NCERT Textbook: Basic Principles of Inheritance | Biotechnology for Class 11 - NEET PDF Download

 Page 1


Genetic Principles 
and Molecular 
Processes
Unit III
Chapter 6 
Basic Principles of Inheritance 
Chapter 7 
Basic Processes
Chapter 8 
Genetic Disorders
The idea of inheritance patterns emerged from the 
work of Mendel and other scientists who followed 
him. What was not clear was the nature of the 
‘factors’ which are responsible for determining a 
particular phenotype. It became crucial to have an 
understanding of the structure of genetic material 
and the patterns of inheritance. The foundation 
of molecular biology and genetics was laid down 
by many eminent scientists of that time, such as 
Watson, Crick, Nirenberg, Khorana, Monod, Benzer, 
etc. The contributions of these scientists and the 
concepts explained by them have been discussed in 
three chapters of this unit.
Chapter 6.indd   145 10/01/2025   14:26:52
Reprint 2025-26
Page 2


Genetic Principles 
and Molecular 
Processes
Unit III
Chapter 6 
Basic Principles of Inheritance 
Chapter 7 
Basic Processes
Chapter 8 
Genetic Disorders
The idea of inheritance patterns emerged from the 
work of Mendel and other scientists who followed 
him. What was not clear was the nature of the 
‘factors’ which are responsible for determining a 
particular phenotype. It became crucial to have an 
understanding of the structure of genetic material 
and the patterns of inheritance. The foundation 
of molecular biology and genetics was laid down 
by many eminent scientists of that time, such as 
Watson, Crick, Nirenberg, Khorana, Monod, Benzer, 
etc. The contributions of these scientists and the 
concepts explained by them have been discussed in 
three chapters of this unit.
Chapter 6.indd   145 10/01/2025   14:26:52
Reprint 2025-26
Gregor Johann Mendel  
(1822-1884) 
Gregor Johann Mendel  was born on July 
22, 1822 in Austria. His pioneering work 
laid the foundation of science of genetics and 
therefore, he is known as ‘father of genetics’. 
In 1843, Mendel began studying even while 
being a monk at St. Thomas Monastery 
in Brno. There he was exposed to the lab 
facilities and got interested in research 
and teaching. His experiments focussed on 
cross-breeding of pea plants and gathering 
data on the variations for several generations. 
Based on his experiments on a total of seven 
characteristics in garden pea, he established 
Law of segregation and Law of independent 
assortment. Decades after his death in 1884, 
his work got recognition by other researchers. 
His research is now considered to be the basis 
of modern genetics. 
Chapter 6.indd   146 10/01/2025   14:26:54
Reprint 2025-26
Page 3


Genetic Principles 
and Molecular 
Processes
Unit III
Chapter 6 
Basic Principles of Inheritance 
Chapter 7 
Basic Processes
Chapter 8 
Genetic Disorders
The idea of inheritance patterns emerged from the 
work of Mendel and other scientists who followed 
him. What was not clear was the nature of the 
‘factors’ which are responsible for determining a 
particular phenotype. It became crucial to have an 
understanding of the structure of genetic material 
and the patterns of inheritance. The foundation 
of molecular biology and genetics was laid down 
by many eminent scientists of that time, such as 
Watson, Crick, Nirenberg, Khorana, Monod, Benzer, 
etc. The contributions of these scientists and the 
concepts explained by them have been discussed in 
three chapters of this unit.
Chapter 6.indd   145 10/01/2025   14:26:52
Reprint 2025-26
Gregor Johann Mendel  
(1822-1884) 
Gregor Johann Mendel  was born on July 
22, 1822 in Austria. His pioneering work 
laid the foundation of science of genetics and 
therefore, he is known as ‘father of genetics’. 
In 1843, Mendel began studying even while 
being a monk at St. Thomas Monastery 
in Brno. There he was exposed to the lab 
facilities and got interested in research 
and teaching. His experiments focussed on 
cross-breeding of pea plants and gathering 
data on the variations for several generations. 
Based on his experiments on a total of seven 
characteristics in garden pea, he established 
Law of segregation and Law of independent 
assortment. Decades after his death in 1884, 
his work got recognition by other researchers. 
His research is now considered to be the basis 
of modern genetics. 
Chapter 6.indd   146 10/01/2025   14:26:54
Reprint 2025-26
Basic Principles 
of Inheritance
6.1 Introduction to 
Inheritance
6.2 Linkage and 
Crossing Over
6.3 Recombination
6.4 Sex-linked 
Inheritance
6.5 Extrachromosomal 
Inheritance
6.6 Polyploidy
6.7 Reverse Genetics
6.1  Introduct Ion to Inher Itance Have you ever noticed that all the members of your family 
have several features in common like facial features, hair 
colour, skin colour, etc.? Why is it 
so? Why do you resemble in certain 
characters with your mother and 
certain characters with your father? 
Characteristics that run in families 
have a genetic basis, meaning that 
they depend on genetic information 
a person inherits from his or her 
parents. The same is true for all 
plants and animals.
This transmission of characters 
from one generation to the next, or 
the phenomenon of the offsprings 
to inherit the parental traits is 
known as ‘Heredity’. The inherited 
characters are present on the chromosomes in the form of 
genes. Further, it is observed that though offsprings inherit 
Gregor Johann Mendel 
(1822–1884), ‘father of 
genetics’
Chapter 6
Chapter 6.indd   147 10/01/2025   14:26:55
Reprint 2025-26
Page 4


Genetic Principles 
and Molecular 
Processes
Unit III
Chapter 6 
Basic Principles of Inheritance 
Chapter 7 
Basic Processes
Chapter 8 
Genetic Disorders
The idea of inheritance patterns emerged from the 
work of Mendel and other scientists who followed 
him. What was not clear was the nature of the 
‘factors’ which are responsible for determining a 
particular phenotype. It became crucial to have an 
understanding of the structure of genetic material 
and the patterns of inheritance. The foundation 
of molecular biology and genetics was laid down 
by many eminent scientists of that time, such as 
Watson, Crick, Nirenberg, Khorana, Monod, Benzer, 
etc. The contributions of these scientists and the 
concepts explained by them have been discussed in 
three chapters of this unit.
Chapter 6.indd   145 10/01/2025   14:26:52
Reprint 2025-26
Gregor Johann Mendel  
(1822-1884) 
Gregor Johann Mendel  was born on July 
22, 1822 in Austria. His pioneering work 
laid the foundation of science of genetics and 
therefore, he is known as ‘father of genetics’. 
In 1843, Mendel began studying even while 
being a monk at St. Thomas Monastery 
in Brno. There he was exposed to the lab 
facilities and got interested in research 
and teaching. His experiments focussed on 
cross-breeding of pea plants and gathering 
data on the variations for several generations. 
Based on his experiments on a total of seven 
characteristics in garden pea, he established 
Law of segregation and Law of independent 
assortment. Decades after his death in 1884, 
his work got recognition by other researchers. 
His research is now considered to be the basis 
of modern genetics. 
Chapter 6.indd   146 10/01/2025   14:26:54
Reprint 2025-26
Basic Principles 
of Inheritance
6.1 Introduction to 
Inheritance
6.2 Linkage and 
Crossing Over
6.3 Recombination
6.4 Sex-linked 
Inheritance
6.5 Extrachromosomal 
Inheritance
6.6 Polyploidy
6.7 Reverse Genetics
6.1  Introduct Ion to Inher Itance Have you ever noticed that all the members of your family 
have several features in common like facial features, hair 
colour, skin colour, etc.? Why is it 
so? Why do you resemble in certain 
characters with your mother and 
certain characters with your father? 
Characteristics that run in families 
have a genetic basis, meaning that 
they depend on genetic information 
a person inherits from his or her 
parents. The same is true for all 
plants and animals.
This transmission of characters 
from one generation to the next, or 
the phenomenon of the offsprings 
to inherit the parental traits is 
known as ‘Heredity’. The inherited 
characters are present on the chromosomes in the form of 
genes. Further, it is observed that though offsprings inherit 
Gregor Johann Mendel 
(1822–1884), ‘father of 
genetics’
Chapter 6
Chapter 6.indd   147 10/01/2025   14:26:55
Reprint 2025-26
Biotechnology 148
characters from their parents, they are 
unique and differ from their parents 
in certain aspects. These differences 
between the offsprings and their parents 
are known as variations. The study of 
scienti??c facts of heredity and variation 
is referred to as Genetics.
The major objective of biotechnology is 
the manipulation of the living organisms 
or to modify the genetic constitution 
of an organism to manufacture 
products intended to improve the 
quality of human life. In order to use 
biotechnological tools for manipulating 
the genes, understanding of the genetics 
and heredity of the traits is essential. 
It is essential to identify the genetic 
constituents (genes and their allelic 
forms in the population) regulating a 
trait, for its manipulation. In this chapter 
we will study about the principles of 
inheritance. 
6.1.1 Mendel’s work: The   
foundation
Our modern understanding of 
inheritance of traits through 
generations comes from the studies 
made by Gregor Mendel, an Austrian 
monk. He selected pea plants (Pisum 
sativum) for his breeding experiments 
as a good model system because it is 
an annual plant with perfect bisexual 
??owers and having many contrasting pair 
of characters. He selected seven pairs 
of contrasting characters for his breeding experiments 
and produced pure line for each trait by self-pollinating 
for several generations (Fig. 6.1). He performed arti??cial 
cross pollination in plants with contrasting traits by 
transferring pollen from one ??ower to another with a 
small brush. He grew a large number of plants for each 
cross and collected data for several generations. 
Fig. 6.1: Seven pairs of contrasting traits of pea 
plants used by Mendel
Character Dominant trait Recessive trait
Seed shape
Seed colour
Flower colour
Green
Pod shape
Pod colour
Yellow
Flower
position
Axial Terminal
Stem
height
Tall Dwarf
Constricted
White
Green
Wrinkled
In?ated
Violet
Yellow
Round
Chapter 6.indd   148 10/01/2025   14:26:56
Reprint 2025-26
Page 5


Genetic Principles 
and Molecular 
Processes
Unit III
Chapter 6 
Basic Principles of Inheritance 
Chapter 7 
Basic Processes
Chapter 8 
Genetic Disorders
The idea of inheritance patterns emerged from the 
work of Mendel and other scientists who followed 
him. What was not clear was the nature of the 
‘factors’ which are responsible for determining a 
particular phenotype. It became crucial to have an 
understanding of the structure of genetic material 
and the patterns of inheritance. The foundation 
of molecular biology and genetics was laid down 
by many eminent scientists of that time, such as 
Watson, Crick, Nirenberg, Khorana, Monod, Benzer, 
etc. The contributions of these scientists and the 
concepts explained by them have been discussed in 
three chapters of this unit.
Chapter 6.indd   145 10/01/2025   14:26:52
Reprint 2025-26
Gregor Johann Mendel  
(1822-1884) 
Gregor Johann Mendel  was born on July 
22, 1822 in Austria. His pioneering work 
laid the foundation of science of genetics and 
therefore, he is known as ‘father of genetics’. 
In 1843, Mendel began studying even while 
being a monk at St. Thomas Monastery 
in Brno. There he was exposed to the lab 
facilities and got interested in research 
and teaching. His experiments focussed on 
cross-breeding of pea plants and gathering 
data on the variations for several generations. 
Based on his experiments on a total of seven 
characteristics in garden pea, he established 
Law of segregation and Law of independent 
assortment. Decades after his death in 1884, 
his work got recognition by other researchers. 
His research is now considered to be the basis 
of modern genetics. 
Chapter 6.indd   146 10/01/2025   14:26:54
Reprint 2025-26
Basic Principles 
of Inheritance
6.1 Introduction to 
Inheritance
6.2 Linkage and 
Crossing Over
6.3 Recombination
6.4 Sex-linked 
Inheritance
6.5 Extrachromosomal 
Inheritance
6.6 Polyploidy
6.7 Reverse Genetics
6.1  Introduct Ion to Inher Itance Have you ever noticed that all the members of your family 
have several features in common like facial features, hair 
colour, skin colour, etc.? Why is it 
so? Why do you resemble in certain 
characters with your mother and 
certain characters with your father? 
Characteristics that run in families 
have a genetic basis, meaning that 
they depend on genetic information 
a person inherits from his or her 
parents. The same is true for all 
plants and animals.
This transmission of characters 
from one generation to the next, or 
the phenomenon of the offsprings 
to inherit the parental traits is 
known as ‘Heredity’. The inherited 
characters are present on the chromosomes in the form of 
genes. Further, it is observed that though offsprings inherit 
Gregor Johann Mendel 
(1822–1884), ‘father of 
genetics’
Chapter 6
Chapter 6.indd   147 10/01/2025   14:26:55
Reprint 2025-26
Biotechnology 148
characters from their parents, they are 
unique and differ from their parents 
in certain aspects. These differences 
between the offsprings and their parents 
are known as variations. The study of 
scienti??c facts of heredity and variation 
is referred to as Genetics.
The major objective of biotechnology is 
the manipulation of the living organisms 
or to modify the genetic constitution 
of an organism to manufacture 
products intended to improve the 
quality of human life. In order to use 
biotechnological tools for manipulating 
the genes, understanding of the genetics 
and heredity of the traits is essential. 
It is essential to identify the genetic 
constituents (genes and their allelic 
forms in the population) regulating a 
trait, for its manipulation. In this chapter 
we will study about the principles of 
inheritance. 
6.1.1 Mendel’s work: The   
foundation
Our modern understanding of 
inheritance of traits through 
generations comes from the studies 
made by Gregor Mendel, an Austrian 
monk. He selected pea plants (Pisum 
sativum) for his breeding experiments 
as a good model system because it is 
an annual plant with perfect bisexual 
??owers and having many contrasting pair 
of characters. He selected seven pairs 
of contrasting characters for his breeding experiments 
and produced pure line for each trait by self-pollinating 
for several generations (Fig. 6.1). He performed arti??cial 
cross pollination in plants with contrasting traits by 
transferring pollen from one ??ower to another with a 
small brush. He grew a large number of plants for each 
cross and collected data for several generations. 
Fig. 6.1: Seven pairs of contrasting traits of pea 
plants used by Mendel
Character Dominant trait Recessive trait
Seed shape
Seed colour
Flower colour
Green
Pod shape
Pod colour
Yellow
Flower
position
Axial Terminal
Stem
height
Tall Dwarf
Constricted
White
Green
Wrinkled
In?ated
Violet
Yellow
Round
Chapter 6.indd   148 10/01/2025   14:26:56
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Basic Princi Ples of i nheritance 
149
Single gene inheritance
When Mendel cross pollinated a pure 
(homozygous) tall pea plant with a pure 
dwarf pea plant, he noticed that the 
progeny of ??rst generation (First ??lial 
or F
1
 generation, which was raised by 
collecting the seeds produced from 
this cross) were all tall. The dwarf 
phenotype was missing. What happened 
to the dwarf trait? When the said F
1
 
offspring were self-pollinated to raise F
2
 
generation, surprisingly both tall and 
dwarf plants appeared in the ratio of 
3:1 (3 tall and 1 dwarf). Since Mendel 
designed this experiment by considering 
only one contrasting trait, i.e., tall and 
dwarf, this cross is called monohybrid 
cross (Fig. 6.2). Interestingly, in all 
such monohybrid crosses involving 
other contrasting pair of characters 
carried out by Mendel, similar ratio of 
approximately 3:1 were obtained in F
2
 
generation. These results prompted 
Mendel to propose that each individual 
has two factors for each character (trait) 
and that one factor (which was later 
named as gene) was inherited from each 
parent through gametes.
This is the reason that the dwarf 
feature which was not there in F
1
 
generation was found in F
2
. Hence, F
1
 tall 
plants are heterozygotes as they contain 
two different alleles (Tt). As F
1 
plants are 
heterozygous tall (Tt), this indicates that 
the tall allele (T) is dominant over dwarf 
allele (t). Thus, dwarf allele (t) is recessive 
to tall allele (T).
Understanding of these crosses can 
be well understood by the graphical 
representation developed by Reginald 
C. Punnett, a British geneticist. Using 
Punnett Square, we can easily calculate 
the probability of all possible genetic 
Rediscovery of Mendel’s Work
Mendel carried out hybridisation 
experiments on pea plants for nine 
long years and published all his 
observations in 1866 in Annual 
Proceedings of Natural History 
Society of Br nn, demonstrating 
the actions of invisible ‘factors’ 
now called gene, in predictably 
determining the traits of an 
organism. Mendel’s conclusions 
were largely ignored by the vast 
majority. In 1900, however, his 
work was ‘rediscovered’ by three 
European scientists, Hugo de 
Vries, Carl Correns, and Erich 
von Tschermak.
Fig. 6.2: Monohybrid cross 
Tall Tall Dwarf
F
generation
2
Tall Tall
X
Sel?ng
generation
F
1
Parents
Tall
Dwarf
X
Tall
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