Science and Environment 10

Arteries are deep seated in the muscle and can bear great pressure. Valves are absent
in the arteries except at their origin in the heart. Inside the arteries, blood flows at high
speed and under high pressure. The aorta and pulmonary arteries are two main arteries
of the human body.

ii) Veins
Veins are thin-walled blood vessels that carry blood towards the heart. All veins carry

deoxygenated blood except the pulmonary veins, which carry oxygenated blood from
the lungs to the left auricle. The wall of a vein also contains three layers as in the arteries,
however, they are thinner as compared to the arterial wall. Veins are comparatively superficial
in position and cannot bear a great pressure. The branches of veins are called venules.
Veins are provided with semi-lunar valves at regular intervals in them. These valves
open towards the heart to stop the reverse flow of blood as the blood moves against
gravity and under low pressure. The lumen of the veins is much wider than that of the
arteries through which blood flows easily at low speed and under low pressure.

Tunica interna

Tunica media
Tunica externa
Valve

Fig.16.7 Detailed structure of a vein

iii) Capillaries
Capillaries are microscopic, extremely narrow and thin-walled blood vessels which can

penetrate every part of the body. They connect arterioles to the venules. The wall of a
capillary is made up of a single layer of endothelium in which tunica media and tunica
externa are absent.

Arteriole Venule

Artery Vein

Tunica interna Capillary network
Fig.16.8 Structure of a capillary Fig.16.9 Showing capillaries and their network

BIOLOGY Oasis School Science - 10 293

The exchange of various materials like oxygen, digested food, carbon dioxide and waste
materials between the blood and body cells takes place through capillaries.

Differences between Arteries and Veins

S.N. Arteries S.N. Veins

1. Arteries carry blood away from 1. Veins bring blood towards the heart.
the heart.

2. They are located deep in the 2. They are located close to the skin.
body away from the skin.

3. They have narrow lumen and 3. They have wide lumen and have

no internal valves. internal valves.

4. Their wall is thick and strong. 4. Their wall is thin and weak.

5. All arteries carry oxygenated 5. All veins carry deoxygenated blood

blood except pulmonary artery. except pulmonary veins.

Reasonable Fact - 6

Veins have valves at intervals in their inner lining whereas arteries do not have valves,
why?
Ans: Inside the veins blood flows at low speed and under low pressure. So valves are
present in the veins at intervals to prevent the backflow of blood. But inside the arteries,
blood flows at high speed and under high pressure. So valves are absent in the arteries, as
there is no chance of backflow of blood.

Reasonable Fact - 7

The wall of the artery is made up of thick muscles, why?
Ans: Inside arteries, blood flows at high speed and under high pressure. To bear the great
pressure, the wall of the arteries is made up of thick muscles.

Reasonable Fact - 8

Explain why the wall of the artery is thicker than that of the veins.
Ans: In the arteries, blood flows at a high speed and under high pressure. This may cause
bursting of the arteries. Therefore, walls of artery is thicker than that of the veins.

16.9 Blood Circulation in Human Body

In human beings, blood flows twice through the heart before the oxygenated blood is pumped
to the body. Such circulation of the blood is called double circulation.

In this process, the blood, before going to the general circulation round the body, enters the
heart twice, first as deoxygenated blood and then as oxygenated blood. The circulation in
which blood travels twice through the heart in one complete cycle in the body is called double
circulation.

294 Oasis School Science - 10 BIOLOGY

Deoxygenated blood is Lung
carried to the lungs
Oxygenated blood is
returned to the heart

Deoxygenated blood Heart Oxygenated blood is
returns to the heart carried to all cells in the
body
CO2 diffuses from the
body cells into the blood Oxygen (O2) diffuses from
the blood to the body cells

Fig. 16.10 Process of blood circulation in human body

The circulation of blood in the human body is of two types:
i. Systemic circulation
ii. Pulmonary circulation

Line figure showing process of blood circulation

Lungs
(Oxygenation of blood)

Pulmonary artery Pulmonary veins
(Oxygenated
Right Left blood)

Vena cavae atrium atrium

(Deoxygenated
blood)

Veins Heart Aorta, arteries and
(Deoxygenated arterioles

blood) (Oxygenated blood)

Organs and
tissues of body

Through capillary
network

BIOLOGY Oasis School Science - 10 295

(i) Systemic Circulation Fact File - 5

The circulation of blood from the left ventricle to the right
auricle of the heart via body tissues (except lungs) is called In systemic circulation,

systemic circulation. In this circulation, the left ventricle blood does not pass
contracts and pumps the oxygenated blood into aorta. through lungs.

The aorta gives off branches (arteries) and sub-branches

(arterioles) to all the organs of the body except the lungs.

From these arterioles, oxygen is diffused into the tissues. Finally, deoxygenated blood

from the tissues is brought to the right auricle by the superior and inferior vena cavae.

The schematic figure of systemic circulation is given below:

Left ventricle Aorta Arteries

Right auricle Arterioles

Vena cavae Capillaries

Veins Venules Capillaries Tissues and
cells

(ii) Pulmonary Circulation

The circulation of blood from the right ventricle of the heart to the left auricle via lungs
is called pulmonary circulation. In this process, the right ventricle pumps deoxygenated
blood to the lungs through the pulmonary artery for purification. The oxygenated blood
from the lungs is brought to the left atrium of the heart by four pulmonary veins. The
schematic figure of pulmonary circulation is given below:

Right ventricle Pulmonary
artery

Lungs

Left auricle Pulmonary
veins

16.10 Blood Pressure

Blood pressure is the force with which blood pushes against the
walls of the arteries. It is generally measured in terms of how high
it can push the mercury column. The normal blood pressure of a
healthy young person is 120/80 mmHg. A sphygmomanometer is Fig. 16.11 Sphygmomanometer
used to measure the blood pressure. The blood pressure is of two
types: systolic blood pressure, or systole, and diastolic blood pressure, or diastole.

systole /ˈsɪstəli/ - the part of the heart's rhythm when the heart pumps blood
diastole /daɪˈæstəli/ - the stage of the heart's rhythm when its muscles relax and the heart fills with blood

296 Oasis School Science - 10 BIOLOGY

(i) Systolic blood pressure

When the ventricles contract, the pressure of the blood inside the arteries is the highest.
This pressure of the blood is termed as systolic blood pressure, or systole. It is also called
the upper limit of the arterial pressure. The systolic blood pressure of a healthy young
person is about 120 millimeters of mercury (120 mm Hg). It may range from 90 mm of
Hg to 130 mm of Hg in adults.

(ii) Diastolic blood pressure

When the ventricles relax, the pressure of the blood inside the arteries is comparatively
less, which is called diastolic blood pressure. It is also called the lower limit of arterial
pressure. The diastolic pressure of a healthy young person is about 80 millimeters of
mercury (80 mm Hg). It may range from 60 mm of Hg to 90 mm of Hg.

Systolic pressure Diastolic pressure

Artery

Stethoscope

Sphygmomanometer

An inflatable cuff is placed around After inflation, there is no sound, but, as Pressure in the cuff continues
the upper arm and inflated until air escapes from the cuff, the pressure to drop until the blood can be
all blood flow, in or out of the arm heard flowing constantly. This
decreases until it falls just below the
stops. The blood flow is monitored pressure of the heart as the contracts. is the diastolic pressure.
using a stethoscope.
This is the systolic pressure.

Fig. 16.12 Method for measuring blood pressure

The main causes of high blood pressure are as follows:

i. Smoking ii. Obesity
iii. Lack of physical exercise iv. Drinking alcohol regularly

v. Age more than 40 years vi. Genetic causes

vii. Hypertension viii. Consumption of more salt in foods

ix. Over weight

Preventive measures of high blood pressure
1. We should avoid the consumption of cold drinks.
2. Smoking and drinking should be avoided.
3. We should conduct physical exercise regularly.
4. We should consume a balanced diet.
5. We should avoid the consumption of fatty, oily and spicy foods.
6. We should avoid the consumption of fatty red meat.

BIOLOGY Oasis School Science - 10 297

16.11 Diabetes

Diabetes is a disease which is caused due to increase in the sugar level in blood. It occurs due
to lack of insulin hormone. Diabetes is commonly known as sugar disease. Insulin hormone
is secreted by the pancreases. Insulin regulates the amount of sugar in blood by converting
extra glucose into glycogen. If there is lack of insulin hormone, the amount of glucose(sugar)
increases in the blood. This condition is called diabetes.

Symptoms of diabetes
i. Frequent urination

ii. Passing out of sugar in urine

iii. Excessive thirst and hunger

iv. Delay in wound healing

v. Tiredness

vi. Blurred vision

vii. Muscle cramping

Preventive measure of diabetes Fig. 16.13 Person suffering from diabetes
i. Obesity should be reduced.

ii. Balanced diet should be taken.

iii. Hypertension should be reduced.

iv. Physical exercise should be taken regularly.

v. We should walk in the morning.

vi. We should avoid smoking and drinking alcohol.
vii. We should consume sufficient amounts of fruits and vegetables.

16.12 Heart Beat

The continuous rhythmic beat throughout life which is produced due to contraction and
relaxation of the heart muscles is called heart beat. One complete contraction and relaxation
of the heart muscles makes up one heart beat. The heart of a healthy young person beats 60 –
72 times per minute. The heart beats vary according to the age, sex and condition of the body.
The rate of heart beat increases with tension, physical exercise, emotion, fear and fever.

16.13 Arterial Pulse

Arterial pulse is the systemic contraction of the heart that can be felt as a jerk in certain arteries
which are superficial in position. Each ventricular systole starts a new pulse. Arterial pulse
proceeds as a wave of expansion throughout the arteries and disappears in the capillaries.
Pulse rate is the same as the heart beat rate, i.e., 72 times per minute.

Pulse can be felt easily in the radial artery at the wrist, temporal artery in front of the ear,
common carotid artery in the neck and facial artery in the corners of the mouth.

298 Oasis School Science - 10 BIOLOGY

Blood groups

According to Karl Landsteiner (1900 AD), a German biochemist, there are two types of
proteins: Agglutinogen or antigen, and Agglutinin, or antibody; two types of antigens:
A and B; and two types of antibodies: a and b, in the human blood. Antigen A and antibody
a are incompatible and antagonistic, and cause self clumping. Similarly, antigen B and
antibody b are incompatible and cause self clumping.

On the basis of the aforementioned facts, a system of blood groups called the ABO system
having four blood groups is recognized in the human blood. These blood groups are – A,
B, AB and O. The given table shows human blood groups and their compatibility.

Blood groups Antigens of Antibodies in Can donate Can receive
RBCs Plasma blood to blood from
A A b A, AB A, O
B B a B, AB B, O
AB AB None AB A, B, AB, O
O None a, b A, B, AB, O O

From the above table, it is clear that blood group O can be given to a person with blood
group O, A, B or AB. So, persons having blood group O are called universal donors.
Similarly, persons with blood group AB can receive blood from A, B, AB or O blood group
as their blood lacks antibodies in their plasma. So, persons with blood group AB are called
universal recipients.

The antigens present in the donor's blood can react with antibodies present in the
recipient's blood and cause clumping or agglutination of RBCs. Therefore, knowledge
of blood groups is essential for safe transfusion so that the antigen of the donor's blood
matches with the antibodies of the recipient's blood.

Rhesus antigen (Rh – Factor)

Depending upon the type of race, the surface of human RBCs contains a protein i.e., Rh
antigen (Landsteiner and Wiener, 1940 AD). This antigen is also found in the RBCs of
Rhesus monkeys. Therefore, it is called Rh - Factor.

In Black Africans, 100 percent population has Rh - antigen and they are called Rh+. In
Black Americans, about 95% are Rh+ and 5% are Rh–. Depending upon the type of race,
85 – 99% of the white population is Rh+ and remaining population is Rh–. Both Rh+ and
Rh– persons are quite normal. But the problem arises when Rh+ blood comes in contact
with Rh– blood, either due to blood transfusion or during pregnancy.

Rh– blood can be given safely to a person with Rh+ blood. When Rh– blood is transfused
into Rh+ person, the recipient forms antibodies in his/her body. However, no complica-
tions develop after the first transfusion. In case of a second transfusion of Rh+ blood to an
Rh– person, the latter's anti Rh factors attack and destroy the donor's RBC.

A serious problem arises if an Rh– mother is carrying a Rh+ foetus. The Rh+ blood of the
foetus stimulates the formation of anti- Rh factors in the mother's blood. During the first
pregnancy, enough antibodies are not produced to harm the foetus. But during the second
pregnancy, if the foetus is Rh+, a large number of RBCs of the foetus are destroyed by anti
Rh factors of the mother, which may damage the child's brain.

BIOLOGY Oasis School Science - 10 299

16.14 Uric acid

Uric acid is a compound of carbon, nitrogen, oxygen and hydrogen with the formula
C5H4N4O3. It forms ions and salts known as urates and acid urates, suchs as ammonium acid
urate. Uric acid is a product of the metabolic breakdown of purine nucleotides (proteins) and
it is a normal component of urine. High blood concentrations of uric acid can lead to gout,
diabetes, kidney stones, etc.

Fact File - 6

In human blood plasma, the reference range of uric acid is typically 3.4-7.2 mg/dL (200-430
umol/L) for men, and 2.6-6.1 mg/dL(140-360 umol/L). Uric acid concentrations in blood
plasma above and below the normal range are called hyperuricemia and hypouricemia.

(a) Gout or arthritis (b) Obese person

Fig. 16.14

Hyperuricemia can lead to gout (a type of arthritis), cardiovascular disease, obesity,
hypertension, etc. The skin becomes red. It causes swelling of the skin and joints of the legs
with burning sensation. The person cannot walk properly.

The amount of uric acid increases in the blood when the kidney cannot work effectively. In
this condition, the use and exrection of uric acid does not occur properly. Uric acid removes
toxins and protects the inner part of blood vessels. Uric acid is produced by the kidneys. More
metabolism of purine produces more uric acid in our body.

The amount of uric acid can be reduced in our body by the following methods:

1. We should drink water several times a day.

2. We should consume baking soda.

3. We should eat 10-40 cherries a day.

4. We should avoid the consumption of fatty red meat, sea food, pulses, etc.

300 Oasis School Science - 10 BIOLOGY

SUMMARY

• The group of organs through which blood circulates throughout the body is called
the circulatory system. This system consists of the heart, blood and blood vessels.

• Blood is a red - colored and loose type of connective tissue made up of plasma
(55%) and blood cells (45%).

• Erythrocytes (RBCs) are red-colored, biconcave and enucleated disc-like structures
present in the plasma. RBCs contain haemoglobin which transports oxygen and
carbon dioxide in the body.

• When there is a long term decrease in the number of RBCs or in the amount of
haemoglobin or in both, the condition is called anaemia. It is mainly caused due
to malnutrition.

• Leucocytes, or WBCs, are larger, amoeboid cells having a nucleus. They fight
against microbes and protect our body from various diseases.

• Leukaemia or blood cancer is a malignant disease of one or other variety of WBCs,
the number of which are greatly increased at the cost of RBCs.

• Platelets, or thrombocytes, are colorless, oval or round non-nucleated cytoplasmic
fragments formed from the giant cells of the bone marrow. They play an important
role in the coagulation of blood.

• Haemophilia is a genetic disease in which blood loses its ability to clot, leading to
slow and persistant bleeding.

• Arteries are thick-walled blood vessels that carry blood away from the heart.

• Veins are the thin-walled blood vessels that carry blood towards the heart.

• Capillaries are microscopic, extremely narrow and thin-walled blood vessels that
connect the arterioles and venules.

• Blood pressure is the force with which the blood pushes against the wall of the artery.

• Heart beat is the continuous rhythmic beat throughout life produced due to the
contraction and relaxation of the heart muscles.

• The circulation of blood from the left ventricle to the right auricle of the heart via
body tissues (except lungs) is called systemic circulation.

• The circulation of blood from the right ventricle to the left auricle via lungs is
called pulmonary circulation.

• Diabetes is a disease which is caused due to increase in the sugar level in the
blood. It occurs due to lack of insulin hormone. Diabetes is commonly known as
sugar disease.

• The continuous rhythmic beat throughout life which is produced due to the
contraction and relaxation of the heart muscles is called heart beat.

• Arterial pulse is the systemic contraction of the heart that can be felt as a jerk in
certain arteries which are superficial in position. Each ventricular systole starts a
new pulse.

• Uric acid is a compound of carbon, nitrogen, oxygen and hydrogen with the
formula C5H4N4O3.

BIOLOGY Oasis School Science - 10 301

Exercise

Group-A
1. What is blood circulatory system?
2. What is blood?
3. What are the components of blood?
4. What is plasma?
5. What is haemoglobin? Write down its function?
6. What is blood cancer?
7. Write down the functions of blood related to transportation.

8. Name the blood vessels that carry pure blood to the left auricle and impure blood to the
right auricle of the heart.

9. Where are aortic valve and pulmonic valve located?
10. State the functions of aortic valve and pulmonic valve.
11. Name the blood vessels that take part in blood circulation.
12. What is blood circulation?
13. What is systemic circulation?
14. What is blood pressure?
15. Define diastolic blood pressure.
16. What is the blood pressure of a healthy young adult?
17. What are the factors that affect blood pressure?
18. Name the device which is used for measuring blood pressure.
19. What is diabetes?
20. What is uric acid?
21. Which instrument is used to take the reading of the blood pressure?
22. Define heart beat.
23. Under what condition does heart beat increase?
24. Which blood cells’ number is less than average in a person suffering from anaemia?
25. Name the blood vessel which carries blood from heart to lungs.
26. Which blood vessel carries impure blood from heart to lungs? Write.
27. Give the name of blood vessel which supplies pure blood from lungs to heart.

Group-B

1. Write any two differences between erythrocytes and leucocytes.
2. Write any two differences between white blood cells and thrombocytes.
3. Write any two differences between auricle and ventricle.
4. Write any two differences between bicuspid valve and tricuspid valve.
5. Write any two differences between pulmonary artery and pulmonary vein.

302 Oasis School Science - 10 BIOLOGY

6. Write any two differences between artery and vein.
7. The wall of the ventricles is thicker than that of the auricles, why?
8. The wall of the left ventricle is thicker than that of the right ventricle, why?
9. Right auricle is larger than the left auricle, why?
10. Valves are present in veins but not in arteries, why?
11. The wall of arteries is thicker than that of the veins, why?
12. Arteries are deeply seated inside muscles, why?
13. A person suffering from anaemia feels tired in short walk, why?
14. Write two differences between systemic circulation and pulmonary circulation.
15. The blood pressure of a healthy man is 120/80 mm Hg. What does it mean?
16. A person with low level of haemoglobin feels tired soon during working. Clarify the reason.
17. All the artery carry pure blood but pulmonary artery carries impure blood, why?
18. The wall of ventricles is thicker than that of the auricles in the heart, why?

Group-C
1. Draw a neat diagram showing the composition of blood.
2. Write any three characteristics of blood.
3. Write any three functions of plasma.
4. Name the valves present in between auricles and ventricles of human heart. Also, write

down their structure and functions.
5. State three major functions of blood circulation in human body.

Group-D
1. What is shown in the given diagram? Name the parts A, B, C and D. Also, mention the

function of each.

A
B

C

D

2. Describe the structure of human heart with a labeled diagram.
3. Draw a neat and labeled diagram showing the internal structure of human heat.

4. What is an artery? Describe its structure in brief with a neat figure.

5. What are veins? Describe the structure of veins in brief.

6. Write short notes on:

i) Uric acid ii) Diabetes

7. Draw a figure showing systematic and pulmonary circulation.

BIOLOGY Oasis School Science - 10 303

UNIT 17 Estimated teaching periods

Theory 5

Practical 1

CHROMOSOME AND

SEX DETERMINATION Anton von Leeuwenhoek

Objectives (1632-1723 AD)

After completing the study of this unit, students will be able to:

• introduce chromosomes and describe their structure, types and functions.
• explain the method of sex-determination

17.1 Introduction

A cell is defined as the fundamental, structural and functional unit of life. The bodies of living
organisms are made up of microscopic units known as cells. Cells are made up of a life giving
substance called protoplasm, cell organelles and inclusions. A cell is capable of independent
existence and performs essential functions of life.

All organisms start their life as a single cell. New cells always arise from pre-existing cells. A
cell consists of various cell organelles and inclusions. Among them, the nucleus is the most
important cell organelle, which controls various activities of a cell. It also consists of thread-
like structures called chromosomes.

17.2 Chromosomes: The Vehicles of Heredity

Chromosomes (Gk., chrome–colour and soma–body) are thread-like microscopic structures
present in the nucleus of a cell which contain genes (i.e. sub-microscopic units of heredity).
Chromosomes are made up of the genetic material DNA and protein.

Each chromosome consists of two identical strands called chromatids. Those chromatids are
separate but firmly attached to each other at a constricted part called the centromere. The
centromere is also called kinetochore. Each cell of plants and animals contain a network of
fine threads which is called chromatin reticulum. During cell division, the fine threads of
chromatin reticulum become shorter and thicker. These shorter and thicker structures are
called chromosomes.

gamete /ˈɡæmiːt/ - a male/female sex cell produced by a male or female organ

304 Oasis School Science - 10 BIOLOGY

Chromatids

Centromere
Chromosome

Fig.17.1. Chromosome

The number of chromosomes varies from species to species, however, it is constant for all
individuals in a species. The number of chromosomes in the somatic cells of organisms is
called diploid number and it is represented by '2n' whereas the number of chromosomes in
gametes (sperm and ovum) is called haploid number and it is represented by 'n'. In human
beings, each somatic cell contains 23 pairs of chromosomes and each gamete contains 23
chromosomes. The following table shows the number of chromosomes found in the somatic
cells of some organisms.

Organisms No. of chromosomes Organisms No. of chromosomes
in somatic cells in somatic cells
1. Human beings (in pairs) 8. Dog (in pairs)
2. Frog 23 9. Horse
3. Rat 13 10. Rice 39
4. Tiger, lion, cat 20 11. Sugarcane 32
5. Garden pea 19 12. Onion 12
6. Chimpanzee (Gorilla) 7 13. Housefly 40
7. Yeast 24 14. Solanum 8
1 15. Pine 6
8
12

17.3 Types of Chromosomes

Chromosomes vary greatly in their shape, size and position of the centromere. On the basis of
the position of the centromere, chromosomes are of three types: metacentric, acrocentric and
telocentric. In metacentric chromosome, the centromere is located at the middle. In acrocentric
chromosome, the centromere is located away from the centre whereas the centromere is
located very near the end in telocentric chromosome.

diploid /ˈdɪplɔɪd/ - containing two complete sets of chromosomes, one from each parent
haploid /ˈhæplɔɪd/ - containing a set of chromosomes from one parent only

BIOLOGY Oasis School Science - 10 305

Metacentric Sub-metacentric Acrocentric Telocentric
chromosome chromosome chromosome chromosome

Fig.17.2 Various shapes of chromosomes

17.4 Functions of Chromosomes

1. Chromosomes act as hereditary vehicles. They transmit characteristics of parents to their
offspring with the help of genes.

2. Chromosomes control protein synthesis and thus induce cell division, cell growth and
cell repair.

3. Sex chromosomes determine the sex of living organisms.

4. Chromosomes form a link (bridge) between parents and their offspring.

5. Chromosomes induce variation by crossing over and undergo mutation, which leads to
evolution of life.

6. Chromosomes control cell metabolism by directing the synthesis of enzymatic proteins.

17.5 Determination of Sex

Each cell of living beings contains two types of chromosomes. They are autosomes and
sex chromosomes. Autosomes determine various characteristices of the body whereas sex
chromosomes determine the sex of an individual.

In human beings, sex is determined by genetic inheritance. Genes inherited from the parents
determine whether an offspring will be a boy or girl.

Genes for all the characteristics are lineraly on chromosomes. These include the genes for
sexual characteristics. Generally, characters related to the reproductive system are called
sexual characters and those that are not are called vegetative characters. The chromosomes
that carry genes for sexual characters are called sex chromosomes and those that carry genes
for the vegetative characters are called autosomes.

A sex chromosome that carries the genes for male characters is called Y-chromosome and one
which carries the genes for female characters is called X-chromosome. Each cell of human
has a total of 46 chromosomes. Half of them come from the mother and the rest, from the
father. Out of these 46 chromosomes, 44 are autosomes and 2 are sex chromosomes. The sex

306 Oasis School Science - 10 BIOLOGY

chromosomes are not always a perfect pair.

In females, there are 44 autosomes and two X-chromosomes. Similarly, in males, there are 44
autosomes, one X-chromosome and one Y-chromosome.

So, the chromosomes in a woman are 44+XX, while the chromosomes in man are 44+XY. Let us
see the inheritance pattern of X and Y chromosomes.

During gamete formation, the normal diploid chromosome number is havled. This is called
the haploid condition. All the eggs of a female have 22+X chromosome. A male produces two
types of sperms–one type bears the 22+X composition and the other, 22+Y. Therefore, in 100
sperms 50 have Y chromosome and 50 have X-chromosome.

Parents → Father Mother

44+XY 44+XX

Gametes →

22+X 22+Y 22+X 22+X

Sperm Sperm Ovum Ovum

Result in F1-generation

Sperms (male) 22+X 22+Y
Ovum (female)

22+X 44+XX 44+XY

Girl (daughter) Boy (son)

22+X 44+XX 44+XY

Girl (daughter) Boy (son)

Any one of the two types of sperms can fertilize Fig. X-Y system of sex determination in man
the egg. If a Y-bearing sperm fertilizes the egg,
the zygote has the 44+XY composition, and the
resulting embryo grows to be a boy or son. When
an X-bearing sperm fertilizes the egg, the resulting
zygote has the 44+XX composition. This embryo
develops into a girl or daughter. All the children
inherit one X-chromosome from the mother.
Therefore, sex is always determined by the other
sex chromosome that they inherit from the father.
One who inherits X-chromosome of the father is a
girl, while one who inherits Y-chromosome of the
father is a boy.

BIOLOGY Oasis School Science - 10 307

17.6 Chromosome Disorder

Chromosomes determine the development of various characteristics in the body. A gene
located in a chromosome determines the development of a certain characteristic. Various
disorders may occur in the body due to change in the sequence of genes in the chromosomes
or dilation, duplication or translocation of chromosomes, Haemophilia is one of the disorders
among many disorders in the body. It is a sex-linked disease in which continuous bleeding
occurs through a cut or wound of the person suffering form haemophilia. Such type of disease
transmits from one generation to another. A disease that occurs only in a particular type of sex
(either male or female) is called a sex-linked disease.

A disorder that occurs in the human body due to increase or decrease in the number of
chromosomes is called chromosomal disorder. It mainly occurs due to dilation, duplication or
translocation in the chromosomes. Actually, a chromosome disorder results from a change in
the number or structure of chromosomes during cell division. Some chromosome disorder are
harmless but some are associated with clinical disorders like Down's syndrome, Klinefelter's
syndrome, Turner's syndrome, Eduward's syndrome, etc.

Fact File - 1

Chromosomal disorder usually occur, when there is an error in cell deviation, resulting in cells with
too few or too many copies of chromosomes.

Fact File - 2

Most chromosome abnormalities originate in the egg or sperm but some happen during the
development of the embryo.

Fact File - 3

The number of chromosomes may increase or decrease when chromosomes cannot separate
during cell division. This process is called aneuploidity.

1. Downs' Syndrome

Down's syndrome is a genetic disorder caused by the presence of all or part of an extra
copy of chromosome 21, i.e. trisomy of chromosome 21. It was first described by Langdon
Down. It occurs in one in every1000 live births, and the risk of having Down's syndrome
increases significantly with maternal age to 1 in
50 if the mother is over 45 of age.

The major symptoms in Down's syndrome are :

1. Mild to moderate intelectual disability

2. Difficulty while sitting, standing, walking,
talking and reading

3. Limited growth

4. Slanting eyes and small ears

Fig. 17.3 Person having Down's syndrome

308 Oasis School Science - 10 BIOLOGY

5. An open mouth with protruding tongues and small nose

6. Short-sightedness

7. Deafness

8. Congenital heart defects

9. Digestive disorders such as constipation

10. Thick palms with creases in the skin

2. Klinefelter's Syndrome

Klinefelter's syndrome is a genetic disorder caused Fact File - 4
due to increase in the sex chromosomes in males. It is
Klinefelter's syndrome is a
also known as 47, XXY or XXY. It is a set of symptoms chromosomal condition that af-
that result from two or more X chromosomes in males. fects the physical and cognitive
Many people do not realize that they are affected. development in males.

Sometimes symptoms are more prominent and may

include weaker muscles, greater height, less body

hair, smaller genitals, breast growth and less interest in sex. The affected males are often

sterile or may have reduced fertility.

Figure: 17.4 Person suffering from Klinefelter's syndrome

3. Turner's Syndrome
Turner's syndrome is a chromosomal disorder in which a female is partly or completely

mission X-chromosomes. It is also called 45+XO disorder. Signs and symptoms vary

BIOLOGY Oasis School Science - 10 309

among those affected. The common symptoms include short stature or height, swelling
of the hands and feet of the newborn, broad chest, low-set ears, obesity, poorly developed
ovaries, lack of menstrual periods, infertility, webbed neck, visual impairment, etc.
Turner's syndrome affects only girls and women.

Fig. 17.5 Person having Turner's syndrome

SUMMARY

• A cell is defined as the fundamental, structural and functional unit of life. The
bodies of living organisms are made up of microscopic units known as cells. Cells
are made up of a life giving substance called protoplasm, cell organelles and
inclusions.

• Chromosomes (Gk., chrome–color and soma–body) are thread-like microscopic
structures present in the nucleus of a cell which contains genes (i.e., sub-
microscopic units of heredity).

• The number of chromosomes varies from species to species, however, it is constant
for all individuals in a species.

• Chromosomes vary greatly in their shape, size and position of centromere. On
the basis of the position of the centromere, chromosomes are of three types, viz.
metacentric, acrocentric and telocentric.

• Chromosomes act as hereditary vehicles. They transmit characteristics of the
parents to their offspring with the help of genes.

• Chromosomes induce variation by crossing over and undergo mutation which
leads to evolution of life.

310 Oasis School Science - 10 BIOLOGY

• Chromosomes control cell metabolism by directing the synthesis of enzymatic
proteins.

• Each cell of living beings contains two types of chromosomes. They are autosomes
and sex chromosomes.

• The chromosomes that carry genes for the sexual characters are called sex
chromosomes and those that carry genes for the vegetative characters are called
autosomes.

• The disease that occurs only in a particular type of sex (either male or female) is
called a sex-linked disease.

• A disorder that occurs in the human body due to the increase or decrease in the
number of chromosomes is called chromosomal disorder.

• Klinefelter's syndrome is a genetic disorder caused due to increase in the sex
chromosomes in males. It is also known as 47, XXY or XXY.

Exercise

Group-A

1. What is a chromosome?

2. Where is chromosome found?

3. What is chromosome made of?

4. What is gene?

5. What is gene made of?

6. What is chromatic reticulum?

7. Define centromere and chromatid.

8. What is kinetochore?

9. How many types of chromosomes are there on the basis of position of centromere?

10. Define sub-metacentric chromosome.

11. What is telocentric chromosome?

12. Write down the number of chromosomes found in somatic cell and gametic cell of given
organisms:

i) Housefly ii) Gorilla

13. Write down the number of chromosomes found in somatic cell and gametic cell of given
organism:

i) Solanum ii) Onion

14. Write down the number of chromosomes found in somatic cell and gametic cell of given
organisms:

i) Pine ii) Yeast

15. What is sex determination?

BIOLOGY Oasis School Science - 10 311

16. What is haemophilia?
18. Write any two characteristics of haemophilia.
19. Write any two characteristics of chromosome.
20. What is Down’s syndrome?
21. Write down the main cause of Down’s syndrome.
22. What is Klinefelter’s syndrome?
23. What is Turner’s syndrome?

Group-B
1. What is meant by metacentric chromosome? Write with figure.
2. Differentiate between metacentric chromosome and sub-metacentric chromosome.
3. Differentiate between acrocentric chromosome and telocentric chromosome.
4. What do you mean by somatic cell and gametic cell?
5. Differentiate between diploid and haploid chromosome.
6. What is sex-linked disease? Write with an example.
7. What is meant by chromosomal disorder?
8. What is meant by aneuploidy? Write with an example.
9. Why is Down’s syndrome called chromosomal disorder?
10. Differentiate between autosome and sex chromosome.
11. How many chromosomes are there in ovum and zygote and why?
12. How many chromosomes are there in human sperm and zygote? Why?

Group-C
1. Draw a neat and labeled figure showing the structure of a chromosome.
2. What type of chromosome is called acrocentric chromosome? Write with figure.
3. Draw a figure showing metacentric chromosome and define it.
4. Draw a figure showing sub-metacentric chromosome and define it.
6. Draw a chart showing the process of sex determination in human beings.
7. Write down the major symptoms of Down’s syndrome.
8. How does Klinefelter’s syndrome occur? Write.
9. Write down the major symptoms of Turner’s syndrome.
10. What symptom occurs due to lack of chromosome X?

Group-D
1. Describe the structure of chromosome with a labeled figure.
2. Write down the type of given chromosomes.

AB C D

3. How is sex determined in human beings? Describe with figure.

312 Oasis School Science - 10 BIOLOGY

UNIT 18 Estimated teaching periods

Theory 10

Practical 2

ASEXUAL AND

SEXUAL REPRODUCTION Flower

Objectives

After completing the study of this unit, students will be able to:

• describe various methods of asexual and sexual reproduction that takes place
in plants and animals with examples.

• explain the importance of asexual and sexual reproduction.
• explain the artificial methods of vegetative reproduction in plants.

18.1 Introduction

Living organisms cannot survive for ever. Each living being remains alive in this nature for
a limited period of time and then it dies. So, living organisms produce their own kinds for
continuity of their race on earth. The biological process in which living organisms produce
their own kind by asexual and sexual method is called reproduction. The continuity of life on
earth has been possible only due to the phenomenon of reproduction.

Reproduction is the process of creation of a new life from a similar form of life. Thus,
reproduction can also be defined as the creation or production of new organisms from the
pre-existing organisms of the same species. Living organisms reproduce asexually or sexually
to maintain the existence of their species on earth.

18.2 Types of Reproduction

There are various methods by which living organisms produce their offspring. Some
unicellular organisms like the amoeba, plasmodium, etc. reproduce by splitting their body
into two or more daughter cells. Some organisms like the hydra, yeast reproduce by means of
buds. Similarly, frogs, snakes, birds, etc. reproduce by laying eggs, and mammals directly give
birth to their young ones. Some plants reproduce from roots, stems or leaves whereas some
plants reproduce with the help of flowers. It clearly indicates that living beings reproduce by
various methods. All the different methods of reproduction can be divided into two main types:

i. Asexual reproduction
ii. Sexual reproduction

18.3 Asexual Reproduction

It is a common method of reproduction in lower plants and animals. In this method, a single
organism is capable of reproduction. The method of reproduction which takes place from

BIOLOGY Oasis School Science - 10 313

a single parent without the fusion of a male gamete and a female gamete is called asexual
reproduction. In this process, no sex cells, i.e., gametes are involved. So, this method is
called asexual reproduction. The new offspring produced by asexual method are genetically
identical to their parents. Asexual reproduction involves the production of offspring from a
single parent without the involvement of sex cells. Binary fission in all amoeba, budding in
hydra, regeneration in planaria, vegetative propagation in a rose, etc. are some examples of
asexual reproduction.

18.4 Basic Features of Asexual Reproduction

i. In asexual reproduction, only a single organism is involved.
ii. It is an easy and rapid method of reproduction.
iii. The offspring produced by asexual reproduction are genetically identical to their

parents.
iv. Gametes are not produced, and fertilization does not take place.
v. All cell divisions during asexual reproduction are either amitotic or mitotic.

18.5 Methods of Asexual Reproduction

The various methods of asexual reproduction are mentioned below:
a. Fission

i. Binary fission

ii. Multiple fission
b. Budding
c. Sporulation
d. Fragmentation or regeneration
e. Vegetative propagation

i. By root

ii. By stem

iii. By leaf

a. Fission
The method of asexual reproduction in which a parent organism divides (splits) into two

or more daughter organisms is called fission. It is common in unicellular organisms like
amoeba, paramecium, plasmodium, bacteria, euglena, etc. On the basis of the number of
daughter organisms produced, fission is of two types:

i. Binary fission and

ii. Multiple fission

fission /ˈfɪʃn/ - the division of a cell into new cells as a method of reproduction

314 Oasis School Science - 10 BIOLOGY

i. Binary fission

The fission in which a single parent divides into two daughter organisms is
called binary fission. In this method, the nucleus divides first into two daughter
nuclei which is followed by the division of cytoplasm. Finally, the cell splits into
two daughter cells. In this way, a parent organism divides into two daughter
organisms. After the binary fission, the parent organism ceases to exist, and
daughter organisms come into existence.

Binary fission occurs during favorable conditions of life, and the offspring
produced are morphologically and genetically identical to the parent. Some
unicellular organisms like bacteria, euglena, amoeba, paramecium, plasmodium,
etc. reproduce by binary fission.

Nucleus divides

Mother Amoeba Daughter Amoeba

Fig.18.1 Binary fission in Amoeba

ii. Multiple fission

Multiple fission is the method of asexual reproduction (fission) in which a parent
organism splits into more than two daughter organisms at the same time. Multiple
fission occurs during unfavorable conditions. Many unicellular algae and some
unicellular animals like the amoeba, plasmodium, etc. reproduce asexually by
multiple fission during unfavourable conditions.

Daughter nuclei

Cyst Daughter
Parent cell Plasmodia

Fig.18.2 Multiple fission in Plasmodium

In multiple fission, a cyst, i.e., a thick protective layer is formed around the cell
of unicellular organism in unfavorable condition. Inside the cyst, the nucleus
divides repeatedly to produce many daughter nuclei. Later on, each nucleus is
surrounded by cytoplasm and cell membrane. As a result, many daughter cells are
formed inside the cyst.

On the return of favorable conditions, the cyst breaks, and small offspring are
released. In this way, multiple fission takes place in unicellular organisms.

BIOLOGY Oasis School Science - 10 315

b. Budding

It is a method of asexual reproduction in which a bulb-like projection, i.e., bud arises
from the parent’s body which detaches and finally develops into a new organism. The
method of asexual reproduction which takes place by the formation of a bud is called
budding. This method is common in unicellular plants like the yeast and multicellular
animals like the hydra.

Chain of buds

Bud grows

Parent yeast cell Bud

Fig.18.3 Budding in yeast

In yeast, a bulb-like outgrowth, i.e., a bud is formed on one side of the cell. The nucleus
divides and one daughter nucleus passes into the bud. The bud grows and finally gets
detached from the parent cell, which grows in size and becomes a new yeast.

In a hydra, a small projection called bud develops on the side of its body by repeated
mitosis cell division. The bud grows and forms a daughter hydra. Finally the daughter
hydra detaches itself from the parent hydra and becomes a new hydra.

Bud separates

Mother hydra Fig. 18.4 Budding in hydra

c. Sporulation

It is a method of asexual reproduction common in the majority of thallophytes and
bryophytes. In this method, the parent plant produces a large number of tiny spores
which can produce new plants. The method of asexual reproduction which takes place
by means of spores is called sporulation. This method is common in mucor, moss, fern,
marchantia, rhizopus, chlamydomonas, etc.

316 Oasis School Science - 10 BIOLOGY

Spores
Sporangium

Fig.18.5 Sporulation in mucor

In mucor, small knob-like structures called sporangia (singular–sporangium) develop at
the top of the hyphae. Numerous spores are produced inside the sporangium. Spores are
the microscopic asexually reproductive bodies covered by a hard protective coat. Spores
germinate into new organisms under favorable conditions. At maturity, sporangia burst
open to release spores, which grow into new organisms on favorable condition.

Activity - 1
To observe sporulation in mucor
• Take a slice of bread and keep it in a moist and warm place for 4-5 days.
• After 4-5 days, you can see a white cottony mass of mucor.
• Keep the slice of bread in sunlight for 2-3 days.
• Observe the surface of mucor under a hand lens and compound microscope.
• Draw labeled figures and write a few comments on it.

d. Fragmentation or Regeneration
Fragmentation is the method of asexual reproduction in which a multicellular organism

breaks into two or more fragments and each fragment develops into a new complete
organism. The ability of an individual fragment of an organism to replace its lost parts
is called regeneration. It can also be defined as the process of getting back a complete
organism from its body parts.
In this method, a mature parent multicellular organism breaks down naturally to produce
two or more pieces. After a certain time, each piece of the organism regenerates itself to
form a complete organism by growing all missing parts. Filamentous algae like spirogyra
and animals like hydra, planaria, tapeworm, sponge, etc. reproduce asexually by this
method. Take note that the term ‘fragmentation’ is generally used for plants and the term
‘regeneration’ is used for animals. It is said spirogyra reproduces by fragmentation and
tapeworm reproduces by regeneration.

BIOLOGY Oasis School Science - 10 317

Fig.18.6 (a) Fragmentation in Spirogyra Fig.18.6 (b) Regeneration in Planaria

e. Vegetative propagation

It is a method of asexual reproduction common in some higher plants. In this method,
a bud grows on the vegetative part of a plant like the stem, root or leaf and develops
into a new plant. On maturity, the new plant detaches itself from the parent plant and
lives as an independent plant. Thus, vegetative propagation is the method of asexual
reproduction in which new plants are produced by cutting, sowing or grafting of the
vegetative parts of plants like the root, stem or leaf.

18.6 Natural Methods of Vegetative Propagation

a. Vegetative propagation by roots

Some flowering plants like the sweet potato, dahlia, mint, etc. reproduce asexually by
means of roots. Roots of those plants contain adventitious buds. When such roots are
planted in the soil, new plants are produced.

Roots Dahlia Root Sweet potato
(a)
(b)

Fig.18.7 Vegetative propagation by roots

b. Vegetative propagation by stem

At maturity, the stem of the plants develops buds on it. The part of the stem having buds
serves as an organ for vegetative propagation. When the stem falls on the ground, it
propagates new plants on favorable condition. For example, a bulb of an onion, stem of
a rose, tuber of potato, etc. reproduce asexually by means of a stem.

318 Oasis School Science - 10 BIOLOGY

The types of stem that reproduce asexually are mentioned below:

i. Underground stem

Some plants like onion, tulip, daffodil, arum, colocasia, gladioulus, ginger, potato, etc.
reproduce asexually by means of an underground stem. Stems of these plants contain
buds on them. Each bud of the stem can grow into a new plant on favorable condition.

Bulb of onion Corm of gladiolus Rhizome of ginger

Underground stem of tulip Tuber of potato

Fig.18.8 Vegetative propagation by underground stem

Reasonable Fact-1

Though a potato is a seed-bearing plant, it reproduces through vegetative propagation, why?
Ans: Though a potato is a seed-beaing plant, it reproduces asexually through vegetative
propagation because a potato does not produce viable seeds.

ii. Sub-aerial stem
Some plants like water hyacinth, water lettuce, etc. reproduce by offset. Strawberry,

raspberry, buttercup, blackberry, gooseberry, bamboo, pineapple, mint, chrysanthemum,
banana, etc. reproduce asexually by means of a sub-aerial stem.

Main shoot
Suckers

New shoot
Suckers

Roots of new plant

Offset Cluster of roots Adventitious roots

(a) Offset of Pistia (b) Sucker of Chrysanthemum
Fig. 18.19

graft /ɡrɑːft/ - a piece cut from a living plant and fixed in a cut made in another plant, so that it grows there

BIOLOGY Oasis School Science - 10 319

iii. Aerial stem
Some flowering plants like a rose, sugarcane, etc. propagate vegetatively by aerial stem.

(a) Sugarcane (b) Rose

Fig. 18.10 Aerial stem of sugarcane and rose

Activity -2

To observe vegetative propagation by a stem in sugarcane
• Bring a mature sugarcane and cut it into small pieces.
• Observe the buds in the nodes of the sugarcane.
• Plant these pieces in the soil and supply water regularly.
• Observe after 2-3 weeks.
• Prepare a small report with labeled figures.

c. Vegetative propagation by leaf

Leaves of some plants like Bryophyllum, Begonia, etc. produce adventitious buds. Those
buds develop into small plantlets when the leaf falls on moist soil. These plantlets get
separated from the leaf and grow into new plants.

(a) Begonia (b) Bryophyllum

Fig. 18.11 Vegetative propagation by leaf

adventitious /ˌædvənˈtɪʃəs/ - occurring elsewhere on the plant except from the leaf axils, stem tips and plant apices

320 Oasis School Science - 10 BIOLOGY

Reasonable Fact-2

A new rose plant is grown by planting a piece of its stem in the soil. What type of axesual
reproduction is it? Give three reasons why such type of reproduction is more beneficial.
Ans: A new rose plant is grown by planting a piece of its stem in the soil. This type of
reproduction is called vegetative propagation by stem.
This type of reproduction is more beneficial than other types of reproduction due to the
following reasons:
(i) Gardeners, farmers and botanists can grow a large number of plants by this method.

(ii) The plants which do not produce viable seeds like a rose, sugarcane, potato, etc. can
easily be produced by this method.

(iii) The plants produced by this method bear flowers and fruits earlier than those
produced from seeds.

Reasonable Fact-3

"Vegetative propagation is beneficial to the farmers." Give any two reasons to justify this
statement.
Ans: Vegetative propagation is beneficial to farmers because of the following reasons:
(i) A large number of plants can be propagated by this method.
(ii) The plants which do not produce viable seeds like a rose, sugarcane, potato, etc. can

easily be produced by this method.

18.7 Artificial Methods of Vegetative Propagation

Besides natural methods of vegetative propagation, artificial methods of plant reproduction
are also common among farmers, gardeners, horticulturists, etc. Various methods like cutting,
grafting, layering, tissue culture, etc. are used to reproduce plants asexually in gardens and
nurseries.

i. Cutting

It is a common artificial method of vegetative propagation in which new plants are
obtained by planting the piece of stem, root or leaf of plants on favorable condition. Each
cutting piece should have buds for propagation.

Some plants like sugarcane, rose, potato, etc. are propagated by stem cutting. Lemon,
tamarind, etc. are propagated by root cutting and Begonia, Bryophyllum, African violets,
etc. are propagated asexually by leaf cutting.

ii. Grafting

In this method, a plant of superior quality is obtained by combining the root system of a
plant and shoot system of another plant. The plant whose root system is taken is called
stock and whose shoot system is taken is called scion. This technique is used in mango,
rose, lemon, apple, peach, plum, etc.

scion /ˈsaɪən/ - a piece of plant, especially one cut to make a new plant

BIOLOGY Oasis School Science - 10 321

In grafting, the ends of a scion and stock to be grafted are cut obliquely (as given in the
fig. 18.12(a) and placed face to face in such a way that the cambia of both plants are in
close contact. Then the scion and stock are bound firmly with a tape (cloth) and covered
with wax. After a few weeks, both the scion and stock combine firmly, resulting in a new
plant.

Scion

Stock

(a) Fig. 18.12 Grafting in mango (b)

Types of grafting

Grafting can be done by various methods. Some of the common methods of grafting are as
follows:

a) Whip grafting b) Tongue grafting c) Cleft grafting

a) Whip grafting

It is an easy and simple method of grafting. Fig. 18.13
In whip grafting, the scion and the stock are
cut slanting (about 3.5 cm) and then joined.
The grafted point is then bound with tape or
plastic and covered with soft wax to prevent
dehydration and germs. It is the most common
method of grafting in commercial fruit trees.
The scion and stock combine within 2-3 months.
Then the plastic or tape can be removed. This
method is applied before the fruit trees sprout.

b) Tongue grafting Fig. 18.14

Tongue grafting is also an easy and simple method
of vegetative propagation. In this method, the lower
portion of the scion and upper portion of stock
is cut obliquly about 3-5 cm. Then, a tongue like
deep structure is cut in both the scion and stock so
that both fit into one another. Then, the connected
portion is sealed with tape or plastic to make air
tight. Both the stock and scion combine within 2-3
months. This method of grafting is very common in
the fruit trees of the mountain and hilly regions.

322 Oasis School Science - 10 BIOLOGY

c) Cleft grafting Fig. 18.15

In cleft grafting, a small cut (about 5-8 cm) is made in the
stalk, and then the pointed end of the scion is inserted in the
stalk. The most common form of grafting is cleft grafting.
This type of grafting is done in the spring season and is
useful for joining a thin scion of about 1cm diameter to a
thicker branch or stock. The branch or stock should be split
carefully down the middle to form a cleft about 3 cm deep.
The end of the scion should be cut cleanly to a long shallow
wedge with a single cut. Then the scion is inserted into the
cleft and sealed with a tape or plastic to make it air tight.
Both the scion and stock combine within 2-3 months.

iii. Layering Fig.18.16 Layering in jasmine

It is an artificial method of reproduction in which
roots are developed on the stem while the stem is still
attached to the parent plant. Layering is commonly
used in jasmine, magnolia, strawberry, raspberry, etc.
Many plantlets can be produced in a short period of
time by this method.

Layering is induced artificially by bending the branch
to the ground and covering it with moist soil. The
apical part of the stem produces leaves whereas the
underground part produces roots. Later the layered
branch is separated from the parent plant.

Types of Layering

Layering can be done by various methods. Some common methods of layering are given
below:

(a) Simple layering (b) Compound layering

(c) Tip layering (d) Mound or stool layering

(e) Air layering

(a) Simple layering

In this method, a low growing stem of more

than one year age is bent downward and the

target region is buried in the soil. However, a

few inches of the leafy stem must remain above

the ground for the bent stem to grow into a

new plant. The buried portion of the stem

develops roots after 2-3 months when watered Fig. 1817

regularly. When the stem develops roots, it is

separated from the parent plant and kept in nurseries. Plants like lemon, citrus, etc. are

propagated by this method.

BIOLOGY Oasis School Science - 10 323

(b) Compound layering

In this method, the entire low growing stem is bent downward and buried in the soil,
keeping its tip out of soil. If the stem is waterd regularly each node produces new
plantlets. This method is commonly applied in apples, pears, walnuts, etc. to produce
many plantlets in a short period of time.

Fig. 18.18 (a) Fig. 18.18 (b)

(c) Tip layering Fig. 18.19

In this method, the tip of the stem of a plant
is buried in the soil to develop new plants.
About a 5 to 8 cm long tip of the stem is buried
in the soil and watered regularly. When the
tip develops roots, the portion of the stem is
separated from the parent plants after 3-4
months. This method is commonly applied in
raspberry, blackberry, etc.

(d) Mound or stool layering

Ground layering, or mound layering, is the typical Fig. 18.20
propagation technique in which the original plants
are set in the ground with the stem nearly horizontal,
which forces side buds to grow upward. After these
are started, the original stem is buried upto 20 to
25 cm from the ground. At the end of the growing
season, the side branches will develop roots and can
be separated after one year.

(e) Air layering

In this method, the target region of the stem is wounded, or Fig. 18.21
a strip of bark is removed and then encased in a moisture
retaining medium, such as moss or cloth, which is further
surrounded in a moisture barrier such as a plastic film. Rooting
hormone, i.e., I.B.A. hormone (Indole-3 butyric acid) is often
applied to encourage the growth of roots. The wounded
portion develops roots within 4-8 weeks. Then, the portion is
separated from the parent plant and planted. Air layering is
the most popular method of artificial vegetative propagation.

324 Oasis School Science - 10 BIOLOGY

iv. Tissue culture

Tissue culture, or micropropagation is a modern technique of producing new plants
from isolated plant cells or a small piece of plant tissue in a culture solution.

Fig. 18.22
The culture solution, or medium, is very important in tissue culture. The solution

contains a special mixture of salts, vitamins and plant hormones. Ornamental plants like
orchids, dahlia, carnation, etc. are propagated artificially by this technique.
In tissue culture, a few cells (or tissues) of the plant to be propagated are taken and kept
into a culture solution under sterile conditions. In this solution, a shapeless lump, i.e.
callus is produced by rapid division of the plant cells. Then the callus is transferred into
another culture solution containing suitable plant hormones, which stimulates the callus
to develop roots. The callus with roots is again transferred into another culture solution
having different hormones that stimulates the development of shoots. The callus having
roots and shoots is separated into many tiny plantlets. The plantlets thus produced are
transplanted into soil or pots to get natural plants.
The main advantages of tissue culture are as follows:
a) A large number of plants having identical features to the parents can be produced

by this method.
b) Adult plants can be produced within a short period of time.
c) Many plantlets can be produced without seeds.
d) Healthy and disease free plants can be propagated by this technique.
Tissue culture is widely used in forestry, horticulture, floriculture and botanical gardens.
The main applications of tissue culture are mentioned below:
a) A large number of plants can be propagated by using meristem of plants.
b) Endangered plants can be conserved by this technique.
c) A large number of sterilized hybrid plants can be propagated by this technique.
stamen /ˈsteɪmən/ - the male organ of a flower that produces pollen grains

BIOLOGY Oasis School Science - 10 325

18.8 Significance of Asexual Reproduction

i. Asexual reproduction produces new individuals with exactly identical qualities as
the parent.

ii. It is a faster, easier and cheaper method of reproduction.

iii. The plants produced by vegetative propagation bear flowers and fruits earlier
than those produced from seeds.

iv. The plants which do not produce viable seeds (like sugarcane, rose, potato, banana,
bamboo, etc.) can easily be propagated by asexual reproduction.

18.9 Advantages of Artificial Vegetative Propagation

i. Plants like sugarcane, rose, potato, grape, banana, etc. that do not produce viable
seeds are propagated by this method.

ii. It is a cheaper, easier and faster method of propagation. Farmers, gardeners and
botanists can grow a large number of plants in a short period by this method.

iii. Seedless fruits like seedless grapes, seedless oranges, etc. can be produced by this
method.

iv. The new plants produced by this method are genetically identical to their parents.

v. The fruit trees or flowers produced by this method start to bear flowers and fruits
much earlier than those obtained from their seeds.

18.10 Disadvantages of Vegetative Propagation

i. Plants produced by vegetative propagation are genetically identical to their
parents. This restricts their adaptability to the changing environment.

ii. It checks the process of evolution as there is no variation.
iii. Plants produced by this method are often overcrowded. Therefore, they lose vigor

and gradually become prone to diseases.

18.11 Sexual Reproduction

It is a complex process of reproduction in which both sexes, the male and female, are
involved. Most animals and higher plants reproduce by sexual method. The process in which
reproduction takes place by the fusion of a male gamete and a female gamete is called sexual
reproduction. In this method, a male gamete and a female gamete unite to form a zygote
which finally develops into a new organism.

In sexual reproduction, haploid gametes or germ cells are produced by meiosis. The male
gamete, or sperm, is produced by the testes/anthers, which are small, active, motile and do
not contain any reserve food. The female gamete, or ovum, is produced by the ovary, which is
large, non-motile and contains reserve food. On favorable condition, a male gamete fuses with

pollination /ˈpɒləˈneɪʃn/ - transfer of pollen grains from anther to stigma of a flower

326 Oasis School Science - 10 BIOLOGY

a female gamete and forms a single diploid zygote. The process of fusion of a male gamete and
a female gamete to form a zygote is called fertilization.

In animals, the fertilization may be external or internal. If the fertilization takes place outside
the body of a female, it is called external fertilization. This process is common in fishes and
frogs. If the male passes sperms inside the female’s body during sexual intercourse and
fertilization takes place there, it is called internal fertilization. This process is common in
reptiles, birds and mammals.

As a result of fertilization, a single diploid cell, i.e. zygote is produced. The zygote divides
mitotically, several times in the early stage of development and forms an embryo. The embryo
further grows and finally develops into a new organism.

18.12 Basic Features of Sexual Reproduction

i. Both male and female organisms are involved in sexual reproduction.

ii. Male organisms produce male gametes, and female organisms produce female
gametes. Those gametes are formed due to meiosis, hence gametes are haploid.

iii. A male gamete and a female gamete fuse to form a diploid zygote.

iv. The zygote divides mitotically several times to form a multicellular organism.

18.13 Sexual Reproduction in Flowering Plants

In flowering plants, sexual reproduction is the most common method of reproduction. In
higher plants, the flower is the sexually reproductive organ. A complete flower contains four
floral whorls, viz. calyx, corolla, androecium and gynoecium.

1. Calyx: It forms the outermost whorl of a flower. It is made up of green, leaf-like structures
called sepals. The calyx protects a flower in the bud stage.

Stamen Anther Stigma Carpel

Style

Filament Ovary

Sepal

Petal

Receptacle

Fig. 18.23 Structure of a complete flower
hermaphrodite /hɜːˈmæfrədaɪt/ - an organism having both male and female sexual organs

BIOLOGY Oasis School Science - 10 327

2. Corolla: The second whorl of a flower is made up of colorful petals, which is called
corolla. The petals are usually scented and beautiful. Petals attract insects for pollination
and also protect the reproductive organs of the flower.

3. Androecium: It forms the third inner whorl of a flower. Androecium consists of a group
of stamen. Each stamen is the male reproductive organ of a flower. It consists of a swollen
upper part called the anther and a slender stalk called the filament. Each anther is made
up of four pollen sacs, which are connected together by a connective. Each pollen sac
is filled with pollen grains. Pollen grains are fine, powdery structures containing male
gametes.

Anther

Connective

Pollen
grains

Pollen
sacs

Anther

Filament

Fig.18.24 Structure of a stamen

4. Gynoecium: Gynoecium is the innermost whorl Stigma
of a flower. It is the female reproductive part. Each Style
member of the gynoecium is called a pistil or carpel. Ovule
Each carpel is made up of three parts: the lowermost Ovary
swollen ovary, middle cylindrical style and the
uppermost spreading the stigma. The stigma receives Fig. 18.25 Structure of a pistil
pollen grains, the style supports stigma and the ovary
contains ovules. The ovules are connected to the
placenta. The ovules produce female gametes and
mature into seeds after fertilization.

18.14 Pollination

At maturity, anthers begin to dry and burst open to release pollen grains. The pollen grains
are transferred to the stigma of a flower by many external agents like insects, wind, birds,
water, animals, etc. Pollination is the transfer of pollen grains from the anther to the stigma of

328 Oasis School Science - 10 BIOLOGY

a flower. It is of two types– self-pollination and cross-pollination.

Self-pollination is the transfer of pollen grains from the anther to the stigma of the same flower
or to the stigma of another genetically identical flower of the same plant. It is also called
autogamy. This process is common in bisexual flowers, e.g., the pea, China rose, etc.

Cross-pollination is the transfer of pollen grains from anther of a flower of one plant to the
stigma of a flower in another plant of the same species. It is also called allogamy. This process
is common in both unisexual and bisexual flowers. External agents of pollination are essential
for cross-pollination. This process leads to cross-fertilization, which brings out variation in the
new offspring. In flowering plants, pollination is followed by fertilization.

Cross-pollination

Self-pollination
Self-pollination

Fig.18.26 Types of pollination

18.15 Fertilization

Fertilization is the process of fusion of a male gamete and a female gamete to form a zygote.
In flowering plants, pollen grains absorb secretions from the stigma and undergo germination
after pollination. The germinating pollen grain produces a pollen tube which grows down
towards the style and finally reaches the ovule in the ovary. An egg cell (female gamete) is
present inside the embryo sac in the ovule. The tip of the pollen tube ruptures in the ovule and
discharges two male nuclei (gametes) into it through the micropyle. One male gamete fuses
with the egg cell and forms a zygote. This fusion is called fertilization (syngamy). Another
male gamete (n) fuses with the diploid (2n) secondary nucleus (fusion product of two polar
nuclei) and forms the endosperm nucleus. This process is called triple fusion.

BIOLOGY Oasis School Science - 10 329

Stigma Pollen grain
Style Pollen tube

Ovary wall Ovule
Embryo sac Antipodal cells
Secondary
Female nucleus
gamete (egg) Micropyle

Fig.18.27 Detailed structure of a pistil and fertilization

Fact File - 1

The fertilization in flowering plants involves the fusion of two male gametes separately, i.e.,
one male gamete with the egg cell (ovum) and another male gamete with the secondary
nucleus. Therefore, the fertilization in flowering plants is called double fertilization.

After fertilization, the zygote divides and redivides mitotically and forms the embryo whereas
the endosperm nucleus develops into the endosperm of the seed.

The embryo is the baby plant in the very early stage of its development before coming out
of its seed whereas the endosperm is the food storage tissue of the seed. The embryo and
endosperm together make a seed. After fertilization, the ovule forms the seed and the ovary
forms the fruit. The seed is a reproductive unit of a flowering plant which can germinate to
produce a new plant on favorable condition.

In this way, flowering plants reproduce by forming seeds through flowers.

Activity -3
To observe various parts of flowers
• Visit a garden and collect different types of flowers.
• Separate the sepals, petals, stamens and pistils of each flower.
• Draw figures and label them.
• Also, write down the similarities and differences between them.

330 Oasis School Science - 10 BIOLOGY

18.16 Sexual Reproduction in Animals

Sexual reproduction is the means of reproduction in vertebrate animals, but it is also found
in the lower group of invertebrates like the paramecium, flatworm, roundworm, arthropod,
molluscs, etc. Among them, some animals are unisexual and some are bisexual. Bisexual
animals can produce both a male and female gametes whereas unisexual animals can produce
either male gamete or female gamete. The process by which animals produce gametes is known
as gametogenesis. In animals, gonads are the primary sex organs which produce gametes, or
sex cells, by meiosis. Testis is the male gonad that produces sperms, or male gametes, by
spermatogenesis. Similarly, the ovary is the female gonad that produces the ova, or egg cells
or female gametes, by oogenesis.

After formation of gametes, fusion of a male gamete and a female gamete takes place which
results in the formation of a zygote. The zygote divides mitotically and forms an embryo
which finally develops into a young animal. However, the method in which a zygote grows
and develops into a complete animal also varies in different animals.

18.17 Male and Female Organism

If an organism produces sperms, or male gametes, it is called a male. If an organism produces
ova, or a female gametes, it is called a female. A male organism contains a set of male sex
organs whereas a female organism contains a set of female sex organs. In males, testes produce
sperms whereas ovaries produce ova in female organisms. In flowering plants, the anther
produces pollen grains which contain male gametes.

18.18 Gametes

Sexual reproduction takes place by the fusion of two special types of haploid cells called
sex cells. Those sex cells are called gametes. Thus, the haploid sex cells involved in sexual
reproduction are called gametes.

Haploid sperm

Diploid zygote

Haploid egg

Fig. 18.28 Gametes and zygote

BIOLOGY Oasis School Science - 10 331

Sex cells, or gametes, are of two types: male gamete and female gamete. The male gamete is
a haploid male sex cell produced by the testis/anther. The male gamete of an animal is called
sperm. It is motile and smaller in size. Sperms are produced by spermatogenesis. The female
gamete, or ovum is the haploid female sex cell produced by the female sex organ, i.e., ovary. It
is also called an egg. It is non-motile and larger in size. It is formed by oogenesis. On favorable
condition, a male gamete and a female gamete fuse together and form a diploid zygote.

18.19 Unisexual and Bisexual Organisms

Involvement of a male organism and a female organism is essential for sexual reproduction.
Sexes are separate in most of the developed animals. The organism having only one kind
of reproductive organ (either male or female) is called unisexual organism. It is also called
dioecious organism. Such organism has one sex only. Human beings, birds, mammals, reptiles,
frogs, fishes, etc. are some examples of unisexual animals. Unisexual organisms produce only
one kind of gametes either male gametes or female gametes. For example, humans are called
unisexual organisms because some of the humans are males as they contain testes whereas
some are females as they contain ovaries. A human being having testes is called the male
(man) and the human being having ovaries is called the female (woman).

Some animals possess both male and female reproductive organs in the same body. The
organism having both male and female reproductive organs in its body is called bisexual, or
hermaphrodite organism. It is also called a monoecious organism. A nermaphrodite organism
can produce both male and female gametes. Some animals like the tapeworm, liver fluke,
earthworm, hydra, leech, etc. and most of the flowering plants are bisexual organisms.

18.20 Significance of Sexual Reproduction

i. Sexual reproduction gives continuity to the race.
ii. New combination of characteristics are formed as the chromosomes are reshuffled during

gametogenesis. It brings out genetic variation in organisms, which leads to evolution.

iii. Increase in genetic variation in a species results in better survival of that species in the
changing environment.

Reasonable Fact-4

"The offspring produced by asexual and sexual reproduction process are dissimilar."
Why?
Ans: In asexual reproduction, crossing over does not take place and gametes are not
produced. So, offspring produced by asexual reproduction are genetically identical to the
parent. But in case of sexual reproduction, crossing over takes place during the formation
of gametes. So variation occurs in the offspring.

332 Oasis School Science - 10 BIOLOGY

Activity -4

• Visit a nearby fish pond and collect eggs, larvae and adults. Observe their structure
and draw labeled figures.

• Collect a few eggs, tadpoles and adult frogs from a nearby stream. Observe their
structure and draw labeled figures.

Differences between Asexual Reproduction and Sexual Reproduction

Asexual Reproduction Sexual Reproduction

1. It takes place without the fusion of a 1. It takes place by the fusion of a male

male gamete and a female gamete. gamete and a female gamete.

2. This method is common in primitive 2. This method is common in developed

plants and animals. plants and animals.

3. Only a single parent is involved in 3. Both male and female organisms are

asexual reproduction. involved in sexual reproduction.

4. Variation in offspring cannot be 4. Variation in offspring can be observed.
observed.

Differences between Self-pollination and Cross-pollination

Self-pollination Cross-pollination

1. It is the transfer of pollen grains from the 1. It is the transfer of pollen grains from

anther to the stigma of the same flower the anther of a flower of one plant to

or to the stigma of another genetically the stigma of a flower of another plant

identical flower of the same plant. of the same species.

2. External agents of pollination are not 2. External agents of pollination are

required. required.

3. Variation does not take place in the 3. Variation takes place in the offspring.

offspring. 4. It is common in both unisexual and

4. It is common in bisexual flowers. bisexual flowers.

Differences between Sperm and Ovum

Sperm or male gamete Ovum or egg or female gamete
1. It is the haploid male sex cell produced 1. It is the haploid female sex cell
produced by the female sex organ.
by the male sex organ.
It is non-motile and larger in size.
2. It is motile and smaller in size. 2.
Less number of ova are produced by
3. Generally, a large number of sperms 3. oogenesis.
are produced by spermatogenesis.

BIOLOGY Oasis School Science - 10 333

SUMMARY

• Reproduction is a biological process in which living beings produce their own
kind asexually or sexually. Living organisms reproduce for the continuity of their
races on the earth.

• Asexual reproduction is a simple method of reproduction which takes place
without the involvement of gametes, or sex cells.

• Fission is the method of asexual reproduction in which a mother organism divides
into two or more daughter organisms. This method is common in unicellular
plants and animals like amoeba, bacteria, etc.

• Budding is the method of asexual reproduction, which takes place by means of a
bud. It is common in hydra, yeast, etc.

• Sporulation is the method of asexual reproduction, which takes place by means of
spores. It is common in moss, marchantia, riccia, etc.

• Fragmentation or regeneration is the method of asexual reproduction in which a
multicellular organism breaks down into two or more pieces (fragments) and each
fragment develops into a new organism. The ability of an individual fragment to
replace its lost parts is called regeneration. It is common in spirogyra, tapeworm,
hydra, planaria, etc.

• Vegetative propagation is the method of asexual reproduction in which new
plants are produced by cutting, sowing or grafting of the vegetative parts of
flowering plants like a root, stem and leaf.

• Tissue culture, or micropropagation, is the modern technique of producing new
plants from isolated plant cells or a small piece of plant tissue in a culture solution.

• Sexual reproduction is the complex method of reproduction which takes place
by the fusion of a male gamete and a female gamete. This method is common in
higher plants and animals.

• Pollination is the process of transfer of pollen grains from the anther to the stigma
of a flower. It may be of two types: self-pollination and cross-pollination.

• Fertilization is the process of fusion of a male gamete and a female gamete to form
a zygote.

• An embryo is a young plant or animal in the very early stage of development
before birth or before coming out of its seed or egg. It is formed by the mitotic cell
division of a zygote in the early stage of its development.

• A male gamete, or sperm, is a haploid male sex cell produced by the male sex
organ (testis/ anther). It is motile and smaller in size.

• A female gamete, or egg, is a haploid female sex cell produced by the female sex
organ, i.e., ovary. It is non-motile and larger in size.

• The organism that produces sperms only is called a male organism whereas the
organism that produces ova(eggs) only is called a female organism.

334 Oasis School Science - 10 BIOLOGY

Exercise

Group-A

1. What is reproduction?

2. Why do living beings reproduce?

3. What is asexual reproduction?

4. Name the various methods of asexual reproduction that occur is plants.

5. What is fission? What are the types of fission?

6. What is binay fission?

7. Name any two organisms that reproduce by binary fission.

8. What is multiple fission?

9. Name one plant and one animal that reproduce by multiple fission.

10. What is budding?

11. Name any two organisms that reproduce by budding.

12. What is sporulation?

13. Name any two organisms that reproduce by sporulation.

14. What is regeneration?

15. What is vegetative propagation?

16. Name one plant each that reproduce asexually by root and stem.

17. Name any two plants that reproduce asexually by leaves.

18. Name an organism which reproduces by either of the given methods:

i) Multiple fission ii) Vegetative propagation by stem

19. What is sexual reproduction?

20. What is pollination?

21. Name two types of pollination.

22. What is fertilization?

23. What is double fertilization?

24. Name the types of fertilization.

25. What is internal fertilization?

26. Name any two animals in which external fertilization takes place.

27. What is an embryo?

28. What is layering?

29. What are types of layering? Name them.

30. What is compound layering?

31. Name any two plants that are propagated by compound layering.

32. What is tip layering?

33. Name any two plants that are propagated by tip layering.

BIOLOGY Oasis School Science - 10 335

34. What is grafting?
35. Name any two plants that are propagated by grafting.
36. What is tongue grating?
38. What is tissue culture?

Group-B
39. What is the meaning of the saying that yeast reproduces asexually by budding?
40. What does it mean by the saying that spirogyra reproduces by fragmentation?
41. Write any two differences between asexual reproduction and sexual reproduction.
42. Write any two differences between fission and budding.
43. Write any two differences between binary fission and multiple fission.
44. Write any two differences between self-pollination and cross-pollination.
45. Write any two differences between external fertilization and internal fertilization.
46. Write any two differences between pollination and fertilization.
47. Write any two differences between sporulation and fragmentation.
48. Write any two differences between ovum and zygote.
50. Write any two differences between sperm and zygote.
51. Write any two differences between male gamete and female gamete.
52. The number of chromosome in gametes is haploid, why?
53. The fertilization in frog is called external fertilization, why?
54. The number of chromosome is different in gametes and zygote, why?
55. Write any two differences between stool layering and air layering.
56. Write any two differences between compound layering and mound layering.
57. Write any two differences between layering and tissue culture.
58. Which method is widely used in Nepal out of grating, layering and tissue culture, why?

Group-C
1. How does amoeba reproduce by binary fission? Describe in brief.
2. Draw a neat diagram showing multiple fission in Plasmodium.
3. How does hydra reproduce by budding? Describe in brief.
4. Which type of asexual reproduction is shown in the given diagram?

Write down any two advantages of such reproduction.
5. Write any three advantages of asexual reproduction.
6. Which method of sexual reproduction is shown in the given diagram? Also, label the

parts A, B, C and D.

336 Oasis School Science - 10 BIOLOGY

A
B
C

D
E

7. Write any three reasons for reproducing plants by artificial vegetative propagation.
8. How is mound layering done? Explain with figure.
9. Write any three reasons for carrying out tissue culture in plants.
10. How are scion and stock selected? Describe.

Group-D
1. Draw a chart showing the process of sexual reproduction in developed organisms.
2. How do flowering plants reproduce? Describe in brief.
3. How is simple layering done? Explain with figure.
4. Describe the method of mound layering with a labeled figure.
5. Describe in brief the method of tissue culture.
6. How is whip grafting done? Explain with figure.
7. Describe in brief the importance of tissue culture.
8. Describe in brief the importance of artificial reproduction in plants.
9. How does sexual reproduction take place in animals? Explain in brief.

BIOLOGY Oasis School Science - 10 337

UNIT 19 Estimated teaching periods

Theory 5

Practical 0

HEREDITY

Objectives T. H. Morgan
(1866-1945 AD)
After completing the study of this unit, students will be able to:

• state Mendel’s laws and experiments with phylogenetic charts.

• identify dominant and recessive traits.

• enlist the causative factors of inheritance and describe the process of inheritance
of characteristics.

19.1 Introduction

Living organisms produce their offspring either by asexual reproduction or sexual
reproduction. Reproduction is the basic characteristic of all living organisms, which involves
the transmission of genetic material from one generation to another. Living beings have the
capacity to produce offspring of their own kind. For example, cats produce kittens, dogs
produce puppies and orange seeds grow into orange plants. The reproduction ensures the
continuity of a race or species.

The offspring produced asexually are identical to their parent as well as to one another because
in asexual reproduction, all the offspring and the parent have identical genes. On the other
hand, in sexual reproduction, although the offspring are similar to their parents they are not
completely identical to them or one another. It is because, in sexual reproduction, offspring
receive some genes from the father and some from the mother. Identical twins look alike as
they have the same genetic material or hereditary material. Actually, the hereditary material
is present in the gametes, or sex cells. Thus sex cells constitute the link from one generation to
the next. The relation that continues to exist between successive generations is referred to as
heredity. The phenomenon by which living organisms transmit parental characteristics to
their offspring is called heredity, and the characteristics of the parents which are transmitted
to their offspring are called hereditary characteristics. Heredity is also known as inheritance.

Hereditary information is present in the gametes. When a male gamete fuses with a female
gamete, a zygote is formed, which finally develops into a new organism of the same kind. It
is possible due to the presence of hereditary information in the gametes. Therefore, a crow
always hatches crows and not parrots, a cat gives birth to kittens, not puppies, a seed of a
mango always grows into a mango tree and not an orange tree.

heredity /həˈredəti/ - the process by which characteristics are passed by parents to their offspring

338 Oasis School Science - 10 BIOLOGY

Though the offspring inherit the traits, or characteristics, of their parents and resemble them
very closely, they are not identical to their parents in all respects. The offspring are never
a true copy of their parents. Even though twins show resemblance in many respects, they
also show differences in many characteristics. There may be morphological, physiological and
behavioristic differences among the individuals of a species. Those differences are known as
variations. So, variation can be defined as the differences in the characteristics, or traits, among
the individuals of a species. For example, the height of a human being is a trait which shows
variations because some people are very tall, some have medium height whereas some are
dwarfs. Similarly, the complexion of a human being, type of hair, color of the eyes, shape of
the nose, etc. are the traits of human beings. These traits bring out variation. The difference in
the traits, or variation, is necessary for organic evolution.

Heredity and variation are two fundamental factors in the process of organic evolution and
formation of new species. The branch of biological science which deals with the study of
heredity and variation is called genetics. It can also be defined as the biological science of
inheritance.

Genetics is the modern experimental study of the laws of inheritance (variation and heredity).
It deals with the mechanisms responsible for similarities and differences among closely related
species. The term genetics was first coined by W. Batesman in 1905 AD [Gk., genesis – grow
into or to become). The first scientific study of genetics was carried out by an Austrian monk
Gregor Johann Mendel (1822 – 1884 AD). He introduced the concept of genes as the basic unit
of heredity.

Fig. 19.1 Gregor Johann Mendel: The father of genetics

Mendel carried out his scientific investigation on hybridization of pea plants. He established
the laws of inheritance of characteristics in 1865 – 66 AD. The results of his eight-year breeding
experiments were read before the Natural History of Society of Brunn (Austria) in 1865 AD.
However, his work remained unnoticed until 1900 AD, when three scientists Hugo De Vries
in Holland, Tschermak in Austria and Correns in Germany discovered its significance. Since
then Mendel's work has formed the basis of the study of genetics. Mendel died in 1884 AD
before he could see his work accepted and appreciated.

trait /treɪt/ - a particular quality or characteristic

BIOLOGY Oasis School Science - 10 339

19.2 Genes and Gene Concept

Genes are tiny units of heredity located in the chromosomes. The concept of gene was
introduced by Sutten, and the term 'gene' was introduced by Johannson. The essential features
of the modern gene concept are given below:

Genes are located in chromosomes, and they are arranged in a linear fashion. Each gene
occupies a fixed position in a fixed chromosome. The position of a gene in a chromosome
is called locus (pl.–loci). Genes determine the physical, anatomical and physiological
characteristics of organisms. These characteristics are transmitted from one generation to
another. Each chromosome contains a large number of genes. It is estimated that about 60000
genes are present in 23 pairs of chromosomes of human beings. A single gene may occur in
several forms and a gene may undergo a sudden change in expression. This phenomenon is
called mutation. A gene is a segment of DNA which can form exact copies by the process of
replication.

Chemically, a gene is a segment of a large polynucleotide molecule called DNA(deoxyribonucleic
acid), which forms the most important constituent of a chromosome.

Functions of genes

1. Genes are responsible for the inheritance of characteristics from one generation to
another.

2. They code for proteins to control phenotype and metabolism.

3. Genes help in evolution by mutation and genetic recombination.

4. Genes code for different types of RNAs for protein synthesis.

19.3 Terminology Related to Genetics

• Allele: An allele is one form of a gene or pair of matching genes. There may be two
or more alleles of a gene. Diploid organisms contain two alleles, one on each pair of
chromosomes. For example, in pea plants, the gene for the height of the plant may have
a tall allele and a dwarf allele. In genetics, different alleles are denoted by letters, e.g., TT,
Tt, Rr, etc.

• Homozygous: When both alleles of a particular gene are identical, it is called
homozygous. Such an individual is called pure breeding because this type of individual
always produces the same phenotype of offspring as it does not hide a recessive allele.
For example, TT (pure tall), RR (pure red), rr (pure white), etc.

• Heterozygous: When two alleles of a gene are not identical, it is called heterozygous.
A heterozygous individual contains one dominant allele and another recessive allele. It is
also called a hybrid. For example, Rr (hybrid red). It contains one dominant allele, i.e. R and
another recessive allele, i.e., r, where R = Red and r = white. Generally, the capital letter stands
for a dominant allele, and the small letter stands for a recessive allele in a set of allele.

• Dominant characteristics: The characteristics which are prominent and appear in
successive generations are called dominant characteristics, e.g., in the cross-pollination

allele /əˈliːl/ - one or two or more possible forms of a gene that are found at the same place on a chromosome

340 Oasis School Science - 10 BIOLOGY

between a tall pea plant and a dwarf pea plant, tall is a dominant characteristic as it
appears in F1–generation.

• Recessive characteristics: The suppressed characteristics, which remain unexpressed in
successive generations, are called recessive characteristics, e.g. in the cross-pollination
between a tall pea plant and a dwarf pea plant, dwarf is a recessive characteristic as it
remains hidden in F1–generation.

• Phenotype: The external appearance of an organism for a contrasting characteristic is
called phenotype. It is expressed in words, e.g., tall, dwarf, etc. Parental characteristics
of an organism cannot be known from the phenotype of an organism.

• Genotype: The genetic make up or genetic constitution of an organism is called genotype.
It is expressed in letters, e.g., TT (pure tall), Tt (hybrid tall), tt (dwarf), etc. Parental
characteristics of an organism can be found from the genotype of an organism.

• Hybrids: Hybrids are the organisms produced after cross fertilization between two
genetically different organisms. The process by which hybrids are formed is called
hybridization (i.e., the process of crossing between organisms having two contrasting
characteristics). Hybrids are of various types, e.g., monohybrids, dihybrids, polyhybrids,
etc.

• Monohybrid cross: The cross involving only one pair of contrasting characteristics or
allelomorphs is called monohybrid cross. As a result of monohybrid cross, offspring
having phenotypic ratio of 3:1 are produced in F2 - generation, e.g., cross between a tall
plant with a dwarf plant.

• Dihybrid cross: The cross involving two pairs of contrasting characteristics is called
dihybrid cross. As a result of dihybrid cross, offspring having phenotypic ratio of 9:3:3:1
are produced in F2 - generation, e.g., cross between a red - flowered tall pea plant with a
white - flowered dwarf pea plant.

• F1–generation: The generation obtained by crossing two parental stocks is called

F1– generation. It is also called the first filial generation.

• F2–generation: The generation obtained by crossing two parental stocks of

F1 - generation is called F2 - generation. It is also called the second filial generation.

19.4 Mendel's Material for His Experiment

Gregor Mendel conducted his experiments over a period of eight years on the common edible
garden pea plant (Pisum sativum) in his Monastery garden at Brunn, Austria. Mendel chose
garden pea plant for his experiment because of the following reasons:

1. Pea plants have several distinct varieties having a large number of contrasting
characteristics in pairs, such as tall (T) and dwarf (t), round (R) and wrinkled (r)
seeds, smooth (I) and constricted (i) pea pods, etc. Mendel observed seven pairs
of contrasting characteristics in pea plants. The seven pairs of traits present in pea
plants are given in table 19.1.

breeding /ˈbriːdɪŋ/ - the keeping of animals or plants in order to reproduce
contrasting /kənˈtrɑː.stɪŋ/ - different in characteristics, e.g. colour, height, etc.

BIOLOGY Oasis School Science - 10 341

2. Pea plants can be cultivated easily, and they have a short life cycle. So it is possible to
study several generations within a few years.

3. Breeding can be controlled easily in pea plants because of the structure of the flowers.

4. They produce a large number of offspring after fertilization, and the hybrids produced
after cross– pollination are also fertile.

5. In pea plants, it is easy to obtain pure lines for several generations due to self-pollination.

6. The flowers of a pea plant are adequate in size and are easy to handle.

7. In pea plants, petals of flowers completely enclose the male and female reproductive
organs until fertilization which ensures self-pollination.

Table 19.1: Seven pairs of contrasting traits in pea plants studied by Mendel

Traits Contrasting forms

Dominant Recessive

1. Height of the plant Tall Dwarf

2. Color of flower Purple (Red) White

3. Shape of seeds Round Wrinkled

4. Color of pods Green Yellow

5. Shape of pods Inflated Constricted

6. Position of flowers Axial Terminal

7. Color of seeds Yellow Green

Height Seed shape Seed colour Seed coat Pod shape Pod colour Flower
position

Dominant Tall Round Yellow Green Inflated (full) Green Axial
trait

Recessive
trait

Short Wrinkled Green White Constricted Yellow Terminal
(Dwarf) (flat)

Fig. 19.2 Mendel's material for his experiment with its traits

342 Oasis School Science - 10 BIOLOGY