18.15
gamete or egg cell and a
Fertilization in flowering plants
GREEN Science (Biology) Book-10 335
diploid secondary nucleus are found. When Do You Know
the pollen tube enters embryo sac, the apex of
the tube dissolves and two male gametes are The fertilization in flowering plants is
released. One of the male gamete fuses with called double fertilization because the
the female gamete and forms a diploid zygote. fertilization involves the fusion of two
This process is called fertilization. The second male gametes separately, i.e. one male
male gamete (n) fuses with secondary nucleus gamete with the egg cell and another
(2n) and forms endosperm nucleus (3n). This male gamete with the secondary nucleus.
process is called triple fusion.
Later on the zygote develops into embryo
and endosperm nucleus develops into endosperm of the seed. After fertilization, ovule
develops into seeds and ovary develops into fruit.
In this way, flowering plants reproduce sexually.
Activity 4
Collect different types of flowers from the school garden.
Identify calyx, corolla, androecium and gynoecium in these flowers.
Also, separate unisexual and bisexual flowers.
Draw a neat figure showing each whorl of the flower and label the main parts.
Differences between self-pollination and cross-pollination
Self-pollination Cross-pollination
1. In this method, pollen grains transfer 1. In this method, pollen grains transfer
from anther to the stigma of the same from anther of one plant to the stigma
flower. of another plant of the same species.
2. It occurs in the same plant. 2. It occurs in between the plants of the
same species.
3. It occurs in bisexual flowers. 3. It occurs in both unisexual as well as
bisexual flowers.
4. Agents of pollination are not essential. 4. Agents of pollination are essential.
Sexual Reproduction in Animals Do You Know
Vertebrate animals reproduce sexually by The fusion of a male gamete and a
means of gametes. However, invertebrates female gamete to form a zygote is
like arthropods, molluscs, annelids, round called fertilization. It is of two types,
worms, flatworms, etc. reproduce by sexual viz. external fertilization and internal
method. Male organisms produce male fertilization.
gametes and female organisms produce
female gametes. On favourable condition, The process by which gamete are
fusion of a male gamete and female gamete formed is called gametogenesis.
takes place to form a zygote. This process
336 GREEN Science (Biology) Book-10
is called fertilization. In animals, fertilization is of two types, i.e. external fertilization
and internal fertilization. If the fusion of a male gamete and a female gamete takes place
outside the body of a female organism, it is called external fertilization. It occurs in fishes
and frogs. If the fusion of a male gamete and female gamete takes place inside the body
of a female organism, it is called internal fertilization. It occurs in insects, reptiles, birds
and mammals.
After formation of a zygote, it divides repeatedly and forms embryo which finally
develops into a new organism. In this way, animals reproduce by sexual method.
Difference between Asexual reproduction and Sexual reproduction
Asexual reproduction Sexual reproduction
1. Gametes are not involved. 1. Gametes are involved.
2. It occurs in lower organisms. 2. It occurs in higher organisms.
3. A single organism can reproduce by 3. A single organism cannot reproduce
this method. by this method.
4. It does not help in evolution of 4. It helps in evolution of organisms.
organisms.
5. All offspring are identical to their 5. All offspring are not exactly identical
parents. to their parents.
Differences between external fertilization and internal fertilization
External fertilization Internal fertilization
1. Fusion of a male gamete and a female 1. Fusion of a male gamete and a female
gamete takes place outside the body gamete takes place inside the body of
of a female. a female.
2. This method is common in fishes and 2. This method is common in insects,
amphibians. reptiles, birds and mammals.
Advantages of Sexual Reproduction
1. It gives continuity to the generations of a species.
2. It brings out variation among the members of the same species.
3. It helps in evolution of organisms.
Difference between gamete and zygote
Gamete (male/female) Zygote
1. Gamete is a haploid sex cell. 1. Zygote is a diploid sex cell.
2. It is produced by male or female sex 2. It is produced by the fusion of a male
organ. sex cell with a female sex cell.
GREEN Science (Biology) Book-10 337
B. Artificial Vegetative Propagation in Plants
Most flowering plants reproduce sexually through seeds. However, some flowering
plants like apple, pear, rose, potato, plum, etc. do not produce viable seeds. Such type of
plants can be propagated artificially by using their vegetative parts (root, stem or leaves).
This process is called artificial vegetative propagation.
Now-a-days, farmers, gardeners, horticulturists and botanists produce a large number of
plantlets artificially by vegetative propagation. This method of propagation of plants is
mainly applied in fruits, vegetables and flowers.
Some of the common artificial methods of vegetative propagation are as follows:
1. Layering 2. Grafting 3. Tissue culture
1. Layering
Layering is the artificial vegetative propagation in which new plants are obtained in the
stem of parent plants. Different types of fruit plants like strawberry, raspberry, lemon,
citrus, apple, pear, etc. are propagated by this method.
Types of Layering
i. Simple layering Parent plant
Vigorous shoot
Simple layering is applied in lemon,
citrus, etc. to propagate new plants.
In simple layering, a long and
flexible stem more than one year age Stem
is bent down and the stem is buried
Fig. Fig.
in the soil about 20cm away from
its tip. After 2-3 months, the buried
portion develops roots. Then the 18.16
plant is separated from the parent Simple layering
plant. Generally, new plants are
separated from the parent plant at the end of rainy season and planted in a new place.
ii. Compound layering Parent plant New plant
Soil cover Roots
This type of layering is commonly
applied in walnut, apple, pear, etc. 18.17
to propagate new plants. In this
method, the entire flexible stem is Compound layering
burried in the soil leaving its tip
outside the soil. The nodes in the
stem produce new plantlets within
2-3 months when watered regularly.
Then the plantlets are separated
from the parent plant.
338 GREEN Science (Biology) Book-10
iii. Air layering
Flowering plants like lemon, orange,
peach, plum, etc. are propagated
artificially by air layering. In air
layering, the bark of the stem of
about 2 year age is removed in the
shape of a ring. Then the portion of Fig. Fig.
the stem is covered with moist soil
and cloth. We can give IBA hormone
to enhance the growth of roots in the 18.18
stem. The portion of the stem can Air layering
also be covered with moss and plastic to make it airproof. The covered portion develops
roots within 1-2 months. Then the plantlets can be separated from the parent plant.
iv. Tip layering
Plants like raspberry, blackberry,
etc. are propagated artificially by tip
layering. The tip of the plant is bent
and burried in the soil to grow roots.
The tip develops roots within 3-4
months. Then the plant is separated
from the parent plant and cultivated.
18.19
Tip layering
v. Stool layering or mound layering
Flowering plants like mango, guava, apple, peach, plum, etc. are propagated artificially
by stool layering. In this method, the selected plant is cut at the height of 5-10cm from
the ground. Then the plant produces many branches. When these branches grow to a
certain height (20-25cm), stem is covered with soil or saw dust upto height of 10-15cm and
watered regularly. These branches develop roots within 3-4 months. Then these plants
can be separated from the parent plant.
Fig.
18.20 Mound layering
Stool layering GREEN Science (Biology) Book-10 339
2. Grafting
Grafting is the method of artificial vegetative propagation in which shoot system of one
plant is combined with root system of another plant. Most of the fruit plants like peach,
apple, mango, plum, guava, lemon, orange, etc. are propagated by this method. The plant
whose root system is used is called stock. Similarly, the plant whose shoot system is used
is called scion. Generally, scion of plant of superior quality is grafted on the stock of
closely related plant.
Types of grafting
i. Whip grafting
In this method, the scion and stock of closely related plants are cut obliquely. The length
of the cut portion should be 3-5cm. Then both stock and scion are combined together and
sealed with a tape. After 3-4 months, both stems combine together. This method should
be done before spring season. Whip grafting is a simple and very common method of
grafting which is generally done in commercial fruit trees.
Fig. Fig. 18.21
Whip grafting
ii. Cleft grafting
Cleft grafting is the most common method of grafting. In this method, the stock is cut and
split down the middle making a cleft about 5-8 cm deep. The end of scion should be cut
slanted in the shape of wedge and inserted into the cleft. The portion is sealed with a tape
making air tight. If the stock and scion are not disturbed, they combine within 2-3 months.
Cleft grafting is generally done in spring season.
18.22 Cleft grafting
340 GREEN Science (Biology) Book-10
iii. Tongue grafting Fig.
Tongue grafting is the popular method of grafting in 18.23
hilly and Himalayan region. In this method, a tongue
like deep structure (about 3 to 5 cm) is cut into scion
and stock. Then they are joined together. The joint
is sealed with a tape or a plastic until they combine
firmly. If remain undistributed and air tight, both
scion and stock combine within 2-3 months.
Tongue grafting
Activity 5
Visit the school garden with your science teacher.
Select some suitable plants and apply layering and grafting in some plants.
Observe these plants after 2 months. What do you find?
3. Tissue culture
The scientific method of propagation of new plants from cell, tissue or organ of a parent
plant keeping them in a culture solution is called tissue culture. The plants with exactly
identical characteristics to the parent can be produced by tissue culture.
Now-a-days, tissue culture is very common among botanists because of the following
reasons:
1. Exactly identical plants to the parents can be produced by this method.
2. A large number of new plants can be produced within a short time.
3. The plants which do not produce viable seeds can be propagated by this method.
4. Disease free plants can be produced.
5. Hybrid plants of desired quality can be produced.
6. Endangered plants can be protected.
Method of tissue culture
In laboratory, an artificial medium is prepared for tissue culture. The artificial medium is
called culture solution. The culture solution consists of nutrients and plant hormones. In
this method, a small lump is tissue is taken and kept in the culture solution under sterile
conditions. This lump is called callus. When the callus gets nutrients and hormones (auxin
and cytokinin), it grows and develops roots and shoot. Please note that auxin enhances
the growth of roots and cytokinin enhances the growth of the shoot. When the callus
develops roots and shoot, it is cut into several tiny plantlets. The plantlets with root and
shoot are planted into soil. In this way, new plantlets are propagated by tissue culture.
GREEN Science (Biology) Book-10 341
Tissue sample scraped Tissue samples placed in Agar
from parent plant growth medium containing
nutrients and auxins
Samples develop into
tiny plantlets
Fig.
18.24 Plantlets planted into compost
Tissue culture
Key Concepts
1. Reproduction is a biological process in which living beings produce their own
kinds asexually or sexually.
2. The reproduction which takes place without the fusion of a male gamete and a
female gamete is called asexual reproduction.
3. A fission is the method of asexual reproduction in which a unicellular organism
divides into two or more daughter organisms.
4. The method of asexual reproduction in which one unicellular organism divides
into more than two daughter organisms is called a multiple fission.
5. The method of asexual reproduction which takes place with the help of a bud is
called budding.
6. The method of asexual reproduction which takes place by means of spores is called
sporulation.
7. A fragmentation is the method of asexual reproduction in which a multicellular
organism splits into two or more fragments and each fragment develops into a new
organism.
8. A regeneration is the method of asexual reproduction in which each fragment of an
organism regains its lost body parts and develops into a complete organism.
9. A vegetative propagation is the method of asexual reproduction in which new
plants are produced by vegetative parts like roots, stem and leaf. This process
occurs in some flowering plants which do not produce viable seeds.
10. The method of reproduction which takes place by the fusion of a male gamete and
a female gamete is called sexual reproduction.
11. A bisexual flower consists of four whorls. They are: calyx, corolla, androecium and
gynoecium.
12. The transfer of pollen grains from anther to the stigma of the same flower or
different flower is called pollination.
13. Pollination is of two types, viz. self-pollination and cross-pollination.
342 GREEN Science (Biology) Book-10
14. The process of fusion of a male gamete and a female gamete to form a zygote is
called fertilization.
15. The fusion of a male gamete and a female gamete to form a zygote is called
fertilization.
16. The process by which gametes are formed is called gametogenesis.
17. Layering is the artificial vegetative propagation in which new plants are obtained
in the stem of parent plants.
18. Grafting is the method of artificial vegetative propagation in which shoot system of
one plant is combined with root system of another plant.
19. The plant whose root system is used is called stock. Similarly, the plant whose
shoot system is used is called scion.
20. Whip grafting is a simple and very common method of grafting which is generally
done in commercial fruit trees.
21. Tongue grafting is the popular method of grafting in hilly and Himalayan region.
22. The scientific method of propagation of new plants from cell, tissue or organ of a
parent plant keeping them in a culture solution is called tissue culture.
Sequential General Exercise 1
1. Choose the best answer from the given alternatives.
a. Which of the given asexual methods is common in amoeba?
fission budding
sporulation fragmentation
b. Planaria reproduces asexually by ........... regeneration
fragmentation
sporulation vegetative propagation
c. Which of the given part of a flower produces female gametes?
sepal stamen
petal pistil
d. Which of the given organisms cannot reproduce by asexual method?
hydra rose
cow starfish
e. Which method of artificial vegetative propagation requires culture solution?
air layering whip grafting
tissue culture tip layering
GREEN Science (Biology) Book-10 343
2. Answer the following questions.
a. Define reproduction.
b. What is asexual reproduction? Name any five organisms that reproduce asexually.
c. Define:
- fission - budding - sporulation
- fragmentation - regeneration
d. Write down the advantages of asexual reproduction.
e. What is vegetative propagation? Write its types.
f. Define sexual reproduction.
g. What is meant by a complete flower? Explain with a neat figure.
h. Define pollination and write its types.
i. What is fertilization? How is zygote formed? Describe.
j. What is artificial vegetative propagation?
k. Define layering and write its types.
l. What is grafting? How is grafting done.
m. Define tissue culture and write its advantages.
3. Differentiate between:
a. Asexual and sexual reproduction
b. Budding and sporulation
c. Self-pollination and cross-pollination
d. Layering and grafting
e. External and internal fertilization
f. Ovum and sperm
g. Simple layering and compound layering
h. Stock and scion
4. How does amoeba reproduce by fission? Describe.
5. Draw a neat and labelled figure showing:
a. Budding in hydra
b. Complete flower
c. Cross-pollination
d. Whip grafting
6. How is simple layering done? Explain with figure.
7. What is cleft grafting? Explain with figure.
8. Artificial vegetative propagation is beneficial for farmers. Justify this statement.
9. Write down the technique of tissue culture in brief.
10. The existence of variety of plants and animals would be impossible in the absence
of sexual reproduction. Justify this statement.
11. Out of layering, grafting and tissue culture, which method of artificial propagation
is suitable in the context of Nepal? Why?
344 GREEN Science (Biology) Book-10
Grid-based Exercise 2
Group ‘A’ (Knowledge Type Questions) (1 Mark Each)
1. What is asexual reproduction?
2. What is fission? What are the types of fission?
3. Write the name of any two organisms that reproduce by binary fission.
4. Write the name of a plant and one animal that reproduce by multiple fission.
5. What is budding?
6. What is regeneration?
7. Write the name of a plant each that reproduce asexually by root and stem.
8. What is artificial vegetative propagation?
9. Name an organism which reproduces by either of the given methods:
i. Binary fission ii. Sporulation
10. What is vegetative propagation?
11. What is sexual reproduction?
12. What is internal fertilization?
13. What is simple layering?
14. What is air layering?
15. What is tissue culture?
Group ’B’ (Understanding Type Questions) (2 Marks Each)
16. Write any two differences between asexual reproduction and sexual reproduction.
17. Write any two differences between fission and budding.
18. What does it mean by the saying that spirogyra reproduces by fragmentation?
19. Write any two differences between self-pollination and cross-pollination.
20. Why is layering done in plants? Give any two reasons.
21. Which method is widely used in Nepal out of grafting, layering and tissue culture?
Why?
22. The number of chromosome is different in gametes and zygote, why?
23. Write any two differences between pollination and fertilization.
24. The number of chromosome in gametes is haploid, why?
25. Write two differences between external and internal fertilization.
Group ‘C’ (Application Type Questions) (3 Marks Each)
26. How does amoeba reproduce by binary fission? Describe in brief.
27. Draw a diagram showing various parts of a bisexual flower.
28. How does hydra reproduce by budding? Describe in brief.
29. Write any three advantages of asexual reproduction.
30. Write any three reasons for reproducing plants by artificial vegetative propagation.
Group ‘D’ (Higher Abilities Type Questions) (4 Marks Each)
31. Describe in brief the importance of artificial reproduction in plants.
32. How is compound layering done? Explain with figure.
33. Draw a chart showing the process of sexual reproduction in developed organisms.
34. How do flowering plants reproduce? Describe in brief.
35. How is cleft grafting done? Describe with figure.
GREEN Science (Biology) Book-10 345
UNIT Heredity
19
Weighting Distribution Theory : 5 Practical: 0
Before You Begin
A variety of plants and animals are found in our surroundings.
These organisms differ from each other in many aspects.
Though many organisms look similar, they differ in many
characteristics. The offspring produced by asexual reproduction
look identical to each other but there are many dissimilarities
in the offspring produced by sexual reproduction. It is
because, offspring get some genes from father and some genes
from mother. The characteristics of parents are transmitted to
their offspring through genes. The process of transmission of
parental heredity. Genes are the tiny units of heredity present
in chromosomes. In this unit, we will study about heredity,
Mendel’s laws of heredity, variation and mutation.
Learning Objectives Syllabus
After completing the study of this unit, students will be able to:
i. introduce heredity and state Mendel's laws of • Introduction to heredity
• Mendel's experiment
heredity. • - Law of dominance
• - Law of segregation
ii. explain Mendel's experiment with figures and charts. • Genetic characteristics in
iii. list the causes of heredity. human beings
• Variation and mutation
iv. differentiate between mutation and variation.
Glossary: A dictionary of scientific/technical terms
heredity : the phenomenon by which parental characteristics are transmitted to their
offspring
genes
hybrid : the tiny units of heredity located in chromosomes
variation : the offspring produced by two genetically different parents
mutation : the differences among the members of the same species
: the sudden heritable change in the genetic material of an organism
346 GREEN Science (Biology) Book-10
Heredity
Heredity is the phenomenon by which living beings transmit parental characteristics to
their offspring. Living beings transmit their characteristics with the help of genes. The
nucleus of a cell contains thread like structures called chromosomes. Genes are located
in chromosomes in the nucleus. They are arranged in a linear fashion in a chromosome.
Genes determine physical, anatomical and physiological characteristics of organisms.
These characteristics are transmitted from one generation to another. Each chromosome
consists of a large number of genes. It is estimated that about 60,000 genes are found in 23
pairs of chromosomes of human beings.
A gene is a segment of DNA (deoxyribo nucleic acid) which can form exact copies by the
process of replication. Genes are responsible for inheritance of characteristics from one
generation to another. The characteristics of the parents which are transmitted to their
offspring are called hereditary characteristics. For example, colour of eyes, colour of skin,
shape of face, colour of hair, etc. Genes also help in evolution of organisms by mutation
and genetic recombination.
Terminology related to heredity
1. Allele: An allele is one form of a gene or pair of matching gene. There may be two or
more alleles of a gene. Alleles are denoted by letters, e.g. BB, Bb, Tt, tt, etc.
2. Homozygous or pure organism: The organism having two identical alleles for a
particular characteristics is called homozygous or pure organism, e.g. TT (tall) BB
(black), rr (white), etc.
3. Heterozygous organism: The organism having two different alleles for a particular
characteristics is called heterozygous organism, e.g. Tt (hybrid tall, Bb (hybrid black),
Rr (hybrid red), etc.
4. Phenotype: The external appearance of an organism is called phenotype.
5. Genotype: The genetic make up of an organism is called genotype.
6. Dominant characteristic: The characteristic which is prominent and appears in
successive generation is called dominant characteristics.
7. Recessive characteristic: The characteristics which remains hidden in successive
generation is called recessive characteristic. For example, in cross between a pure tall
pea plant and a dwarf pea plant, tall characteristic and dwarf is the recessive one.
8. First filial generation: The generation produced by crossing two pure parents is
called first filial or F1 generation.
9. Second filial generation: The generation obtained by crossing two parents of F1-
generation is called second filial generation.
GREEN Science (Biology) Book-10 347
10. Monohybrid cross: The cross made between two pure organisms or considering
only one pair of contrasting characteristics is called monohybrid cross, e.g. cross
between a pure tall pea plant with a dwarf pea plant.
11. Dihybrid cross: The cross made by considering two pairs of contrasting characteristics
is called dihybrid cross. e.g. the cross between a pure tall pea plant with pure red
flowers and a dwarf pea plant with pure white flowers.
Symbols given to the genes
Genes are symbolized by letters of English alphabet. The alphabet used is the first letter of
the character, .e.g. T for tall, R for red, etc. The dominant gene is symbolized with capital
letter and recessive gene is symbolized with small letter. For example, in genotype Tt, 'T'
represents tall character and 't' represents dwarf character.
Mendel's experiment
The branch of biology in which we study about genes and
heredity is called genetics. The first scientific study of genes
was done by an Austrian scientist Gregor Mendel (1822 -
1884 AD). He established the concept of genes as the basic
unit of heredity. Therefore, Gregor Mendel is considered as
the father of genetics.
Fig.
Fig.
Gregor Johann Mendel conducted experiment on
hybridization of garden pea plants. On the basis of his
research, he formulated the laws of inheritance. These 19.1
laws are popularly known as Mendel's laws of inheritance. Gregor Mendel
Gregor Mendel selected common edible garden pea plants
(Pisum sativum). He cross-pollinated pea plants having different traits in his Monastery
garden at Brunn, Austria. He found seven pairs of contrasting characteristics (traits) in
pea plants which are as follows:
1. Height of plant : Tall (Dominant) and dwarf (recessive)
19.2 Dwarf pea plant
Tall pea plant
348 GREEN Science (Biology) Book-10
2. Colour of flower: Red (purple) (Dominant) and white (Recessive)
Fig. Fig. Fig.19.3 White flowers of pea
Purple flowers of pea
3. Colour of seed: Yellow (Dominant) and green (Recessive)
19.4 Green seeds
Yellow seeds
4. Colour of pod: Green (Dominant) and yellow (Recessive)
19.5 Yellow pods of pea
Green pods of pea
5. Shape of seed: Round (Dominant) and wrinkled (Recessive)
Fig.
19.6 Wrinkled seeds of pea
Round seeds of pea
GREEN Science (Biology) Book-10 349
6. Shape of pod: Smooth (Dominant) and constricted (Recessive)
Fig. Fig.19.7 Constricted pods of pea
Smooth pods of pea
7. Position of flower: Axial (Dominant) and terminal (Recessive)
19.8 Terminal flowers of pea
Axial flowers of pea
Mendel selected pea plants for his experiment because of the following reasons:
1. Pea plants contain a large number of contrasting characteristics in pairs.
2. They are easy to cultivate.
3. They have a short life cycle. So it is possible to study several generations of pea plants
within a few years.
4. In pea plants, cross-pollination can be done easily.
5. The flowers of pea plants are closed and bisexual. So they can reproduce naturally.
6. The flowers of pea plants are easy to handle.
7. In pea plants, breeding can be controlled easily due to structure of flowers.
8. Pea plants produce a large number of offspring after pollination.
9. The offspring of pea plants produced after cross-pollination are also fertile.
While conducting experiment on pea plants, Mendel concentrated his attention on only
one pair of contrasting characteristics at a time and observed the offspring produced
by cross-pollination and self-pollination for several generations. He controlled the
experimental conditions ensuring that all pea plants were pollinated only by the pollen
grains that he transferred.
Mendel cross-pollinated pure (homozygous) red-flowered pea with a pure white-flowered
pea plants. He transferred pollen grains from the stamen of red-flower to the stigma of
white-flower preventing self-pollination.
350 GREEN Science (Biology) Book-10
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