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The work of Mendel and others who followed him gave us an
idea of inheritance patterns. However the nature of those ‘factors’
which determine the phenotype was not very clear. As these
‘factors’ represent the genetic basis of inheritance, understanding
the structure of genetic material and the structural basis of
genotype and phenotype conversion became the focus of
attention in biology for the next century. The entire body of
molecular biology was a consequent development with major
contributions from Watson, Crick, Nirenberg, Khorana, Kornbergs
(father and son), Benzer, Monod, Brenner, etc. A parallel problem
being tackled was the mechanism of evolution. Awareness in the
areas of molecular genetics, structural biology and bio informatics
have enriched our understanding of the molecular basis of
evolution. In this unit the structure and function of DNA and the
story and theory of evolution have been examined and explained.
Chapter 4
Principles of Inheritance
and Variation
Chapter 5
Molecular Basis of Inheritance
Chapter 6
Evolution
Rationalised 2023-24
Page 2


The work of Mendel and others who followed him gave us an
idea of inheritance patterns. However the nature of those ‘factors’
which determine the phenotype was not very clear. As these
‘factors’ represent the genetic basis of inheritance, understanding
the structure of genetic material and the structural basis of
genotype and phenotype conversion became the focus of
attention in biology for the next century. The entire body of
molecular biology was a consequent development with major
contributions from Watson, Crick, Nirenberg, Khorana, Kornbergs
(father and son), Benzer, Monod, Brenner, etc. A parallel problem
being tackled was the mechanism of evolution. Awareness in the
areas of molecular genetics, structural biology and bio informatics
have enriched our understanding of the molecular basis of
evolution. In this unit the structure and function of DNA and the
story and theory of evolution have been examined and explained.
Chapter 4
Principles of Inheritance
and Variation
Chapter 5
Molecular Basis of Inheritance
Chapter 6
Evolution
Rationalised 2023-24
James Dewey Watson was born in Chicago on 6 April 1928. In 1947, he
received B.Sc. degree in Zoology. During these years his interest in
bird-watching had matured into a serious desire to learn genetics. This
became possible when he received a Fellowship for graduate study in
Zoology at Indiana University, Bloomington, where he received his Ph.D.
degree in 1950 on a study of the effect of hard X-rays on bacteriophage
multiplication.
He met Crick and discovered their common interest in solving the
DNA structure. Their first serious effort, was unsatisfactory. Their second effort
based upon more experimental evidence and better appreciation of
the nucleic acid literature, resulted, early in March 1953, in the proposal
of the complementary double-helical configuration.
Francis Harry Compton Crick was born on 8 June 1916, at Northampton,
England. He studied physics at University College, London and obtained
a B.Sc. in 1937. He completed Ph.D. in 1954 on a thesis entitled “X-ray
Diffraction: Polypeptides and Proteins”.
A critical influence in Crick’s career was his friendship with J. D.
Watson, then a young man of 23, leading in 1953 to the proposal of
the double-helical structure for DNA and the replication scheme. Crick
was made an F.R.S. in 1959.
The honours to Watson with Crick include: the John Collins Warren
Prize of the Massachusetts General Hospital, in 1959; the Lasker Award,
in 1960; the Research Corporation Prize, in 1962 and above all, the
Nobel Prize in 1962.
JAMES WATSON
FRANCIS CRICK
Rationalised 2023-24
Page 3


The work of Mendel and others who followed him gave us an
idea of inheritance patterns. However the nature of those ‘factors’
which determine the phenotype was not very clear. As these
‘factors’ represent the genetic basis of inheritance, understanding
the structure of genetic material and the structural basis of
genotype and phenotype conversion became the focus of
attention in biology for the next century. The entire body of
molecular biology was a consequent development with major
contributions from Watson, Crick, Nirenberg, Khorana, Kornbergs
(father and son), Benzer, Monod, Brenner, etc. A parallel problem
being tackled was the mechanism of evolution. Awareness in the
areas of molecular genetics, structural biology and bio informatics
have enriched our understanding of the molecular basis of
evolution. In this unit the structure and function of DNA and the
story and theory of evolution have been examined and explained.
Chapter 4
Principles of Inheritance
and Variation
Chapter 5
Molecular Basis of Inheritance
Chapter 6
Evolution
Rationalised 2023-24
James Dewey Watson was born in Chicago on 6 April 1928. In 1947, he
received B.Sc. degree in Zoology. During these years his interest in
bird-watching had matured into a serious desire to learn genetics. This
became possible when he received a Fellowship for graduate study in
Zoology at Indiana University, Bloomington, where he received his Ph.D.
degree in 1950 on a study of the effect of hard X-rays on bacteriophage
multiplication.
He met Crick and discovered their common interest in solving the
DNA structure. Their first serious effort, was unsatisfactory. Their second effort
based upon more experimental evidence and better appreciation of
the nucleic acid literature, resulted, early in March 1953, in the proposal
of the complementary double-helical configuration.
Francis Harry Compton Crick was born on 8 June 1916, at Northampton,
England. He studied physics at University College, London and obtained
a B.Sc. in 1937. He completed Ph.D. in 1954 on a thesis entitled “X-ray
Diffraction: Polypeptides and Proteins”.
A critical influence in Crick’s career was his friendship with J. D.
Watson, then a young man of 23, leading in 1953 to the proposal of
the double-helical structure for DNA and the replication scheme. Crick
was made an F.R.S. in 1959.
The honours to Watson with Crick include: the John Collins Warren
Prize of the Massachusetts General Hospital, in 1959; the Lasker Award,
in 1960; the Research Corporation Prize, in 1962 and above all, the
Nobel Prize in 1962.
JAMES WATSON
FRANCIS CRICK
Rationalised 2023-24
CHAPTER 4
Have you ever wondered why an elephant always gives
birth only to a baby elephant and not some other animal?
Or why a mango seed forms only a mango plant and not
any other plant?
Given that they do, are the offspring identical to their
parents? Or do they show differences in some of their
characteristics? Have you ever wondered why siblings
sometimes look so similar to each other? Or sometimes
even so different?
These and several related questions are dealt with,
scientifically, in a branch of biology known as Genetics.
This subject deals with the inheritance, as well as the
variation of characters from parents to offspring.
Inheritance is the process by which characters are passed
on from parent to progeny; it is the basis of heredity.
Variation is the degree by which progeny differ from their
parents.
Humans knew from as early as 8000-1000 B.C. that
one of the causes of variation was hidden in sexual
reproduction. They exploited the variations that were
naturally present in the wild populations of plants and
animals to selectively breed and select for organisms that
possessed desirable characters. For example, through
artificial selection and domestication from ancestral
PRINCIPLES OF INHERITANCE
AND VARIATION
4.1 Mendel’s Laws of
Inheritance
4.2 Inheritance of One Gene
4.3 Inheritance of Two Genes
4.4 Sex Determination
4.5 Mutation
4.6 Genetic Disorders
Rationalised 2023-24
Page 4


The work of Mendel and others who followed him gave us an
idea of inheritance patterns. However the nature of those ‘factors’
which determine the phenotype was not very clear. As these
‘factors’ represent the genetic basis of inheritance, understanding
the structure of genetic material and the structural basis of
genotype and phenotype conversion became the focus of
attention in biology for the next century. The entire body of
molecular biology was a consequent development with major
contributions from Watson, Crick, Nirenberg, Khorana, Kornbergs
(father and son), Benzer, Monod, Brenner, etc. A parallel problem
being tackled was the mechanism of evolution. Awareness in the
areas of molecular genetics, structural biology and bio informatics
have enriched our understanding of the molecular basis of
evolution. In this unit the structure and function of DNA and the
story and theory of evolution have been examined and explained.
Chapter 4
Principles of Inheritance
and Variation
Chapter 5
Molecular Basis of Inheritance
Chapter 6
Evolution
Rationalised 2023-24
James Dewey Watson was born in Chicago on 6 April 1928. In 1947, he
received B.Sc. degree in Zoology. During these years his interest in
bird-watching had matured into a serious desire to learn genetics. This
became possible when he received a Fellowship for graduate study in
Zoology at Indiana University, Bloomington, where he received his Ph.D.
degree in 1950 on a study of the effect of hard X-rays on bacteriophage
multiplication.
He met Crick and discovered their common interest in solving the
DNA structure. Their first serious effort, was unsatisfactory. Their second effort
based upon more experimental evidence and better appreciation of
the nucleic acid literature, resulted, early in March 1953, in the proposal
of the complementary double-helical configuration.
Francis Harry Compton Crick was born on 8 June 1916, at Northampton,
England. He studied physics at University College, London and obtained
a B.Sc. in 1937. He completed Ph.D. in 1954 on a thesis entitled “X-ray
Diffraction: Polypeptides and Proteins”.
A critical influence in Crick’s career was his friendship with J. D.
Watson, then a young man of 23, leading in 1953 to the proposal of
the double-helical structure for DNA and the replication scheme. Crick
was made an F.R.S. in 1959.
The honours to Watson with Crick include: the John Collins Warren
Prize of the Massachusetts General Hospital, in 1959; the Lasker Award,
in 1960; the Research Corporation Prize, in 1962 and above all, the
Nobel Prize in 1962.
JAMES WATSON
FRANCIS CRICK
Rationalised 2023-24
CHAPTER 4
Have you ever wondered why an elephant always gives
birth only to a baby elephant and not some other animal?
Or why a mango seed forms only a mango plant and not
any other plant?
Given that they do, are the offspring identical to their
parents? Or do they show differences in some of their
characteristics? Have you ever wondered why siblings
sometimes look so similar to each other? Or sometimes
even so different?
These and several related questions are dealt with,
scientifically, in a branch of biology known as Genetics.
This subject deals with the inheritance, as well as the
variation of characters from parents to offspring.
Inheritance is the process by which characters are passed
on from parent to progeny; it is the basis of heredity.
Variation is the degree by which progeny differ from their
parents.
Humans knew from as early as 8000-1000 B.C. that
one of the causes of variation was hidden in sexual
reproduction. They exploited the variations that were
naturally present in the wild populations of plants and
animals to selectively breed and select for organisms that
possessed desirable characters. For example, through
artificial selection and domestication from ancestral
PRINCIPLES OF INHERITANCE
AND VARIATION
4.1 Mendel’s Laws of
Inheritance
4.2 Inheritance of One Gene
4.3 Inheritance of Two Genes
4.4 Sex Determination
4.5 Mutation
4.6 Genetic Disorders
Rationalised 2023-24
54
BIOLOGY
wild cows, we have well-known Indian
breeds, e.g., Sahiwal cows in Punjab. We
must, however, recognise that though our
ancestors knew about the inheritance of
characters and variation, they had very
little idea about the scientific basis of these
phenomena.
4.1 MENDEL’S LAWS OF INHERITANCE
It was during the mid-nineteenth century that
headway was made in the understanding of
inheritance. Gregor Mendel, conducted
hybridisation experiments on garden peas for
seven years (1856-1863) and proposed the
laws of inheritance in living organisms. During
Mendel’s investigations into inheritance
patterns it was for the first time that statistical
analysis and mathematical logic were applied
to problems in biology. His experiments had a
large sampling size, which gave greater
credibility to the data that he collected. Also,
the confirmation of his inferences from
experiments on successive generations of his
test plants, proved that his results pointed to
general rules of inheritance rather than being
unsubstantiated ideas. Mendel investigated
characters in the garden pea plant that were
manifested as two opposing traits, e.g., tall or
dwarf plants, yellow or green seeds. This
allowed him to set up a basic framework of
rules governing inheritance, which was
expanded on by later scientists to account for
all the diverse natural observations and the
complexity inherent in them.
Mendel conducted such artificial
pollination/cross pollination experiments
using several  true-breeding pea lines. A true-
breeding line is one that, having undergone
continuous self-pollination, shows the stable trait inheritance and
expression for several generations. Mendel selected 14 true-breeding pea
plant varieties, as pairs which were similar except for one character with
contrasting traits. Some of the contrasting traits selected were smooth or
wrinkled seeds, yellow or green seeds, inflated (full) or constricted green
or yellow pods and tall or dwarf plants (Figure 4.1, Table 4.1).
Figure 4.1 Seven pairs of contrasting traits in
pea plant studied by Mendel
Rationalised 2023-24
Page 5


The work of Mendel and others who followed him gave us an
idea of inheritance patterns. However the nature of those ‘factors’
which determine the phenotype was not very clear. As these
‘factors’ represent the genetic basis of inheritance, understanding
the structure of genetic material and the structural basis of
genotype and phenotype conversion became the focus of
attention in biology for the next century. The entire body of
molecular biology was a consequent development with major
contributions from Watson, Crick, Nirenberg, Khorana, Kornbergs
(father and son), Benzer, Monod, Brenner, etc. A parallel problem
being tackled was the mechanism of evolution. Awareness in the
areas of molecular genetics, structural biology and bio informatics
have enriched our understanding of the molecular basis of
evolution. In this unit the structure and function of DNA and the
story and theory of evolution have been examined and explained.
Chapter 4
Principles of Inheritance
and Variation
Chapter 5
Molecular Basis of Inheritance
Chapter 6
Evolution
Rationalised 2023-24
James Dewey Watson was born in Chicago on 6 April 1928. In 1947, he
received B.Sc. degree in Zoology. During these years his interest in
bird-watching had matured into a serious desire to learn genetics. This
became possible when he received a Fellowship for graduate study in
Zoology at Indiana University, Bloomington, where he received his Ph.D.
degree in 1950 on a study of the effect of hard X-rays on bacteriophage
multiplication.
He met Crick and discovered their common interest in solving the
DNA structure. Their first serious effort, was unsatisfactory. Their second effort
based upon more experimental evidence and better appreciation of
the nucleic acid literature, resulted, early in March 1953, in the proposal
of the complementary double-helical configuration.
Francis Harry Compton Crick was born on 8 June 1916, at Northampton,
England. He studied physics at University College, London and obtained
a B.Sc. in 1937. He completed Ph.D. in 1954 on a thesis entitled “X-ray
Diffraction: Polypeptides and Proteins”.
A critical influence in Crick’s career was his friendship with J. D.
Watson, then a young man of 23, leading in 1953 to the proposal of
the double-helical structure for DNA and the replication scheme. Crick
was made an F.R.S. in 1959.
The honours to Watson with Crick include: the John Collins Warren
Prize of the Massachusetts General Hospital, in 1959; the Lasker Award,
in 1960; the Research Corporation Prize, in 1962 and above all, the
Nobel Prize in 1962.
JAMES WATSON
FRANCIS CRICK
Rationalised 2023-24
CHAPTER 4
Have you ever wondered why an elephant always gives
birth only to a baby elephant and not some other animal?
Or why a mango seed forms only a mango plant and not
any other plant?
Given that they do, are the offspring identical to their
parents? Or do they show differences in some of their
characteristics? Have you ever wondered why siblings
sometimes look so similar to each other? Or sometimes
even so different?
These and several related questions are dealt with,
scientifically, in a branch of biology known as Genetics.
This subject deals with the inheritance, as well as the
variation of characters from parents to offspring.
Inheritance is the process by which characters are passed
on from parent to progeny; it is the basis of heredity.
Variation is the degree by which progeny differ from their
parents.
Humans knew from as early as 8000-1000 B.C. that
one of the causes of variation was hidden in sexual
reproduction. They exploited the variations that were
naturally present in the wild populations of plants and
animals to selectively breed and select for organisms that
possessed desirable characters. For example, through
artificial selection and domestication from ancestral
PRINCIPLES OF INHERITANCE
AND VARIATION
4.1 Mendel’s Laws of
Inheritance
4.2 Inheritance of One Gene
4.3 Inheritance of Two Genes
4.4 Sex Determination
4.5 Mutation
4.6 Genetic Disorders
Rationalised 2023-24
54
BIOLOGY
wild cows, we have well-known Indian
breeds, e.g., Sahiwal cows in Punjab. We
must, however, recognise that though our
ancestors knew about the inheritance of
characters and variation, they had very
little idea about the scientific basis of these
phenomena.
4.1 MENDEL’S LAWS OF INHERITANCE
It was during the mid-nineteenth century that
headway was made in the understanding of
inheritance. Gregor Mendel, conducted
hybridisation experiments on garden peas for
seven years (1856-1863) and proposed the
laws of inheritance in living organisms. During
Mendel’s investigations into inheritance
patterns it was for the first time that statistical
analysis and mathematical logic were applied
to problems in biology. His experiments had a
large sampling size, which gave greater
credibility to the data that he collected. Also,
the confirmation of his inferences from
experiments on successive generations of his
test plants, proved that his results pointed to
general rules of inheritance rather than being
unsubstantiated ideas. Mendel investigated
characters in the garden pea plant that were
manifested as two opposing traits, e.g., tall or
dwarf plants, yellow or green seeds. This
allowed him to set up a basic framework of
rules governing inheritance, which was
expanded on by later scientists to account for
all the diverse natural observations and the
complexity inherent in them.
Mendel conducted such artificial
pollination/cross pollination experiments
using several  true-breeding pea lines. A true-
breeding line is one that, having undergone
continuous self-pollination, shows the stable trait inheritance and
expression for several generations. Mendel selected 14 true-breeding pea
plant varieties, as pairs which were similar except for one character with
contrasting traits. Some of the contrasting traits selected were smooth or
wrinkled seeds, yellow or green seeds, inflated (full) or constricted green
or yellow pods and tall or dwarf plants (Figure 4.1, Table 4.1).
Figure 4.1 Seven pairs of contrasting traits in
pea plant studied by Mendel
Rationalised 2023-24
55
PRINCIPLES OF INHERITANCE AND VARIATION
4.2 INHERITANCE OF ONE GENE
Let us take the example of one such
hybridisation experiment carried out by
Mendel where he crossed tall and dwarf pea
plants to study the inheritance of one gene
(Figure 4.2). He collected the seeds produced
as a result of this cross and grew them to
generate plants of the first hybrid generation.
This generation is also called the Filial
1
progeny or the F
1
. Mendel observed that all
the F
1 
 progeny  plants were tall, like one of
its parents; none were dwarf (Figure 4.3). He
made similar observations for the other pairs
of traits – he found that the F
1 
always
resembled either one of the parents, and that
the trait of the other parent was not seen in
them.
Mendel then self-pollinated the tall F
1
plants and to his surprise found  that in the
Filial
2
 generation some of the offspring were
‘dwarf’; the character that was not seen in
the F
1
 generation was now expressed. The
proportion of plants that were dwarf were
1/4
th
 of the F
2
 plants while 3/4
th
 of the F
2
 plants were tall. The tall and
dwarf traits were identical to their parental type and did not show any
blending, that is all the offspring were either tall or dwarf, none were of in-
between height (Figure 4.3).
Similar results were obtained with the other traits that he studied:
only one of the parental traits was expressed in the F
1
 generation while at
the F
2
 stage both the traits were expressed in the proportion 3:1. The
contrasting traits did not show any blending at either F
1
 or F
2
 stage.
Figure 4.2 Steps in making a cross in pea
Table 4.1: Contrasting Traits Studied by
Mendel in Pea
S.No. Characters Contrasting Traits
1. Stem height Tall/dwarf
2. Flower colour Violet/white
3. Flower position Axial/terminal
4. Pod shape Inflated/constricted
5. Pod colour Green/yellow
6. Seed shape Round/wrinkled
7. Seed colour Yellow/green
Rationalised 2023-24
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FAQs on NCERT Textbook: Principles of Inheritance & Variation - Biology Class 12 - NEET

1. What are the principles of inheritance and variation?
Ans. The principles of inheritance and variation are the fundamental concepts in genetics that explain how traits are passed on from parents to offspring. The principles include Mendel's laws of segregation and independent assortment, which describe how genetic traits are inherited and how they can be combined in different ways during reproduction.
2. How do genes play a role in inheritance?
Ans. Genes are segments of DNA that contain instructions for the development and functioning of living organisms. They play a crucial role in inheritance as they carry the genetic information that determines the traits passed on from parents to offspring. Different versions of genes, known as alleles, can result in variations in inherited traits.
3. What is the role of variation in evolution?
Ans. Variation is essential for the process of evolution. It refers to the differences in traits among individuals of the same species. These variations can arise due to genetic mutations, recombination, and other genetic processes. Natural selection acts on these variations, favoring traits that provide an advantage in a particular environment, leading to the accumulation of beneficial traits in a population over generations.
4. How does the environment influence inheritance and variation?
Ans. The environment can influence inheritance and variation in several ways. Environmental factors, such as temperature, nutrition, and exposure to certain substances, can affect gene expression and contribute to variations in traits. Additionally, the environment can also play a role in natural selection, as certain traits may be favored or disadvantaged depending on the specific environmental conditions.
5. How can the principles of inheritance and variation be applied in agriculture and animal breeding?
Ans. The principles of inheritance and variation have significant applications in agriculture and animal breeding. By understanding the inheritance patterns of specific traits, breeders can selectively breed individuals with desirable traits to improve crop yield, disease resistance, or animal productivity. This process is known as artificial selection and has played a crucial role in the development of many domesticated plant and animal species.
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