Class 10 Science Solutions Nepal

ii. They neutralize the toxic substances produced by the germs.
iii. Neutrophils destroy bacteria by engulfing them.
iv. Basophyl secrete anticoagulant.

MEMORY TIPS

The increase in the number of WBCs above the normal value leads to leukaemia (blood
cancer) and decrease in the number of WBC leads to the leucopenia.

(c) Platelets

They are the smallest blood cells which are oval
or round having no nucleus. They are produced
in the bone marrow and after the completion
of a lifespan of about 2-3 days, they die in the
spleen. The number of platelets in blood is about
2.5 lakh to 4.5 lakh per cubic mm.

Functions

i. They help in the clotting the blood and thus prevent the excessive loss of blood
from the wounds.

ii. They help in healing of wounds.

MEMORY TIPS

Blood clot is a thick jelly which closes ruptured blood vessels so that the flow of blood is
checked. The general mechanism of blood clotting is :

Damaged tissue + Blood platelets on exposure release thromboplastin (proteins)

to air

Prothrombin Thromboplastin Thrombin (enzyme)

Ca++

Fibrinogen Thrombin Fibrin (protein)

Fibrin + RBC Blood clot (thick mass of fibres to which RBCs are enlongled

ACTIVITY

Clean the fingertip with spirit. Prick the fingertip using a sterilized needle. Put a drop of blood on the slide and
make it a thin film. Let it to dry. Observe it under microscope. Did you see RBCs? Put few drops of leishmans’
strain on it and wait for 5 minutes. Wash the slide and observe under the microscope. Did you see WBCs?

QUESTIONS

# What are the functions of different types of blood corpuscles?
# How are anaemia and leukemia caused?

New Creative Science, Class 10 | 247

Function of blood

There are mainly three functions of blood. (c) Protection
(a) Transportation (b) Regulation

(a) Transportation
i) It transports oxygen from the lungs to different cells of the body and CO2
from different parts of the body to the lungs for excretion.
ii) It transports nutrients absorbed in the intestine to all parts of the body.
iii) It brings the waste materials from the body parts to their respective
excretory organs.
iv) It also transports hormones to the different body parts.

(b) Regulation
(i) It maintains the body temperature.
(ii) It regulates the amount of water in the body.
(iii) It also maintains different chemicals in the body.

(c) Protection
(i) It helps in the defense system of the body by producing antibodies which
fight against foreign toxic germs and substances.
(ii) It helps to prevent the loss of blood from the body by forming blood clot.

Blood vessels

The continuous muscular tubules through which blood flows throughout the body
are called blood vessels. There are three types of blood vessels in the human body.

a) Arteries
b) Veins
c) Capillaries

a) Arteries

These are the blood vessels which always carry pure (except pulmonary artery) blood
from the heart to different parts of the body. The exception is the pulmonary artery
which carries impure blood from the heart to the lungs for purification. The blood
pressure is very high in the arteries. So they are deep seated in the body. They are not
provided with the valves. They are scarlet in colour due to the flow of oxygenated
blood through them. They are thick-walled blood vessels.

Detail structure of artery

248 | Blood Circulatory System in Human Body

b) Veins
These are the blood vessels which carry impure blood from different parts of the body
to the heart. The pulmonary vein in the body carries pure blood/oxygenated blood
from the lungs to the heart. The blood pressure is low in the veins, so they are not
deep seated in the body. They have valves in them which prevent the back flow of
blood. They are violet in colour as they carry deoxygenated blood. They are thin-
walled blood vessels.

Detail structure of vein

c) Capillaries
They are very fine networks of the blood vessels that connect arteries to the veins. The
exchange of oxygen and CO2, nutrients and waste products take place in capillaries.

Differences between Vein and Artery

S.N. Vein S.N. Artery

1. It carries blood from different parts 1. It carries blood from the heart to

of the body to the heart. different parts of the body.

2. The blood pressure is low. 2. The blood pressure is high.

3. It has valves which help the blood 3. It does not have valves.
to flow in a backward direction.

4. It is superficial in the body. 4. It is deep seated in the body.

5. They are thin-walled. 5. They are thick-walled.

New Creative Science, Class 10 | 249

MEMORY TIPS
The three layers of blood vessel are tunica interna (inner layer), which is squamous
epithelium tunica externa (middle layer) which is an involunary smooth muscle and tunica
adventitia (outer layer), which is a tough fibrous connective tissue.

QUESTIONS
# Why are arteries deep seated in the body and veins are superficial?
# Why are veins provided with valves but arteries do not have valves?

(iii) Heart

The human heart is a hollow, muscular organ, roughly the size of a ‘clenched fist’. It
weighs about 300 gm in males and about 250 gm in females. It is the main pumping
organ of the body, which pumps blood to different parts of the body. The heart is
situated between the two lungs in the thoracic cavity. It is surrounded by a double
membranous structure known as pericardium. The pericardium has a pericardial
cavity which is filled with pericardial fluid. It reduces the friction during heartbeat
and protects the heart from shocks and injuries.

External structure of heart

Internal structure of heart
The heart is made up of special tissues known as cardiac muscles. It is divided by
septa into two halves- the right and the left. Each half consists of two chambers- upper
auricle and lower ventricle. The auricles are thin-walled, separated from each other by
the inter auricular septum whereas the ventricles are thick-walled chambers separated
from each other by the inter ventricular septum.
Different blood vessels enter and leave the heart. The superior venacava and the
inferior venacava bring impure blood from all parts of the body to the right auricle.
The left auricle is connected to two pairs of pulmonary veins, one from each lung,

250 | Blood Circulatory System in Human Body

which bring the oxygenated blood. The pulmonary aorta arises from the right ventricle
and the systemic aorta arises from the left ventricle.

Internal structure of heart

There are four valves in the heart. They are:
(i) Right autrio-ventricular valve or tricuspid valve: It is a valve between the right

auricle and the right ventricle. It consists of three lobes. It prevents the back
flow of blood from the right ventricle to the right auricle and blood flows to the
pulmonary artery.
(ii) Left autrio-ventricular valve or Bicuspid valve or mitral valve: It is a valve
between the left auricle and the left ventricle. It consists of two lobes. It prevents
the back flow or blood from the left ventricle to the left auricle and blood flows
to the aorta.
(iii) Pulmonic valve / Pulmonary value: It lies between of the right ventricle and the
pulmonary artery. It is half moon-shaped and helps to flow blood to the lungs.
(iv) Aortic valve: It is a valve between the left ventricle and the aorta. It is also half
moon-shape. It helps to flow the blood from the ventricle to the aorta.

MEMORY TIPS
 The auricles are thin-walled as they have to pump blood to a shorter distance i.e. up to

ventricles whereas ventricles are thick-walled as they have to pump blood to the lungs
and different parts of the body. The left ventricle has the thickest wall to pump blood to
all parts of the body.
 The coronary sinus brings deoxygenated blood from the walls of the heart to the right
auricles.
 The coronary artery supplies oxygenated blood from the aorta to the muscular wall of
the heart.

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QUESTIONS

# Name the valve between the right auricle and the right ventricles and between the left
auricle and the left ventricle.

# Why are there valves in the heart?

ACTIVITY

1. Bring a heart of a goat from butcher’s shop and observe its external structure and internal
structure (by cutting it longitudinally).

2. Draw a neat and labeled diagram showing the external and internal structure of the heart.

Differences between auricles and ventricles.

S.N. Auricles S.N. Ventricles

1. They are thin-walled upper heart 1. They are thick-walled lower heart

chambers. chambers.

2. They are smaller. 2. They are bigger.

3. They receive blood from different 3. They receive blood from auricles.

parts of the body.

4. They give rise to veins. 4. They give rise to arteries.

Functions of different chambers of the heart

a) Right auricle: It receives impure/deoxygenated blood from different parts of
the body and pumps it into the right ventricle.

b) Right ventricle: It receives the deoxygenated blood from the right auricle and
pumps it into the lungs for purification.

c) Left auricle: It receives pure/oxygenated blood from the lungs and pumps it
into the left ventricle.

d) Left ventricle: It receives pure blood from the left auricle and pumps it to
different parts of the body.

BLOOD CIRCULATION IN HUMAN BODY

The blood circulation in a human body is double circulation. This means that the blood

passes through the heart twice for each circulation of the body. The first circulation

involves the entry of blood from all parts of body into the heart. This impure blood

goes to the lungs for purification. Now, the second circulation involves the entry

of oxygenated/pure blood from the lungs to the heart, from where it is distributed

throughout the body. Thus, blood circulation is of two types in human body:

(i) Systemic circulation (ii) Pulmonary circulation

252 | Blood Circulatory System in Human Body

(i) Systemic circulatiheon

The circulation of blood from the heart to all parts of the body except the lungs is called the
systematic circulation.

When the left ventricle contracts, the pure blood is pushed to the aorta, from where
smaller arteries distribute blood to different parts of the body. The arteries divide into
arterioles and finally to capillaries. Capillaries have a thin wall and through this the
exchange of nutrients and oxygen of the blood with the waste products go to venules
from the capillaries. The venules unite to form veins and finally form venacava which
brings the blood to the right auricle of the heart.

The systemic circulation can be shown in the flowchart as shown below:

Left auricle Aorta Arteries

(pure blood) (pure blood) (pure blood)

Right auricle Arterioles

(impure blood) (pure blood)

Venacava Veins Venules Capillaries

(impure blood) (impure blood) (impure blood) (exchange of blood)

Systemic circulation

(ii) Pulmonary circulation

The circulation of blood between the heart and the lungs is called pulmonary
circulation.

The impure blood on the right ventricle is pumped to the pulmonary artery which
brings the blood to the lungs for purification. In the lungs, the blood releases CO2 and
then mixes with oxygen and gets purified. This pure blood is carried by the pulmonary
vein to the left auricle of the heart. This process can be shown in the chart as:

Right ventricle Pulmonary artery

(impure blood) (impure blood)

Lung

(Blood purified)

Left auricle Pulmonary vein

(pure blood) (pure blood)

Pulmonary circulation

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Blood circulation

BLOOD PRESSURE

The blood exerts force on the walls of the blood vessels. This force is called blood
pressure.

It is measured with the help of the instruments, called sphygmomanometer. There are
two types of blood pressure: systolic and diastolic.

(a) Systolic pressure is the pressure of blood inside the artery during
ventricular contraction. The normal systolic pressure is 120 mm of Hg.

(b) Diastolic pressure is the pressure of the blood inside the artery during
ventricular relaxation. The normal diastolic pressure is 80 mm of Hg.

Thus, the normal blood pressure in a human being is written as 120/80 mm of Hg.

254 | Blood Circulatory System in Human Body

Causes of blood pressure
i. High blood pressure may caused due to continuous smoking.
ii. It may caused due to obesity or heavy weight.
iii. It is due to lack of physical exercise.
iv. It is due to excessive salt in everyday food.
v. It is due to more and regular drinking alcohol.
vi. It is due to adrenal and thyroid disorder.
vii. It is due to heredity character that transfers from parents to the offspring.
viii. It is due to old age (more than 40 years).
ix. It is due to mental tension.
x. It is due to excessive fat in the food.
Preventive measures of the blood pressure
Blood pressure is a disorder of the body. It is not like a communicable disease. It does
not transfer through micro-organisms. It is mainly due to our habit. The following
preventive measures can be taken to avoid high blood pressure.
i. We should avoid smoking and drinking alcohol.
ii. We should control our body weight.
iii. We should do physical exercise regularly.
iv. The amount of salt in everyday food should be controlled.
v. We should avoid mental tension.
vi. The amount of fat in the food should be minimized.

Measuring of blood pressure

HEART BEAT

The rhythmic contraction and relaxation of the cardiac muscles of the heart is called
heartbeat.

New Creative Science, Class 10 | 255

The normal heart beat may increase with diseases, exercises, tension, emotions and
fever.

A heart beat of less than 60 beats per minute is known as bradycardia and more than
100 beats per minute is known as tachycardia.

DIABETES

Diabetes is a condition in which sugar level in the blood is high. It is due to hypo-
secretion of the insulin hormone from the pancreas. After the digestion of food, it
turns into glucose (sugar) molecules. This sugar gets absorbed by our body to produce
energy. The amount of sugar in the blood is controlled and balanced by an insulin
hormone. If the production of insulin is low then the sugar level in the blood is high.
As a result, sugar comes through urine.

Symptoms of diabetes
i. Hunger and fatigue
ii. Peeing more often and being thirstier
iii. Dry mouth and itchy skin
iv. Blurred vision
v. Unplanned weight loss
vi. Slow healing cuts
vii. Pain or numbness in feet and legs
viii. Nausea and vomiting

Preventive measure of diabetes
i. We should change our living style.
ii. We should have a balanced diet.
iii. It is necessary to control our body weight.
iv. We should be free from mental tension.
v. We should avoid drinking alcohol and smoking.
vi. We should have regular green vegetables in our food.
vii. We should do regular physical exercise.

Uric acid

Uric acid is produced from the natural breakdown of the body cells and the food that
we eat.

Most of the uric acid is filtered out by the kidneys, and it passes out of the body through
urine. A small amount of uric acid also passes out of the body through stool. If too
much amount of uric acid is produced in our body or our kidneys are unable to filter
it then the level of uric acid in the blood is high. If there is more amount of uric acid in
the blood then it makes solid crystals within the joints. This causes painful condition
called gout. High level of uric acid may cause kidney stones or kidney failure.

256 | Blood Circulatory System in Human Body

Causes of uric acid
i. It is caused due to poor functioning of the kidneys.
ii. It is due to more metabolism of the purine.
iii. It may be caused due to more amount of red meat, sea food, etc. in the diet.

Effects of uric acid
i. It causes dip pain in joints and muscles.
ii. It causes reddish skin, swelling in the skin and the muscle, burning in joints, etc.
iii. It causes problems in walking, bending and other physical works.

The way to reduce uric acid in blood
i. Drink sufficient water that decreases uric acid in the blood.
ii. Use baking soda that decreases uric acid in the blood.
iii. Eat cherries everyday as they contains anthocyanins that decrease uric acid in

the blood.
iv. Avoid fatty meat, red meat, sea food, excess proteins, etc.

MEMORY TIPS
 The heart in which the impulse for contraction is generated in the specialized collection

of cardiac muscle (sinuatrial node/SA node) is called myogenic heart. Human and
mammalian hearts are myogenic.
 The heart in which the impulse for contraction is generated in nerve cells is called
neurogenic heart. The frog’s heart is neurogenic.
 A pacemaker is the substance which can start electrical activity in the heart required
for cardiac contraction. So, the node is the natural pacemaker.

BLOOD GROUPS

Human blood can be divided into different blood groups according to the presence
or absence of certain protein molecules called antigens and antibodies. There are
four different kinds of blood groups A, B, AB and O. Blood group O +ve is called a
universal donor and blood group ‘AB’ +ve is called universal acceptors. (+) and (–) are
the rhesus (Rh) factor of the blood.

MEMORY TIPS
The systemic contraction of the heart can be felt as a jerk in the certain arteries like the
radial artery at the wrist, temporal artery in front of the ears, artery in the neck and facial
artery in the corners of the mouth. It is known as pulse. Its rate is the same as that of the
heart beat.

New Creative Science, Class 10 | 257

MODEL QUESTIONS ANSWER

1. Define blood. Write its main constituents.
Ü Blood is a fluid connective tissue which is red in colour. The main components

of blood are plasma- 55%, blood cells (RBC, WBC,Platelets)- 45%.

2. Differentiate between RBCs and WBCs.

S.N. RBCs S.N. WBCs

1. They are oval, biconcave 1. They are round cells with nucleus

structure without nucleus. and capable of changing shapes.

2. They are smaller than WBCs. 2. They are bigger than RBCs.

3. They are red in colour due to the 3. They lack haemoglobin and are white

presence of haemoglobin. in colour.

4. They help in respiration. 4. They help in defending and
protecting the body against foreign
toxic substances.

3. The walls of arteries are thicker than those of veins. Why?

Ü The blood pressure in arteries is very high than in veins. So, to balance the
pressure and to prevent the arteries from rupturing due to more force, the walls
of arteries are thicker than those of veins.

4. Veins have valves but arteries do not have. Why?

Ü The blood pressure in veins is low. So, there is the chance of back flow of blood.
So, valves are provided to prevent the back flow of blood. However, due to
more pressure of blood in the artery, there is no danger of back flow of blood
and valves are not necessary in arteries.

5. What would be the consequences of a deficiency of haemoglobin in our
bodies?

Ü Deficiency of haemoglobin causes anaemia in our bodies. We will not be able to get
sufficient oxygen for respiration and so that we get easily tired, lose weight, etc.

6. The wall of the left ventricle is thicker than the wall of the right ventricle. Why?

Ü The left ventricle has to pump blood to a long distance i.e. to all parts of the body
but the right ventricle has to pump blood to only a short distance i.e. to lungs.
Due to this, more pressure is required for the left ventricle and its wall is thicker.

258 | Blood Circulatory System in Human Body

7. The blood should be treated with sodium citrate after the donation. Why?
Ü The donated blood is treated with sodium citrate to avoid the blood clotting as

it removes fibrinogen from the blood and prevents blood clotting.

8. Name the valve between the left auricle and the left ventricle.
Ü Bicuspid valve/mitral valve or the left autrio-ventricular valve.

9. Name the diseases caused due to the:
i) Lack of haemoglobin/RBC
ii) Lack of WBC
iii) Systolic pressure becomes more than 120 mm of Hg
iv) Over production of WBC
v) Inability to clot blood

Ü i) Anaemia
ii) Leucopenia
iii) High blood pressure
iv) Leukaemia
v) Haemophilia

10. Why does blood not clot inside the body?
Ü Blood does not clot inside the body due to the presence of heparin or

antiprothrombin (anticoagulant) produced in the liver.

SUMMARY
 The circulatory system is the system of the body which helps in the transportation and

distribution of materials throughout the body.
 The circulatory system comprises blood, blood vessels and the heart.
 Blood is a fluid connective tissue which consists of 55% of blood plasma and 45% of blood

cells.
 Erythrocytes, leucocytes and thrombocytes are three types of blood cells.
 RBCs are red in colour due to the presence of an iron pigmented substance known as

haemoglobin.
 WBCs help in the defense of the body by producing antibodies and antigens.
 Platelets help in blood clotting.
 The main functions of blood are transportation, protection and regulation.
 The heart is the main pumping organ, which pumps blood to different parts of the body.
 Blood vessels are the muscular tubes, through which blood flows throughout the body. There

are three types of blood vessels i.e. arteries, veins and capillaries.
 Arteries are thick-walled blood vessels, which are deep seated in the body. They carry mainly

pure blood away from the heart.
 Veins are thin-walled blood vessels, which are superficial. They carry mainly impure blood

towards the heart.
 Capillaries are narrow, thin-walled blood vessels, where the exchange of substances takes place.

New Creative Science, Class 10 | 259

 The force, by which blood exerts pressure against the wall of blood vessels is known as blood
pressure.

 The normal blood pressure in human body is 120/80 mm of Hg.
 Heart beat is the rhythmic contraction and relaxation of the heart. The normal heart beats

72 times per minute.
 There are four different types of blood groups i.e. A, B, AB and O.
 There are two types of blood circulations in the human body i.e. i) Systemic circulation ii)

Pulmonary circulation.
 A heart consists of four valves, which prevent the back flow of blood.

EXERCISE

1. Define blood. Mention its various components.
2. What is plasma? Write the functions of plasma.
3. Differentiate between RBCs and WBCs.
4. What will happen if the numbers of RBCs decrease in the blood? Why is blood

red in colour?
5. Differentiate between:

(a) artery and vein
(b) pulmonary artery and pulmonary vein
(c) tricuspid valve and bicuspid valve
(d) auricles and ventricles
6. Mention the functions of blood.
7. Define pulmonary circulation and systemic circulation.
8. Give reasons.
(a) The left ventricle has a thicker wall than the right ventricle.
(b) Arteries are deep seated in the body.
(c) Arteries do not have valves.
(d) Blood in the pulmonary artery is violet and in the pulmonary vein it is scarlet.
9. Define heart beat. Why does it increase during physical exercises?
10. Draw a well-labeled diagram of the external structure of the heart.
11. Explain the working mechanism of the heart.
12. What do you mean by systemic circulation and pulmonary circulation?
13. The blood pressure of a person is 120/80 mm/Hg. What do you mean by it?

260 | Blood Circulatory System in Human Body

14. Study the given figure and answer the following questions.

(a) Label (A), (B) and (C) B
(b) Write the functions of (C) and (B). A

(c) Which of them is related to disease resistant?

15. State the functions of the following structures: C
(a) Tricuspid valve

(b) Pulmonary artery

(c) Pericardial fluid

(d) Mitral value

16. Why is our pulse taken in the wrist or the neck?

17. Write some functions of the blood circulatory system.

18. Define blood groups. Which blood group is known as ‘Universal donor’ and
which is known as ‘Universal acceptor’?

19. Which blood vessel brings pure blood from the lungs to the heart?

A

B GLOSSARY
C

Pulmonary : related to the lungs
Arterioles : branches of arteries

Venules : branches of veins

Bone marrow : the fluid found inside the bone which makes blood cells

Cardiac : connected to the heart

Deficiency : shortage

Emotions : strong feelings
Haemophilia
: a medical condition that causes severe bleeding from even a slight injury because the
blood fails to clot.

Propagate : to spread



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UNIT

17 Chromosomes and Sex Determination

About the Scientist INTRODUCTION

Robert Hooke All living organisms are made up of cells and cells are
known as the structural and functional units of life. The
Robert Hooke’s father was John growth and development of every organism depend on
Hooke, who was a curate at All Saints the multiplication and enlargement of its cells. All cells of
Church in Freshwater on the Isle of the body arise from the division of pre-existing cells. Thus,
Wight. Although formally a curate, the cell division provides the basis for the growth and
since the minister was also Dean of reproduction in living organisms.
Gloucester Cathedral and of Wells,
John Hooke was left in charge of All The division of cells takes place in two fundamental steps
Saints. It was a well-off church being i.e. (i) Karyokinesis (division of nucleus) which may or may
in the patronage of St John’s College, not be followed by (ii) Cytokinesis (division of cytoplasm).
Cambridge. As well as his duties in
the church, John Hooke also ran a Thus, cell division can be defined as the process in which a
small school attached to the church pre-existing cell divides into daughter cells by the process of
and acted as a private tutor. Robert karyokinesis and cytokinesis.
had a brother named John, the same
as his father, who was five years older. As we know, the nucleus of the cell plays an important role
Relatively few details of Robert’s in cell division. Let’s be familiar with its parts. The nucleus
childhood are known. What we is one of the most important parts of the cell. It contains
record here is information which he nucleoplasm, nucleolus and chromosome bound together
mentioned to his friends later in his by a double membranous nuclear membrane.
life. Robert, like many children of
his day, had poor health and was Eachorganismhas a fixed number of chromosomes.The small
not expected to reach adulthood. His change in the chromosome may also lead to vast changes in
father was from a family in which the living organisms. The number of chromosomes present
it was expected that all the boys in various living organisms is given below:
joined the Church (John Hooke’s
three brothers were all ministers) so S.N. Organism No. of chromosomes
had Robert enjoyed good health as a 1. Human beings 23 pairs (46)
child there is no doubt that he would 2. Frog 13 pairs (26)
have followed the family tradition. 3. Monkey 27 pairs (54)
As it was Robert’s parents did begin 4. Rice 12 pairs (24)
to set up his education with this in 5. Tiger 19 pairs (38)
mind but he continually suffered 6. Pea 7 pairs (14)
from headaches which made studying 7. Horse 32 pairs (64)
hard. Lacking confidence that he 8. Sugarcane 40 pairs (80)
would reach adulthood, Robert’s 9. Dog 39 pairs (78)
parents gave up on his education, 10. Lion 19 pairs (38)
leaving him much to his own devices.

262 | Chromosomes and Sex Determination

The chromosome is made up of nucleic acids and proteins. Genes are the structural
and functional units of chromosomes. Chromosomes are of two kinds.

SEX CHROMOSOMES

They are the single paired chromosomes which determine the sex of an individual.
They are represented by X and Y.

AUTOSOMES

They are other chromosomes which determine various other characters in the
individual. For example, the human body contains 22 pairs of autosomes and one pair
of sex chromosomes. The sex chromosomes in females are indicated and understood
as XX and that in males as XY.

The main functions of chromosomes are:
i. They transfer the genetic characters from parents to the offspring with the
help of gene.
ii. Sex chromosomes determine the sex or gender of the organisms.
iii. Due to the crossing over in chromosome, it may lead to variation and
evolution.

Types of cell division

In animals and plants, the following three types of cell division have been distinguished:
1) Direct cell division or amitosis
2) Indirect cell division or mitosis
3) Reductional division or meiosis

CHROMOSOME

We know that the body of an animal
and plant is made up of millions
and billions of cells. In each cell,
there are various cell organelles like
mitochondria, ribosomes, lysosomes,
nucleus, Golgi body, etc. In the
nucleus, there are many thread-like
structures called chromatin network.
They are very long and thin structures.
These chromatin structures cannot be
seen by our naked eyes but they can
be seen under the microscope. During
cell division, these structures become
short and thick. They are called chromosomes. Thus, chromosomes are the organized
structure of DNA and protein, which are present in the nucleus of the cell.

New Creative Science, Class 10 | 263

STRUCTURE OF THE CHROMOSOMES

The study of the chromosome revels that each chromosome is made up of DNA
molecule along with many histone proteins. Each molecule of DNA contains thousands
and thousands of molecules of nucleotides. In the DNA molecule, these nucleotides
join together to make a long chain. The length of the DNA molecule is about 8.5 cm. In
the chromosome, there is a constriction called the centromere or kinetochore. On the
basis of the position of the centromere, there are four types of chromosomes. They are:

Metacentric chromosomes

The chromosomes in which the centromere is present in the centre of the
chromosomes making equal arms are called metacentric chromosomes. In
these chromosomes, the arms are equal sized.

Sub-metacentric chromosomes

The chromosomes in which the centromere is present slightly at one side from
the centre making unequal arms are called sub-metacentric chromosomes. In
these chromosomes, the arms are not have equal sized.

Acrocentric chromosomes

The chromosomes in which the centromere is present towards the end of the
chromosomes are called acrocentric chromosomes. In these chromosomes, one
arm is very long and another arm is very short.

Telocentric chromosomes

The chromosomes in which the centromere is present at the tip of the
chromosomes are called telocentric chromosomes. In these chromosomes, one
arm is very long and another arm is invisible.

NUMBER OF CHROMOSOMES

The number of chromosomes in each cell of an animal or a plant is always fixed. But,
they vary from plant to plant and animal to animal. The number of chromosomes in
the gamete cell is just half as in the somatic cell. For example, in a somatic cell of the
human body, there are 46 chromosomes but in the gamete cell (ovum or sperm), there
are only 23 chromosomes. The chromosomes present in the somatic cell are called
diploid chromosomes and the chromosomes present in the gamete cell are called
haploid chromosomes. The diploid chromosomes are represented by “2n” and the
haploid chromosomes are represented by “n”.

264 | Chromosomes and Sex Determination

The number of chromosomes in different animals and plants

Organism Number of chromosomes

Human being 46

House fly 12

Frog 26

Gorilla 48

Allium cepa 16

Onion 16

Pinus 24

Mushroom 2

Function of chromosomes

i. Chromosomes are the storage genes.
ii. They transfer parental characters to the offspring.
iii. They determine sex of the offspring.
iv. They help in protein synthesis.

SEX DETERMINATION

In human beings, there are 23
pairs of chromosomes. Among
them, 22 pairs of chromosomes
are called autosomes and one
pair of chromosomes is called
sex chromosomes. Autosomes
determine the characteristics
of the body and the sex
chromosomes determine the sex
of the offspring.

In females, the sex chromosomes
produce same types of gametes,
which are called homogametes.
They are denoted by “x” and
“x”. So, the eggs of females
contain “x” chromosome.
Similarly, in males two different
types of gametes are produced,
which are called heterogametes. They are denoted by “x” and “y”. So, the sperms
of males contain “x” and “y”. The female gametes contain 100 % “x” chromosomes
whereas 50% male gametes contain “x” chromosomes and 50% “y” chromosomes.

New Creative Science, Class 10 | 265

During fertilization, if the male gamete of “x” chromosome fuses with the egg of the
female, it produces the female zygote. Similarly, if the male gamete of “y” chromosome
fuses with the egg of the female, it produces the male zygote. The female zygote
contains “xx” chromosomes and the male zygote contains “xy” chromosomes. The
male zygote develops into a boy and the female zygote develops into a girl. From
the above discussion, it is clear that, the female has no role in sex determination in
the offspring because the female produces homogametes. It is also clear that the
production of the male and female offspring has 50-50 chance as 50% male gametes
contain “y” chromosomes.

Sex linked disease

Sometimes, the number and structure of chromosomes get changed in an animal
or a plant cell. It is called chromosome disorder. Due to chromosome disorder,
different types of diseases may occur in animals. Some diseases are located in the sex
chromosomes. These diseases are called sex linked diseases. Thus, the diseases which
are present in sex chromosomes and which transfer from one generation to another
generation are called sex linked diseases. For example, colourblindness, haemophilia,
etc. The patients suffering from haemophilia show continuous bleeding through the
wound. Thereis no blood clotting. So, patients may die due to the deficiency of blood
in their body. Sex linked diseases occur only in one particular sex (either male or
female). The blind is a sex linked disease, which occurs in man only and breast cancer
is a sex linked disease which occurs only woman.

Chromosomal disorder

The problems which are seen in the human body due to change in number of
chromosomes are called chromosomal disorder. Chromosomal disorder occurs due to
increase or decrease of one or more chromosomes. Sometimes it may occur due to an
unusual combination or deletion of chromosomes. During the cell division, sometimes
the chromatids cannot be splatted in equal number. So, the number of chromosomes
may increase or decrease. This condition is called aneuploidy. The various syndromes
like down’s syndrome, klinefelter’s, syndrome turner’s syndrome, etc. occur due to
chromosomal disorder.

Down’s syndrome

In every cell in the human body, there is a nucleus. In the nucleus, there are
chromosomes. In the chromosomes, there are many genes. Genes carry the hereditary
information from one generation to the next. Each cell of the human body contains
23 pairs of chromosomes. In case of Down’s syndrome, there is one extra copy of
chromosome in 21 pair of chromosomes. Thus, the disorder in the human body, which
occurs due to one extra copy of chromosome in 21 pair of chromosomes is called
down’s syndrome.

266 | Chromosomes and Sex Determination

Down’s syndrome alters the course of development. It causes the characteristics
associated with Down syndrome. The children suffering from Down’s syndrome have
low muscle tone. They have small stature. They have an upward slant to the eyes
and a single deep crease across the center of the palm. Although each person with
Down’s syndrome is a unique individual they may possess the above characteristics
of different degrees.

Klinefelter’s syndrome

The disorder in the human body which occurs due to one extra copy of “x” chromosome
in sex chromosomes is called Klinefelter’s syndrome. In this syndrome the person
has xxxy chromosomes in 23 pair of chromosomes. As a result of this syndrome, the
person seems like a male but he has breasts. They are impotent in nature.

Turner’s syndrome

The disorder in the human body which occurs due to deletion of “x” chromosome
in sex chromosomes of the girls is called Turner’s syndrome. Turner syndrome is a
condition that affects only girls and women. It results when a sex chromosome (the
X chromosome) is missing or partially missing in 23 pair of chromosomes. Turner
syndrome can cause a variety of medical and developmental problems. It includes
a short height, failure to start puberty, infertility, heart defects, certain learning
disabilities and social adjustment problems.

Nearly all girls and women with Turner syndrome need ongoing medical care from a
variety of specialists. Regular checkups and appropriate care can help most girls and
women lead relatively healthy and independent lives.

MODEL QUESTIONS ANSWER

1. What is a haploid pair of chromosomes?

Ü The cells which have half number of chromosomes in their nucleus are called
haploid chromosomes. For example, in the sperms and ovum of the human
body, there are only 23 chromosomes. Such types of cells are represented by “n”.

2. What is a diploid pair of chromosomes?

Ü The cells which have diploid number of chromosomes in their nucleus are called
diploid chromosomes. For example, in the somatic cells of the human body,
there are only 46 chromosomes. Such types of cells are represented by “n”.

3. What are genes?

Ü The segments of DNA which are responsible to carry hereditary characters
from one generation to the next generation are called genes. They are the unit
of heredity. One gene carries only one character from one generation to the next
generation.

New Creative Science, Class 10 | 267

4. What is the main difference between the autosome and sex chromosomes?

Ü The chromosomes which determine the body characters of the organisms are
called autosomes. For example, in the human body, each cell has 22 pairs of
autosomes. Similarly, the pair of chromosomes which determines the sex of the
offspring is called the sex chromosome. For example, in the human body, each
cell has only one pair of sex chromosomes. It is represented by “xy”.

5. What is syndrome?

Ü The chromosomal disorder which causes different kinds of problems in the
human body is called syndrome.

6. What is the chromosomal disorder in different kinds of syndromes?

Ü The chromosomal disorder in different kinds of syndromes is

i) Down’s syndromes = xxx in 21 pair of chromosomes
ii) Klinefelter’s syndromes = xxy in 23 pair of chromosomes
iii) Turner’s syndromes = 0x in 23 pair of chromosomes in girls

e SUMMARY
 Chromosomes are the organized structure of DNA and protein, which are present in the

nucleus of the cell.
 Each chromosome is made up of the DNA molecule along with many histone proteins.
 The chromosomes in which the centromere is present in the centre of the chromosomes

making equal arms are called metacentric chromosomes.
 The chromosomes in which the centromere is present slightly at one side from the centre

making unequal arms are called sub-metacentric chromosomes.
 The chromosomes, in which centromere is present towards the end of the chromosomes are

called acrocentric chromosomes.
 The chromosomes, in which the centromere is present at the tip of the chromosomes are

called telocentric chromosomes.
 The chromosomes present in somatic cells are called diploid chromosomes.
 The chromosomes present in gamete cells are called haploid chromosomes.
 The diploid chromosomes are represented by “2n” and the haploid chromosomes are

represented by “n”.
 The diseases, which are present in sex chromosomes and which transfer from one generation

to another generation are called sex linked diseases.
 The problems, which are seen in the human body due to the change in the number of

chromosomes are called chromosomal disorder.
 The disorder in the human body which occurs due to an extra copy of chromosome in 21 pair

of chromosomes, is called Down’s syndrome.
 The disorder in the human body, which occurs due to one extra copy of “x” chromosome in sex

chromosomes is called Klinefelter’s syndrome.
 The disorder in the human body, which occurs due to deletion of “x” chromosome in sex

chromosomes of the girls is called Turner’s syndrome.

268 | Chromosomes and Sex Determination

EXERCISE

1. Define the following terms.

(a) Chromosomes (b) Centromere

(c) Metacentric chromosomes (d) Sub-metacentric chromosomes

(e) Acrocentric chromosomes (f) Telocentric chromosomes

(g) Syndromes (h) Down’s syndrome

(i) Klinefelter’s syndrome (j) Turner’s syndrome

2. Differentiate between:

(a) Klinefelter’s syndrome and Turner’s syndrome

(b) autosomes and sex chromosomes

(c) haploid and diploid

3. Give reason.

(a) Male is responsible for sex determination

(b) Gene is called the unit of heredity

4. Answer the following questions.

(a) What are chromosomes?

(b) Classify chromosomes on the basis of the centromere.

(c) What is chromosome disorder? Write down the names of different types of
syndromes.

(d) How is sex determined? Explain with a figure.

A

B GLOSSARY
C

Autosomes : the chromosomes which determine characters
the chromosomes which determine sex
Sex chromosomes : the constriction of the chromosomes
chromosomes which are present in pair
Centromere : chromosomes which are present in one set only
change in number of chromosomes
Diploid :
Haploid :

Chromosomal disorder :



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UNIT

18 Reproduction in Animals and Plants

About the Scientist INTRODUCTION

Louis Pasteur Reproduction is a special biological process related to the
(1822) production of new generation of individuals of the same
species. Though, the reproduction is not an essential
Louis Pasteur was born on process for the survival of an individual, it is essential for
December 27, 1822 in Dole, in multiplication and continuation of the species.
the region of Jura, France. His
discovery that most infectious Reproduction provides group immortality by replacing
diseases are caused by germs, the dead individuals with new ones for the survival of the
known as the “germ theory species on the earth.
of disease,” is one of the most
important in medical history. Reproduction may be defined as the production of new generation
His work became the foundation of individuals of the same species that are physically independent
for the science of microbiology, of their parents.
and a cornerstone of modern
medicine. A. ASEXUAL AND SEXUAL REPRODUCTION

Pasteur’s phenomenal The reproductive methods of the living organisms are
contributions to microbiology broadly categorized into two types:
and medicine can be summarized
as follows: First, he championed i) Asexual reproduction
changes in hospital practices ii) Sexual reproduction
to minimize the spread of
disease by microbes. Second, he i) Asexual reproduction
discovered that weakened forms
of a microbe could be used as Asexual reproduction involves the participation of a
an immunization against more single individual parent. It occurs without using the sex
virulent forms of the microbe. organs or formation of gametes. It is a common method of
Third, Pasteur found that rabies reproduction in lower plants and animals like in unicellular
was transmitted by agents so organisms; algae, fungi, bryophytes, sponges, hydra, etc.
small they could not be seen under
a microscope, thus revealing the Asexual reproduction may be defined as the production of young
world of viruses. As a result, he ones by a single parent without the formation of gametes or
developed techniques to vaccinate fertilization or meiosis cell division.
dogs against rabies, and to treat
humans bitten by rabid dogs. It occurs due to the mitosis cell division. It is an easy and
And fourth, Pasteur developed rapid method of reproduction in which the young ones
“pasteurization,” a process by posses all the genetic characters of the parents.
which harmful microbes in
perishable food products are
destroyed using heat, without
destroying the food.

270 | Reproduction in Animals and Plants

It occurs in the following ways:
a) Fission
b) Budding
c) Sporulation
d) Fragmentation and regeneration
e) Vegetative propagation

a) Fission

It is a common method of asexual reproduction in unicellular organisms, in which the
organism breaks into two or more halves.

Thus, fission may be defined as the process of breaking of a parent cell into two or more separate
daughter cells.

The fission may also be of two types: Binary fission and multiple fission.

Binary fission

In this process, a parent cell divides into two nearly equal sized similar daughter
cells. In this process, the nucleus divides first followed by the division of cytoplasm.
According to the plane of division, the binary fission may be simple, transverse,
longitudinal and oblique. It takes place in the favourable conditions. This process is
common in amoeba, euglena, paramecium, bacteria, diatom, etc.

Daughter amoeba

Mother amoeba

Binary fission in amoeba

Multiple fission

In the multiple fission, the nucleus of the cell divides very rapidly into many nuclei.
Each daughter nuclei is surrounded by the little mass of the cytoplasm. In this
method, a thick covering or cyst is formed around the cell under the unfavourable
condition and on getting the favourable condition of food and water, the cyst breaks
and small offspring are released and they live their independent life. It is common in
plasmodium, amoeba, monocyst, chlamydomonas etc.

New Creative Science, Class 10 | 271

Multiple fission in plasmodium

MEMORY TIPS
Malaria parasite (plasmodium) produces about 100 daughter cells from one parent at a
time, each capable of invading RBC.

b) Budding Budding in yeast

This type of asexual reproduction is common in yeast, some
protozoans, coelentrates (hydra) and other lower animals. In
this process, the body of the parent gives out a small outgrowth
known as a bud. The bud is supported by the parent body and
it ultimately develops into a new individual. The developing
individual gets food from the body of the parent and when
it becomes fully mature, it is detached from the body of the
parent and leads an individual existence.

Budding in hydra

MEMORY TIPS
The formation of a bud takes place by the amitosis division of the nucleus and cytoplasm.

ACTIVITY

Fill a conical flask with water. Cover the neck of the flask with a wire mesh.
1. Dissolve about 10 gm of sugar in 100 ml of water.
2. Take 20 ml of this solution in a test tube and add a pinch of yeast granules to it.
3. Put a cotton plug on the mouth of the test tube and keep it in a warm place.
4. After 1 or 2 hours, put a small drop of yeast culture from the test tube on a slide and cover it with a

cover slip.
5. Observe the slide under a microscope.

272 | Reproduction in Animals and Plants

QUESTIONS
# How does binary fission differ from multiple fission?

c) Sporulation

Spores are produced by spore mother cells in sporangia.
They gets detached from the parents and give rise directly or
indirectly to a new individual.

This method of reproduction is common in some bacteria and Sporulation in muc or
most of the fungi. During the formation of spores, the fungal

hypha develops on sporangium whose nucleus divides many times. Each nucleus gets

surrounded by cytoplasm and develops into a spore. Each spore grows into a new

hypha after germinating. Rhizopus, mucor; aspergillus, penicillium etc. reproduce by

sporulation.

MEMORY TIPS
Non-flagellated and non-motile spores are called aplanospores and the flagellated and
motile spores are called zoospores.

ACTIVITY

Wet a slice of bread, and keep it in a cool, moist and dark place. Observe the surface of the slice with a
magnifying glass after 2-3 days.

d) Fragmentation and Regeneration

The process in which an organism breaks into fragments and are formed into new
organisms from these fragments is called fragmentation and regeneration.

Regeneration is common in lower plants (e.g; Algae, fungi etc.) and animals (hydra,
planaria and sponges).

A mature filament of spirogyra breaks into two or more pieces and each piece grows
into a new individual.

Mother planaria Daughter
Regeneration of planaria planaria

New Creative Science, Class 10 | 273

MEMORY TIPS
The term ‘fragmentation’ is generally used for plants and the term ‘regeneration’ is used
for animals.

e) Vegetative propagation

It is common in higher plants. In this process, parts of the plants other than seeds are
used as propagules. It is more useful in those plants which have lost their capacity
to produce seeds or produce non-viable seeds, e.g. banana, seedless grapes, rose,
pineapple, etc. It is of two types:

(i) Natural vegetative propagation
(ii) Artificial vegetative propagation

(i) Natural vegetative propagation

This type of vegetative propagation occurs naturally and takes place through the
following parts:

Vegetative propagation through roots: The adventitious roots of some plants like
dahlia, mint, sweet potato, etc. reproduce by means of roots. These roots contain the
adventitious buds through which new leafy shoots can be developed.

Dahlia Sweet potato

Vegetative propagation by roots

Vegetative propagation through stem: Underground
modified stems such as rhizomes (e.g. ginger, water
hyacinth), corms (banana, colocasia), bulbs (e.g.
garlic, onion etc.) also give rise to the new plants.
The rhizome is a short and swollen underground
modified stem having buds, scale leaves and
adventitious roots.

The corm is a swollen underground modified stem
having a swollen base. It remains surrounded by scale leaves with one or more buds
on it.

The bulb is an underground modified stem having a short shoot and fleshly storage
leaves.

274 | Reproduction in Animals and Plants

Some plants like potatoes reproduce asexually by means of a tuber. A tuber is a swollen
underground modified stem having buds. Similarly bamboo, pineapple, banana, rose,
sugarcane also propagate vegetatively from the stem.
Vegetative propagation through leaves: Some plants develop adventitious buds on
their leaves which develop into new plants, e.g. bryophyllum, begonia, ctc.

Vegetative propagation through leaves
Advantages of vegetative propagation
i) The plants which are unable to produce viable seeds can be easily propagated

by this method, e.g. sugarcane, banana, rose, potato etc.
ii) Plants raised by vegetative propagation bear flowers and fruits earlier than

other plants of the same species.
iii) All plants produced by this method are generally similar to the parent plants

and have all its characteristics.
iv) It is a cheap and rapid method of reproduction.
v) It can give disease-free plantlets (through tissue culture).
vi) We can take both the advantages of better root system of one plant and more

efficient shoot system of another plant.
Advantages of Asexual reproduction
i) It is a rapid and cheap method of reproduction, as it does not need two

individuals for reproduction.
ii) It is better for seedless plants and the plants bearing non-viable seeds.
iii) They give their young ones exactly the same characters of their parents.

New Creative Science, Class 10 | 275

QUESTIONS
# Why is vegetative propagation practised for growing some types of plants?
# Write some advantages of asexual reproduction.

Sexual reproduction

The production of the offspring by the fusion of male and female gametes produced due to the
meiosis cell division is called sexual reproduction.

In this process, male and female gametes unite to form a zygote. It finally develops
into a new organism. The male reproductive part of living organisms produces male
gametes and that of female reproductive part produces female gametes.

The fusion of male and female gametes takes place to form a zygote and this process
is known as fertilization. The zygote divides mitotically and grows into an embryo,
which finally develops into a new organism.

The sexual mode of reproduction involves two individuals- one acting as a male and
the other acting as a female. In majority of the higher animals, the male and female are
separate i.e. an individual may be either a male or a female. Such organisms are called
unisexual or dioecious.

When both of the reproductive organs are found in the same individual, they are
called bisexual or hermaphrodite, e.g. tapeworm, earthworm, hydra, and other lower
organisms.

Features of sexual reproduction

1. In the second reproduction male and female gametes are involved in fertilization.
2. The meiosis cell division takes place during the formation of gametes. So, the

gametes are haploid. These gametes are united during fertilization forming a
diploid zygote.
3. It is complicate type of reproduction, which takes a long time to complete.
4. Products of sexual reproduction show variation, i.e. they are not genetically
similar to their parents.

External and Internal fertilization

The fertilization which takes place outside the body of the female is known as external
fertilization. Fishes and amphibians have external fertilization.

If the fusion of male and female gametes takes place inside the body of a female, then it is known
as internal fertilization. Insects, birds, mammals, reptiles have internal fertilization.

MEMORY TIPS
Sexual reproduction may be syngamy, conjugation or autogamy.

276 | Reproduction in Animals and Plants

QUESTIONS
# What are the advantages of sexual reproduction over asexual reproduction?

Sexual reproduction in plants

In flowering plants, all the steps of sexual reproduction occur within the flower. So,
the flower is also known as the main reproductive organ of the flowering plants.

A typical angiosperm flower consists of four whorls attached to the receptacle or

thalamus. The receptacle is the swollen top of the flower stalk. The four whorls of a

flower are:

i) Calyx ii) Corolla iii) Androecium iv) Gynoecium

i) Calyx

It is the outermost whorl of the flower. Each member of the calyx is known as a sepal.
They are usually green in colour. They protect the flower during the bud stage.

ii) Corolla stigma petal
style anther
It is the second whorl of the flower. Each member filament
of the corolla is known as petals. They are usually
bright, colourful and scented. They attract insects
for pollination.

iii) Androecium sepal

It is the third whorl of the flower and also the male ov ary receptacle

reproductive part of the plant. Each member is

known as a stamen. A stamen is divided into anther and filament. The anther consists of

the pollen sac (sporangia) which produces male gametes or microspores or pollen grains.

iv) Gynoecium

It is the fourth and innermost whorl of the flower and also the female reproductive
part of the plant. Each member is known as the pistil or the carpel. The pistil is
differentiated into three different parts stigma, style and ovary. Ovary produces
megaspore or female gametes or ovules.

Parts of flower

New Creative Science, Class 10 | 277

Pollination

The transfer of pollen grains from the anther of stamen to the stigma of pistil is called pollination.

It is an important process that facilitates successful fertilization in plants. The common
agents responsible for successful pollination are wind, water, insects, birds, bats and
animals.

Types of pollination

Pollination is of two types: (i) Self pollination (ii) Cross pollination

i) Self pollination

It involves the transfer of pollen grains from the anther of a flower to the stigma of the
same flower or the stigma of another flower which is born on the same plant.

ii) Cross pollination

It involves the transfer of pollen grains from the flower of one plant to the stigma of
the flower of another plant of the same species.

Cross pollination

Self pollination

Pollination

Fertilization in plants

After the deposition of pollen grains over the stigma,
they absorb water. They swell and then germinate to
produce pollen tubes. One of the pollen tubes grows
into stigma, passes through the style and moves
towards the ovary. The pollen tube finally pierces the
egg through micropyle.
The mature embryo sac consists of an egg apparatus
(one haploid egg and two synergids), two polar nuclei
and three antipodal cells.

278 | Reproduction in Animals and Plants

During fertilization, one male gamete fuses with the egg to form the diploid zygote.
The diploid zygote finally develops into an embryo.
The other male gamete in pollen grain fuses with the polar nuclei to form triploid
(3n) primary endosperm nucleus. This process is called triple fusion and the whole
mechanism is called double fertilization. After fertilization, the zygote develops into an
embryo, the ovule develops into a seed and the ovary develops into a fruit. It can be
summarized as:

Summary of fertilization
MEMORY TIPS
Due to the triple fusion or double fertilization process, fruits are developed in angiosperm
but the gymnosperms lack this process. So, gymnosperms do not bear fruits.

ACTIVITY

Study the different types of flowers near your school or locality and differentiate the
structures of whorls in those flowers.

New Creative Science, Class 10 | 279

Difference between self- pollination and cross-pollination.

S.N. Self Pollination S.N. Cross Pollination

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

from the anther to the stigma of from the anther of one flower to the

the same flower or another flower stigma of another flower in the other

of the same plant. plant.

2. It preserves the parental 2. It does not preserve the parental

characters. characters.

3. External agents of pollination are 3. External agents of pollination are

not required. required.

4. Variation does not take place in 4. Variation takes place in the offspring.

the offspring.

5. It takes place in bisexual flowers. 5. It takes place in unisexual as well as

bisexual flowers.

Sexual reproduction in animals

Sexual reproduction is the only mode of reproduction in higher animals. The higher
animals are usually unisexual. The male produces male gametes or sperms and the
female produces female gametes or ova/eggs. Sperms are motile and have a tail
which is produced by the process known as spermatogenesis in testes. Similarly, ova
are non-motile, larger and do not bear a tail. The process by which eggs are produced
is known as oogenesis. On favourable condition, the male and female gametes fuse
to form a zygote. This process is fertilization and the fertilization may be external
or internal. The zygote is the diploid structure formed after the fertilization of two
haploid gametes. The zygote divides mitotically and forms an embryo which finally
develops into a young animal.

Advantages of sexual reproduction

1. It brings variation and helps the living organisms to get adopted to the particular
environment.

2. It gives continuity to the generation.
3. The offspring of sexual reproduction exhibit diversity of characters.
4. It helps in the origin of new species and leads to variation required for evolution.

MEMORY TIPS
In human females, fertilization takes place inside the fallopian tube.

ACTIVITY

Visit a nearby fishery (fish farm) observe the egg, larva and adult of the fishes and draw their
figures

280 | Reproduction in Animals and Plants

B. ARTIFICIAL VEGETATIVE PROPAGATION IN PLANTS

Artificial vegetative propagation is a commercial technique of reproducing/
propagating plants rapidly by using their vegetative part. It is usually used to
reproduce those plants that produce either very few seeds or do not produce viable
seeds. It is widely used in agriculture, horticulture and floriculture sector for the quick
and large scale production of plants. It provides an opportunity for propagation of
best selected varieties of plants. Bananas, pineapples, oranges, grapes, roses, pears,
etc. are some of the common examples of plants that are propagated by artificial
vegetative propagation.

Layering, grafting and tissue culture are the widely used artificial vegetative
propagation techniques and is therefore they are discussed briefly in this unit.

I. Layering

The development of roots on a stem while the stem is still attached to the parent plant
is called

Layering. A layer is the rooted stem following detachment (removal) from the parent
plant. Layering is popular technique in horticulture. Some types of layering are
explained below.

a. Simple layering: This method produces a new plant
from a low growing flexible stem. The low growing
flexible stem is bent so that its bend get dipped into
the shallow trench of about 10-15 cm. The bending
part in the trench is covered with soil leaving 20-25
cm of the branch tip part above the soil. After 2-3
months of the growing season, the branch develops rooting. Bending the branch
in a vertical position and wound the lower side of the bent branch induce rooting
faster. This method is used in lemon, forsythia, honeysuckle etc.

b. Compound layering: This method produces many new plants from a single
flexible stem. For compound layering, a flexible stem near to the ground is bent
to the shallow trench like simple layering. But here
the number of trenches are more and the buds in the
section of the stem is alternately exposed and covered
with soil in shallow trenches. Roots develop from
each of the buds covered in the soil. This method is
used in sweet potatoes, pears, apples, grapes, heat-
leaf philodendron, etc.

New Creative Science, Class 10 | 281

c. Mound layering: It is used to produce new plants from plants with heavy stems,
closely branched shrubs and rootstocks of tree
fruits. For mound layering, the plant is cut back
to about 3 cm above the soil surface in the
dormant season. Dormant buds produce new
shoots in the spring. As the new shoots grow
soil is mound over them. Roots will develop at
the bases of the young shoots. The layer should
be then removed in the dormant season.

d. Tip layering : Tip layering is a type of artificial
vegetative propagation in which the branch of
a tree is buried into the soil and a new plant is
grown up. In this method, 5-8cm of the branch
of a tree is buried into the soil for about 3-4
months. In this period, roots and branches
come out from the parent branch. After that the parent branch is separated from
the tree for its independent growth.

e. Air layering : Air layering is a type of artificial vegetative propagation, in which
the branch of a tree is cut and buried into the soil to grow a new plant.

It is the most popular method of artificial
vegetative propagation. First of all, a branch
of about two years old is selected for the air
layering. The bark of the branch is removed
making a ring-like structure. It protects the
circulation of food from the leaves to the roots.
It also helps to germinate roots at the part cut.
At the area of the this part, moist soil and some
algae are kept covering with plastic. Some growth hormone is supplied to it.
After about 4-8 weeks, roots start to come from the part cut. After about one
month, the branch is removed from the tree and sown in the ground for further
germination.

ACTIVITY

Produce layers of one of the plants like litchi, grape, sweet potato, etc. using compound layering.

II. Grafting

Grafting is the method of joining the parts of two separate plants so that they will
unite and continue to grow as a single plant. With such a quick method of uniform
propagation, plants with similar characteristics and disease resistance in rootstocks

282 | Reproduction in Animals and Plants

have advantages of grafting. The scion and stock of two compatible plants are joined
in this method. The scion is part of the union to be attached to the rootstock that
forms the bearing parts and the shoot system. The stock is the part of the union, which
contains the root portion of the union that forms the lower trunk and root system of
the tree. For the success of grafting, the cambial layer of the scion and the stock need
to make contact with each other. The cambium is the regenerative tissue or the layer
of cells, located between the bark and the wood of the plant.

Some common methods of grafting are mentioned below.

a. Whip Grafting : The whip graft is useful for plants that unite easily. This method
is useful for apples, mangos and pears. It can be used to graft the root, stem or
the top graft. The diameter of the scion and the rootstock should be the same,
from the size of a pencil to 10-15 mm.

a. Simple whip grafting: This type of grafting practice
includes the process of a simple sloping cut on both
the scion and the rootstock. The two parts should
overlap each other perfectly. The wider the scion
and the root stock, the longer should be the cut
surface. The joint of the scion and the stock need to
be wrapped air tight. Within 3-4 months, the wound
in the joint gets recovered.

b. Tongue whip grafting : This method is more
common in practice, especially in the case of pear
and apple trees. The scion should have two or three
buds with the graft made below the bottom bud.
The first cut is a 2-5 cm sloping cut at the bottom of
the scion. The second cut is made with a distance of
1/3 cm from the tip of the first cut making a tongue.
The same process is repeated on the rootstock so
that the tongue of the scion fits in the tongue of the stock. The joint is
wrapped air tight using plastic and within 3-4 months, the joint gets
recovered.

b. Saddle grafting : In this method, the stock is cut in the
shape of a wedge (inverted V shaped). The scion is also
cut carefully creating an inward inverted V shaped slot
that exactly fits the stock wedge tightly in it. The scion
and the stock are joined using grating wax and wrapped
air tight by grafting a tape or plastic.

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c. Cleft grafting: In this method, branches of the stock are sawed straight across,
and the stub of the branch is split to a depth
approximating the diameter of the stock. A
temporary wedge sufficient to open the slit is
inserted at the centre of the split. Two scions, each
having several buds, are inserted at the edges of
the split, then adjusted so that the lowest bud is
close to the top of the stock facing outward. The
wedge is removed without displacing the scions,
and the cut is covered with grafting wax and wrapped air tight by plastic.

ACTIVITY

Take a scion and a stock of a local fruit or flower. Propagate it by saddle grafting and transfer it
to your school garden/ nursery.

TISSUE CULTURE

Tissue culture is the method of producing a number of plants in a cultured medium
under sterile condition by using small pieces of plant tissues (explant). It is also
known as micro-propagation. Mass production of plants with no seeds or non-viable
seeds, production of identical plants with identical plants with common features of
sizes, colours, quality, etc., production of disease free plants and production of mature
plants in s short time are the major reasons for the widespread development of tissue
culture.

Tissue culture involves six major steps. They are:

Preparation and sterilization of growing medium

Murashige Skoog (MS) media is the growing medium for the plant tissue which is
prepared in sterile distilled water. Agar, sucrose and growth hormone along with
other substances are added to it. The growing media is kept in tubes, capped tightly
and sterilized using autoclave or boiling in water.

Preparation of sterile transfer chamber and equipment

The working desk is sterilized using chlorox bleach and other disinfectants. Equipment
like razors, forceps, scalpels, etc. are sterilized autoclave or boiling in water.

Plant preparation: The plant part is washed cleanly with detergent water and sterilized
using chlorox solution. If the plant part is infected with bacteria or fungi, they may
destroy it or may result in diseased plantlets.

284 | Reproduction in Animals and Plants

Transfer of plant material to tissue culture medium

Discard the tissue of prepared plant part if it is destroyed by bleach. Carefully transfer
the sterilized plant tissue/ plant part in the tube containing growing medium using
sterilized forceps and replace the cap of the medium tightly.

Growing plants

A yellow-brown mass of cells called callus develops from the piece of the plant. Small
chunks of the callus are separated using sterilized equipment and each piece is placed
in a sterile Petri dish with a hormone like auxin and cytokinin along with nutrient mix
that stimulates the development of the callus pieces into plants. Auxin promotes shoot
formation and cytokinin promotes root formation.

Potting the plants

The young plants, after the development of shoots and roots, are removed from the
Petri dish and placed in pots with soil. They also can be placed into a controlled
greenhouse or the ground, where they grow to maturity.

ADVANTAGE OF TISSUE CULTURE

i. Propagation of plants from any of vegetative parts or tissues of the plant.
ii. Mass production of healthy plants in a short time.
iii. Preservation of endangered and near to extinct plant species.
iv. Production of plants with same characteristics that are in their parents.

MEMORY TIPS
Parthenogenesis is another type of asexual reproduction in which an organism is developed
from unfertilized eggs.

ACTIVITY

1. Take an average sized potato tuber and observe its surface.
2. From where do the new buds get their food?
3. From which part of the potato gives rise to new buds?

MODEL QUESTIONS ANSWER

1. Explain the various steps of budding in yeast.

Ü During budding, a small bud like outgrowth arises at one end of the parent cell.
Slowly, it enlarges in size. The nucleus at first, enlarges and divides into two,
out of which one remains in the parent cell and the other migrates into the bud.
Later cytoplasm divides and the bud grows into a new individual. It separates
out from the parent body and leaves freely.

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2. What are the advantages of sexual reproduction over asexual reproduction?
Ü Sexual reproduction has the following advantages over asexual reproduction.

i. The offspring shows diversity of characters in sexual reproduction.
ii. It plays a prominent role in the origin of new species and leads to evolution.

3. Regeneration cannot be regarded as reproduction. Why?
Ü In regeneration, an organism is cut or broken up into many pieces. These pieces

then grow into separate individuals. In nature, most of the organisms would not
normally depend on being cut off to be able to reproduce. They would like to
survive themselves as well as reproduce new individuals.

4. Asexual reproduction is advantageous to farmers. Why?

Ü It is because, asexual reproduction is a rapid method of reproduction, in which
flowers and fruits are developed after a short duration. It is an easy and cheap
method of reproduction.

5. Gametes are haploid and zygotes are diploid. Why?
Ü Gametes are formed after meiosis cell division and the zygote is the structure

formed after the fusion of two haploid gametes.

6. The fertilization in amphibians is called external fertilization. Why?
Ü It is so because, the fusion of sperms and eggs takes place in water i.e. outside

the female body.

7. Differentiate between sperm and ova.

S.N. Sperm S.N. Ova

1. It is a male sex cell. 1. It is a female sex cell.

2. It is motile. 2. It is non-motile.

3. It is smaller in size. 3. It is bigger in size.

4. Generally, a large number of 4. Less number of ova are produced

sperms are produced at a time. at a time.

SUMMARY

 Reproduction is the process of production of new young ones from the parent body.
 There are two types of reproduction i.e. sexual and asexual.
 Budding, fission, regeneration, sporulation and vegetative propagation are the types of asexual

reproduction.
 In fission, an organism breaks into two or more nearly equal halves. It may be binary fission or

multiple fission.
 In budding, a small bud develops in a parent body. It is common in hydra, yeast and other fungi.
 In sporulation, spores are produced, which on getting favourable conditions germinate into

new plants.
 The ability of an individual fragment to replace its lost parts is called regeneration.

286 | Reproduction in Animals and Plants

 In vegetative propagation, new plants are developed from the vegetative parts like roots, stem,
leaf, etc.

 Artificial vegetative propagation is a commercial technique of reproducing/ propagating
plants rapidly by using their vegetative part.

 The development of roots on a stem while the stem is still attached to the parent plant is
called Layering.

 Simple layering, compound layering, mound layering, etc. are the methods of layering.
 Grafting is the method of joining the parts of two separate plants so that they will unite and

continue to grow as a single plant.
 Simple whip grafting, tongue whip grafting, saddle grafting, cleft grafting, etc. are different

methods of grafting.
 Tissue culture is the method of producing a number of plants in a cultured medium under

sterile condition by using small pieces of plant tissues (explants).
 Auxin and cytokinin are growth hormones that promotes, shoot formation and root formation,

respectively.
 Tissue culture is a modern technique of asexual reproduction.
 Sexual reproduction is the process, which involves the fusion of male and female gametes.
 Pollination is the process of transfer of pollen grains from the anther to stigma of the flower.
 Fertilization means the fusion of male and female gametes after pollination.
 Fertilization leads to the diploid structure known as the zygote.
 The organism having both male and female sex organs in the same body are known as bisexual

or hermaphrodite or monoecious.
 The organisms having male and female sex organs in different parts of the body are known as

unisexual or dioecious.

EXERCISE

1. Define reproduction. How many types of reproduction are there? Name them.

2. Define asexual reproduction. What are its characteristics?

3. What is fission? Name its types.

4. Describe the multiple fission in plasmodium.

5. Define budding. Describe its process in hydra.

6. Name the method of asexual reproduction common in the given organisms.

(a) Amoeba (b) Hydra (c) Plasmodium (d) Spirogyra

(e) Monocyst (f) Potato (g) Bryophyllum (h) Onion

(i) Sweet potato

7. Differentiate between:

(a) male gametes (sperms) and female gametes (ova)

(b) spores and gametes

(c) pollination and fertilization

(d) spores and zygote

8. Draw a figure to show fertilization in plants.

New Creative Science, Class 10 | 287

9. Write in short about the sexual reproduction in animals.
10. List the advantages of vegetative propagation.
11. List the advantages of asexual reproduction.
12. Study the given diagram and answer the questions asked.

(a) (b)

(c) (d)
(a) Which one of them shows vegetative propagation?
(b) What is the name of the animal shown in (a)? What is the method of

reproduction in it?
(c) What type of reproduction is shown by (c)?
(d) Which one of them is the simplest method of reproduction?
13. ‘Fusion of gametes in angiosperm’ is called double fertilization. Why?
14. Define monoecious and dioecious plants.
15. Define hermaphrodite and give two examples of it.
16. Label the parts ‘A’, ‘B’, ‘C’, ‘D’, ‘E’ and ‘F’ in the following figure. Assign the
role played by ‘B’ and ‘F’

288 | Reproduction in Animals and Plants

17. Only some flowers of cucurbita, pumpkin and bitter gourd set fruits whereas
others fail. Why?

18. What is syngamy? What happens after syngamy?

19. Define.

(a) Layering (b) Layer (c) Grafting (d) Scion
(e) Stock (f) Tissue culture (g) Explants (h) Callus

20. Give reasons.

(a) Auxin and cytokinin are added into the nutrient medium. Why?

(b) Why grapevines are preferred plants for layering?

21. Differentiate between:

(a) layering and grafting

(b) simple layering and compound layering

(c) simple whip grafting and tongue grafting

22. Answer the following questions.

(a) Describe the method of compound layering in plants.

(b) How is tongue grafting done in an apple? Explain it in short.

(c) List out the major steps to be considered in tissue culture.

(d) What are the advantages of tissue culture?

A

B GLOSSARY
C

Bud : a projection formed at the side of the body

Prepagules : the structural unit that is employed in place of seed

Germ cells : sex cells

Immortality : the state of being alive forever

Propagate : to spread



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UNIT

19 Heredity

About the Scientist INTRODUCTION

Gregor Johann Mendel It is commonly seen that members of a species are largely
(1822-1884 alike. A cow resembles other cows, a rose plant looks alike
other rose plants, children resemble their parents. It is a
Gregor Johann Mendel (1822- well-known fact that ‘like begets like’, like, which implies
1884) was born in a peasant family continuity of life. It is, however, not absolutely true as the
of Maravia. Due to poverty, he members of a species are seldom exactly alike i.e. a son may
became a monk in 1843 and resemble his parents or grandparents but he is not exactly
later in 1847, was made an abbot the same as them. The similarities and differences among
(head) of Augustinian monastery the members of a species are not coincidental; they are
of St. Thomas at Brunn, Austria received from their parents by the young ones.
(now Bruna in Czech Republic).
From here, he went in 1851 to Heredity is the passing of characters from one generation to
the University of Vienna where another whereas variations are departure from a complete
he studied natural history and similarity between the successive generations.
mathematics for two years. He also
became interested in the process of ‘Genetics’ is the branch of biology which deals with heredity
hybridization. Mendel returned to and variations. The term ‘genetics’ was coined by William
the monastery in 1853 and also Bateson in 1906.
worked as a teacher in a school.
From 1856 to 1865, Mendel HEREDITY
conducted breeding experiments on
garden pea plant (Pisum sativum) The resemblance of the offspring with their parents is due
in the garden of his monastery. to heredity. A chick cannot be hatched from a duck's egg,
He published his findings in 1866 calves cannot be given birth by a horse, a puppy is similar
in the ‘Annual Proceedings of the to its parents. All of these processes are due to the heredity.
Natural History society of Brunn’. Thus,
His findings remained unknown
until these were rediscovered by Heredity is the process of transmission of characters from parents
three biologists namely Hugo to the offspring, i.e. from one generation to the next. It is also
Devries of Netherland, Karl known as inheritance.
Correns of Germany and Erich
Van Tshermak of Austria VARIATION
independently in 1900. The work
of Mendel was published in 1901. Heredity involves inheritance of the similarities as well as
Mendel’s findings formed the basis some changes in the body of living organisms from one
of the science of genetics and he is generation to the next. Asexual reproduction involves a
credited as the father of genetics. single parent. So, the individuals produced through asexual

290 | Heredity

method would be similar. However, there would be very minor differences between
them, which arise due to the small inaccuracies in DNA copying.
Sexual reproduction, on the other hand, generates greater diversity among individuals.
Thus, the differences in the characteristics shown by the individuals of a species and
also by the offspring of the same parents are referred to as variations.

The causes of variation

(i) Separation of chromosomes during gametes formation
(ii) Crossing over during meiosis
(iii) Mutations
(iv) Environmental factors like scarcity of food, temperature of surroundings, etc.

Variation may be of two types:

(a) Hereditary variation: The differences seen in an organism due to the chromosomal
changes during the meiosis cell division in the sexual reproduction are known
as hereditary variation. These variations pass from generation to generation
through genes. It is an example of continuous variation.

(b) Environmental variation: The organisms having the same genetic make-up
may also seem to be different due to the changes in the environmental factors
like heat, light, food, temperature, etc. These changes in an organism due to the
environmental factors are called environmental variations.

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Significance of variation

i. Variation helps in organic evolution.
ii. It helps to adapt living organisms to a particular environment and hence increases

the change of survival of an organism in a changing environment.
iii. It maintains the individuality (distinct personality) of the organism.

MEMORY TIPS
A larva of a bee fed on bee-bread (pollen mixed with honey) grows into a worker but larvae
fed on royal jelly form a queen. This is an example of the environmental variation. Blood
groups, colour blindness, etc. are the examples of germinal variations.

ACTIVITY

Observe the ears of all the students in the class. Prepare a list of students having free or attached
ear lobes and calculate the percentage of students having each type of ears. Prove variation on
this basis.

Some terms related to genetics

i. Gene: It is a structural unit of chromosome made up of nucleic acids governing a
particular character in a living organism.

ii. Offspring: They are the individuals produced from sexual reproduction. They
are also called filial generation.

iii. Contrasting characters: The characters which always appear in two opposing
conditions are called contrasting characters.

iv. Trait or characteristics: It is the recognizable feature in an individual like the
height of a plant or skin color in humans, etc.

v. Clone: A group of organisms formed, which inherits all its parental characters is
known as clone, e.g. clone of bacteria.

vi. Alleles or allelomorphs: A pair of genes (Homologous pair) which controls two
alternative characters is called alleles. For example, in a cross between tall and
dwarf plant ‘T’ stands for the gene of tallness and ‘t’ stands for the alternative
form of the gene of dwarfness. Here, ‘T’ and ‘t’ are termed as alleles of each other.

vii. Homozygous: A condition when a diploid organism has the same allelic forms of
a gene, e.g. 'tt' or ‘TT’ is homozygous.

viii. Heterozygous: A condition when a diploid organism has different allelic forms of
a gene, e.g. ‘Tt’ (hybrid) is heterozygous.

ix. Genotype: It is the genetic expression of an organism. It cannot be seen or observed
physically.

292 | Heredity

x. Phenotype: It is the physical or observable expression of an organism. It is the
result of genotype.

xi. Dominant characters: The allele that expresses itself in the F1-generation of cross
between two contrasting characters is called dominant characters. It is represented
by capital letters, e.g. ‘T’, ‘R’ for tallness and red colour of flower respectively.

xii. Recessive characters: The allele which cannot express itself when present along
with the dominant allele is called recessive characters. It is represented by small
letters, e.g. ‘t’, ‘r’ for dwarfness and white colour of a flower respectively. Such
character is expressed only when they make a homozygous pair.

xiii. F1-generation: It refers to the 1st filial generation. The offspring produced by
crossing true-breeding parental forms are called F1-generations.

xiv. F2-generations: It refers to the 2nd filial generation produced by the self crossing of
F1-generation.

xv. Punnet square: It is a checker board, which helps to study all the possible results
of various crosses.

xvi. Monohybrid cross: It is the cross between one pair of contrasting characters, e.g.
the cross between tall plants with dwarf plants.

xvii. Dihybrid cross: The cross between two pairs of contrasting characters, e.g. the cross
between tall plants having red flowers with dwarf plants having white flowers.

xviii. Hybrid: The organism which has heterozygous condition in its genotype or the
organism having different genotype and phenotype is called hybrid.

MEMORY TIPS
Twins derived from a single zygote are clones of each other but offspring of their parents.

MENDEL’S EXPERIMENTAL PLANT

Mendel selected garden a pea plant (Pisum sativum) for a series of hybridization
experiments. He had several reasons to choose the pea plant for his experiment, like:
1. It is an annual plant with well-defined characteristics.
2. It could be grown and crossed easily.
3. It has perfect flowers (bisexual) and can be self-pollinated.
4. The life cycle of the plant is short.
5. The plant can produce a large number of offspring at a time.
6. Mating can be controlled in it.
7. When the hybrids are crossed, they result in perfectly fertile progeny.
8. It has various contrasting characters among its different varieties as shown in the

table below.

New Creative Science, Class 10 | 293

Seven contrasting characters noted by Mendel in the garden pea plant.

S. Characters Contrasting characters
N.
Dominant Recessive

1. Plant size or height Tall Dwarf

2. Position of flower on the stem Axial Terminal

3. Colour of the unripened pod Green Yellow

4. Shape of the pod Inflated Constricted

5. Shape of the seed Round (smooth) Wrinkled

6. Colour of the seed Yellow Green

7. Colour of the flower Violet (or, red) White

MENDEL’S EXPERIMENTAL TECHNIQUE

Mendel conducted breeding experiments in three steps.
(a) Selection of pure plants (i.e. plants having the same genotype and phenotype).
(b) Production of first generation of plants by cross breeding (hybridization).
(c) Raising of the second generation by self-fertilization of hybrids.

For getting pure plants, Mendel avoided the self-fertilization between two traits of the

plant. For this, he removed the anther (male parts) of the flower before the maturity

of the female part. Such flowers were covered to prevent the entry of foreign pollen

grains from outside. Now, he crossed it with the pollen grain of a desired plant. The

seeds developed by such crosses were collected and they belonged to the first filial

generation or F1-generation. He then self-pollinated the F1-generation and the products
so formed were called F2-generation and so on.
He maintained all the records of his experiments.

On the basis of the number of characters chosen, Pure tall Pure dwarf

the crosses performed by him may be monohybrid

cross, dihybrid cross, trihybrid cross, etc.

(I) Mendel’s monohybrid cross H yb rid Tall

The cross between only one pair of contrasting in F1
generation

characters or alleles is called monohybrid cross.

In monohybrid cross, Mendel selected pure tall H yb rid Tall

(TT) and dwarf pea plants (tt). He made cross

between these two plants. In the F1-generation, he

found all the pea plants were tall (Tt). Again, he F2 generation
self- pollinated the F1-generation plants and get

F2-generation in which he found, three out of four Pure H yb rid Pure
plants were tall and one was a dwarf, i.e. 75% of tall Tall dwarf

294 | Heredity

the plants were tall and 25% of the plants were dwarfs. In other words, the ratio of
tallness and dwarfness is 3:1 phenotypically.

But, genotypically it was found that 25% of the plants were pure tall (TT), 50% were
hybrid tall (Tt) and 25% were pure dwarf (tt). In other words, the genotypic ratio of
pure tall, hybrid tall and pure dwarf plants is 1 : 2 : 1.

He repeated the same experiment by taking other characters also and every time he
succeeded with his result of phenotypic ratio 3 : 1 and genotypic ratio 1 : 2 : 1 in every
F2-generation. This can be demonstrated as,

Monohybrid cross in Drosophila

T.H Morgan also performed the same experiment in drosophila (a type of fly). He first

crossed between long winged drosophila (LL) and short winged drosophila (ll). In F1-
generation he found all of the drosophila had long wings (Ll) and when F1-generations
are crossed with each other then the ratio of drosophila having long and short wings

was found to be 3 : 1 which proved Mendel’s result.

Pure tall Pure dwarf

TT × tt

Gametes T T tt

F1 generation Tt Tt Tt Tt

A cross between pure a tall and pure dwarf plant

H yb rid Tall × H yb rid Tall
Tt Tt

Gametes Tt Tt

F2 generation TT Tt Tt tt

Result Pure tall H yb rid tall
Phenotype ratio = 3 : 1
Genotype ratio = 1 : 2 : 1 Pure dwarf

A cross between hybrid tall plants

New Creative Science, Class 10 | 295

Monohybrid cross in Guinea Pigs
It is another example of the monohybrid cross. When a guinea pig having white coat
(bb) is crossed with the one having black coat (BB), then it was found that all the
hybrid of F1-geneation were black (Bb) and when the F1-hybrids were crossed with
each other then in F2-of generation the black and white guinea pigs were produced in
the ratio of 3:1 as

296 | Heredity