Genetics (NCERT) Class 12 Notes | EduRev

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Class 12 : Genetics (NCERT) Class 12 Notes | EduRev

 Page 1


Genetics – Nirmaan TYCRP
 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009 0
Genetics term was given by W.Bateson. (1905) (Father of Modern Genetics).
Genetics = Branch of biology which deals with the study of heredity and variation
? Heredity – Transmission of genetic characters from generation to generation.
? Variation – Individuals of same species have some difference, these are called variation.
Variation
Germinal (Blastogenic) variation
transfer from generation to generation (Inheritable)
Two types
Somatic variation
(Non inheritable)
Continuous variation Discontinuous variation
(due to crossing over) (due to mutation)
Substantive Meristic Substantive Meristic
? INHERITANCE : HEREDITY AND VARIATIONS
Heredity : It is the transmission of genetic characters from parents to the offsprings. It deals with the phenomenon of
‘‘like begets like’’ e.g., human babies are like human beings in overall characteristics.
Variations are common in sexually reproducing organisms. Variations are of following two types :
? Somatogenic : These are acquired variations and are non-inheritable in nature. The ability of an organism to alter its
phenotype in response to environment is called phenotypic plasticity.
? Blastogenic Variations : These are germinal variations and are hereditary in nature. They are again of two types:
? Continuous variations : These are the fluctuating variations and can not give rise to new species. These are
further of two types
? Substantive :  Variation in size, shape and colour of organism.
? Meristic : Variation in number of parts e.g. number of grains in an ear of wheat
? Discontinuous Variations : Also known as mutations, sports or saltations. These variation are responsible for
formation of new species and organism thus formed is called mutant.
Types of discontinuous variations
? Substantive variations : These influence shape, colour, size etc., e.g., hairless cat, short legged ancon
sheep.
? Meristic variation : These affect number of parts e.g. polydactyly in humans.
Variations are significance in evolution as they make the organism better suited to modifying environmental
conditions, produce new trait in organism and provide raw material for evolution.
History :
? Muller : Proposed the term ‘‘Cytogenetics’’ (Cytology + Genetics)
Father fo Actinobiology
Actinobiology - study the effect of radiation of living organisms.
GENETICS
GENETICS
Page 2


Genetics – Nirmaan TYCRP
 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009 0
Genetics term was given by W.Bateson. (1905) (Father of Modern Genetics).
Genetics = Branch of biology which deals with the study of heredity and variation
? Heredity – Transmission of genetic characters from generation to generation.
? Variation – Individuals of same species have some difference, these are called variation.
Variation
Germinal (Blastogenic) variation
transfer from generation to generation (Inheritable)
Two types
Somatic variation
(Non inheritable)
Continuous variation Discontinuous variation
(due to crossing over) (due to mutation)
Substantive Meristic Substantive Meristic
? INHERITANCE : HEREDITY AND VARIATIONS
Heredity : It is the transmission of genetic characters from parents to the offsprings. It deals with the phenomenon of
‘‘like begets like’’ e.g., human babies are like human beings in overall characteristics.
Variations are common in sexually reproducing organisms. Variations are of following two types :
? Somatogenic : These are acquired variations and are non-inheritable in nature. The ability of an organism to alter its
phenotype in response to environment is called phenotypic plasticity.
? Blastogenic Variations : These are germinal variations and are hereditary in nature. They are again of two types:
? Continuous variations : These are the fluctuating variations and can not give rise to new species. These are
further of two types
? Substantive :  Variation in size, shape and colour of organism.
? Meristic : Variation in number of parts e.g. number of grains in an ear of wheat
? Discontinuous Variations : Also known as mutations, sports or saltations. These variation are responsible for
formation of new species and organism thus formed is called mutant.
Types of discontinuous variations
? Substantive variations : These influence shape, colour, size etc., e.g., hairless cat, short legged ancon
sheep.
? Meristic variation : These affect number of parts e.g. polydactyly in humans.
Variations are significance in evolution as they make the organism better suited to modifying environmental
conditions, produce new trait in organism and provide raw material for evolution.
History :
? Muller : Proposed the term ‘‘Cytogenetics’’ (Cytology + Genetics)
Father fo Actinobiology
Actinobiology - study the effect of radiation of living organisms.
GENETICS
GENETICS
Genetics – Nirmaan TYCRP
 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009
2
? Morgan : Father of Experimental genetics
He experiment on Drosophila melanogaster & proposed various concepts
Gene theory : According to gene theory; genes are linearly located on chromosome.
Linkage term, Theory of sex linkage, Crossing over term, Criss - cross inheritance.
EARLY SPECULATIONS (PREMENDELIAN) 
(1) Vapour Theory (2) Preformation Theory
(3) Encasement Theory (4) Theory of Epigenesis
(5) Pangenesis Theory (6) Weismann Theory of Germ plasm
(7) Mendelism
(1) Vapour Theory :
? Proposed by a Greek philosopher Pythagoras in 500 B.C.
? Each organ of an animal body emitted some kind of vapour and that a new individual was formed by combination of
the vapour from different organs.
(2) Preformation Theory (1632 – 1723) :
? Dutch scientist ‘Swammerdam’ propounded the preformation theory. This theory holds that the sex cells (sperm
and ova) had the miniature copy of adults and the development of embryo was actually only the enlargement of
parts that were already present in the sperm or egg.
? This minature form of the animal present in the gamete was called ‘Homunculus’.
? This theory was supported by Hartsoeker.
? Those who considered homunculus to be present in sperms were called Spermist.
? Those who considered homunculus to be present in ova (eggs) were called Ovist.
? Antony Van Leeuwenhoek was the first to observe human sperms.
(3) Encasement Theory :
‘Charles Bonnet’ and his supporters presumed that every female contains within her body minature prototypes of all the
creatures which will every descend from her, one generation within the other some what like a series of chinese
bones. (a box inside a box and so on).
(4) Theory of Epigenesis :
? Wolff proposed that germ cells contain definite but undifferentiated substances which, after fertilization becomes
organized into various complex body organs (Differentiated) which form the adult.
This idea was referred to as epigensis.
(5) Pangenesis Theory :
? Charles Darwin propounded pangenesis theory.
? According to this theory every cell, tissue and organ of animal body produces many minute particles known as
pangenes or gemmules.
? These gemmules are discharged in the blood stream and are deposited in the reproductive organs. These reproductive
cells contain these pangenes and a child develops as a result of blending of the pangenes from two parents.
? Thus the individual would represent the mixture of both of the parents.
(6) Theory of Germplasm :
? ‘August Weismann’ (1889) suggested the theory of continuity of germplasm.
? They referred to the reproductive cells as germplasm and to rest of the body as somatoplasm.
? The germplasm forms the bridge of life and passes from one generation to the next.
? Soma is ‘Mortal’ because it eventually dies and disintegrates, the germ is “immortal”, because it continues forever.
Page 3


Genetics – Nirmaan TYCRP
 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009 0
Genetics term was given by W.Bateson. (1905) (Father of Modern Genetics).
Genetics = Branch of biology which deals with the study of heredity and variation
? Heredity – Transmission of genetic characters from generation to generation.
? Variation – Individuals of same species have some difference, these are called variation.
Variation
Germinal (Blastogenic) variation
transfer from generation to generation (Inheritable)
Two types
Somatic variation
(Non inheritable)
Continuous variation Discontinuous variation
(due to crossing over) (due to mutation)
Substantive Meristic Substantive Meristic
? INHERITANCE : HEREDITY AND VARIATIONS
Heredity : It is the transmission of genetic characters from parents to the offsprings. It deals with the phenomenon of
‘‘like begets like’’ e.g., human babies are like human beings in overall characteristics.
Variations are common in sexually reproducing organisms. Variations are of following two types :
? Somatogenic : These are acquired variations and are non-inheritable in nature. The ability of an organism to alter its
phenotype in response to environment is called phenotypic plasticity.
? Blastogenic Variations : These are germinal variations and are hereditary in nature. They are again of two types:
? Continuous variations : These are the fluctuating variations and can not give rise to new species. These are
further of two types
? Substantive :  Variation in size, shape and colour of organism.
? Meristic : Variation in number of parts e.g. number of grains in an ear of wheat
? Discontinuous Variations : Also known as mutations, sports or saltations. These variation are responsible for
formation of new species and organism thus formed is called mutant.
Types of discontinuous variations
? Substantive variations : These influence shape, colour, size etc., e.g., hairless cat, short legged ancon
sheep.
? Meristic variation : These affect number of parts e.g. polydactyly in humans.
Variations are significance in evolution as they make the organism better suited to modifying environmental
conditions, produce new trait in organism and provide raw material for evolution.
History :
? Muller : Proposed the term ‘‘Cytogenetics’’ (Cytology + Genetics)
Father fo Actinobiology
Actinobiology - study the effect of radiation of living organisms.
GENETICS
GENETICS
Genetics – Nirmaan TYCRP
 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009
2
? Morgan : Father of Experimental genetics
He experiment on Drosophila melanogaster & proposed various concepts
Gene theory : According to gene theory; genes are linearly located on chromosome.
Linkage term, Theory of sex linkage, Crossing over term, Criss - cross inheritance.
EARLY SPECULATIONS (PREMENDELIAN) 
(1) Vapour Theory (2) Preformation Theory
(3) Encasement Theory (4) Theory of Epigenesis
(5) Pangenesis Theory (6) Weismann Theory of Germ plasm
(7) Mendelism
(1) Vapour Theory :
? Proposed by a Greek philosopher Pythagoras in 500 B.C.
? Each organ of an animal body emitted some kind of vapour and that a new individual was formed by combination of
the vapour from different organs.
(2) Preformation Theory (1632 – 1723) :
? Dutch scientist ‘Swammerdam’ propounded the preformation theory. This theory holds that the sex cells (sperm
and ova) had the miniature copy of adults and the development of embryo was actually only the enlargement of
parts that were already present in the sperm or egg.
? This minature form of the animal present in the gamete was called ‘Homunculus’.
? This theory was supported by Hartsoeker.
? Those who considered homunculus to be present in sperms were called Spermist.
? Those who considered homunculus to be present in ova (eggs) were called Ovist.
? Antony Van Leeuwenhoek was the first to observe human sperms.
(3) Encasement Theory :
‘Charles Bonnet’ and his supporters presumed that every female contains within her body minature prototypes of all the
creatures which will every descend from her, one generation within the other some what like a series of chinese
bones. (a box inside a box and so on).
(4) Theory of Epigenesis :
? Wolff proposed that germ cells contain definite but undifferentiated substances which, after fertilization becomes
organized into various complex body organs (Differentiated) which form the adult.
This idea was referred to as epigensis.
(5) Pangenesis Theory :
? Charles Darwin propounded pangenesis theory.
? According to this theory every cell, tissue and organ of animal body produces many minute particles known as
pangenes or gemmules.
? These gemmules are discharged in the blood stream and are deposited in the reproductive organs. These reproductive
cells contain these pangenes and a child develops as a result of blending of the pangenes from two parents.
? Thus the individual would represent the mixture of both of the parents.
(6) Theory of Germplasm :
? ‘August Weismann’ (1889) suggested the theory of continuity of germplasm.
? They referred to the reproductive cells as germplasm and to rest of the body as somatoplasm.
? The germplasm forms the bridge of life and passes from one generation to the next.
? Soma is ‘Mortal’ because it eventually dies and disintegrates, the germ is “immortal”, because it continues forever.
Genetics – Nirmaan TYCRP
 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009 2
MENDELISM :
Experiments performed by Mendel on genetics and description of mechanisms of hereditery processes and
formulation of principles are known as Mendelism.
Gregor Johann Mendel (1822 – 1884) : Mendel was born on july 22, 1822 at Heinzendorf in Austria a Silesia
village. Mendel worked in Augustinian Monastery as monk at Brunn city, Austria.
In 1856-57, he started his historical experiments of heredity on pea (pisum sativum) plant. His experimental work
continued on pea plant till 1865 (19th century)
The results of his experiments were published in the science journal, “Nature For schender varien” in 1866.
This journal was in Germen language. Title was “verschue uber Pflangen Hybridan”.
This journal was published by ‘Natural History society of Bruno’.
A paper of Mendel by the name of ‘‘Experiment in plant Hybridization’’ published in this journal. Mendel was unable
to get any popularity. No one understood of him. He died in 1884 without getting any credit of his work due to kidney
disease (Bright disease)
After 16 years of Mendel’s death in 1900, Mendel’s postulates were rediscovered. Rediscovery by three scientists
independently.
? Carl Correns : Germany (Experiment on Maize)
? Hugo deVries (Holland) (Experiment on Evening Primerose)
He republished the Mendel’s results in 1901 in Flora magazine
? Erich von Tschermak Seysenegg - (Austria) (Experiment on different flowering plants)
The credit of rediscovery of Mendelism goes to three scientists.
Correns given two laws fo Mendelism
Law of heredity / Inheritance / Mendel
? Mendel Results Remain Hidden due to :
? At that time Darwin’s book ‘‘Origin of Species’’ published. Scientists were busy in discussion with this book.
? Medel’s ideas were ahead of that time.
? Mendel used higher statistical calculation in his experiments so the results were complicated to understand.
? Mendel also performed his experiments on Hawkeweed (hieracium) and beans (Lablab) plant on suggestion of Karl
Nageli but mendel did not get succeed in Heriacium, Parthenogenesis is present in it.
? Reasons for Mendel’s success :
? Mendel studied the inheretance of one or two characters at a time unlike his predecessors who had considered
many characters at a time (Kolreuter-Tobacco plant, John Goss &  Knight-Pea plant).
? Selection of Material (Pea plant - Pisum sativum)
? Selection of garden pea plant is suitable for studies. Which have following advantage.
? Pea plant is annual plant with short life cycle of 2-3 months so large number of offsprings can be analysed within a
short period of time.
? It has many contrasting traits.
? Natural self pollination is present in pea plant so purity of character is maintained in it.
? Cross pollination can be performed in it artificially so hybridization can be made possible.
? Pea plant easy to cultivate.
? Pea seeds are large. In addition to pea, Mendel worked on rajama.
? Mendel quantitatively analyse the inheritance of qualitative characters.
? He maintained the statistical records of all the experiments.
Page 4


Genetics – Nirmaan TYCRP
 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009 0
Genetics term was given by W.Bateson. (1905) (Father of Modern Genetics).
Genetics = Branch of biology which deals with the study of heredity and variation
? Heredity – Transmission of genetic characters from generation to generation.
? Variation – Individuals of same species have some difference, these are called variation.
Variation
Germinal (Blastogenic) variation
transfer from generation to generation (Inheritable)
Two types
Somatic variation
(Non inheritable)
Continuous variation Discontinuous variation
(due to crossing over) (due to mutation)
Substantive Meristic Substantive Meristic
? INHERITANCE : HEREDITY AND VARIATIONS
Heredity : It is the transmission of genetic characters from parents to the offsprings. It deals with the phenomenon of
‘‘like begets like’’ e.g., human babies are like human beings in overall characteristics.
Variations are common in sexually reproducing organisms. Variations are of following two types :
? Somatogenic : These are acquired variations and are non-inheritable in nature. The ability of an organism to alter its
phenotype in response to environment is called phenotypic plasticity.
? Blastogenic Variations : These are germinal variations and are hereditary in nature. They are again of two types:
? Continuous variations : These are the fluctuating variations and can not give rise to new species. These are
further of two types
? Substantive :  Variation in size, shape and colour of organism.
? Meristic : Variation in number of parts e.g. number of grains in an ear of wheat
? Discontinuous Variations : Also known as mutations, sports or saltations. These variation are responsible for
formation of new species and organism thus formed is called mutant.
Types of discontinuous variations
? Substantive variations : These influence shape, colour, size etc., e.g., hairless cat, short legged ancon
sheep.
? Meristic variation : These affect number of parts e.g. polydactyly in humans.
Variations are significance in evolution as they make the organism better suited to modifying environmental
conditions, produce new trait in organism and provide raw material for evolution.
History :
? Muller : Proposed the term ‘‘Cytogenetics’’ (Cytology + Genetics)
Father fo Actinobiology
Actinobiology - study the effect of radiation of living organisms.
GENETICS
GENETICS
Genetics – Nirmaan TYCRP
 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009
2
? Morgan : Father of Experimental genetics
He experiment on Drosophila melanogaster & proposed various concepts
Gene theory : According to gene theory; genes are linearly located on chromosome.
Linkage term, Theory of sex linkage, Crossing over term, Criss - cross inheritance.
EARLY SPECULATIONS (PREMENDELIAN) 
(1) Vapour Theory (2) Preformation Theory
(3) Encasement Theory (4) Theory of Epigenesis
(5) Pangenesis Theory (6) Weismann Theory of Germ plasm
(7) Mendelism
(1) Vapour Theory :
? Proposed by a Greek philosopher Pythagoras in 500 B.C.
? Each organ of an animal body emitted some kind of vapour and that a new individual was formed by combination of
the vapour from different organs.
(2) Preformation Theory (1632 – 1723) :
? Dutch scientist ‘Swammerdam’ propounded the preformation theory. This theory holds that the sex cells (sperm
and ova) had the miniature copy of adults and the development of embryo was actually only the enlargement of
parts that were already present in the sperm or egg.
? This minature form of the animal present in the gamete was called ‘Homunculus’.
? This theory was supported by Hartsoeker.
? Those who considered homunculus to be present in sperms were called Spermist.
? Those who considered homunculus to be present in ova (eggs) were called Ovist.
? Antony Van Leeuwenhoek was the first to observe human sperms.
(3) Encasement Theory :
‘Charles Bonnet’ and his supporters presumed that every female contains within her body minature prototypes of all the
creatures which will every descend from her, one generation within the other some what like a series of chinese
bones. (a box inside a box and so on).
(4) Theory of Epigenesis :
? Wolff proposed that germ cells contain definite but undifferentiated substances which, after fertilization becomes
organized into various complex body organs (Differentiated) which form the adult.
This idea was referred to as epigensis.
(5) Pangenesis Theory :
? Charles Darwin propounded pangenesis theory.
? According to this theory every cell, tissue and organ of animal body produces many minute particles known as
pangenes or gemmules.
? These gemmules are discharged in the blood stream and are deposited in the reproductive organs. These reproductive
cells contain these pangenes and a child develops as a result of blending of the pangenes from two parents.
? Thus the individual would represent the mixture of both of the parents.
(6) Theory of Germplasm :
? ‘August Weismann’ (1889) suggested the theory of continuity of germplasm.
? They referred to the reproductive cells as germplasm and to rest of the body as somatoplasm.
? The germplasm forms the bridge of life and passes from one generation to the next.
? Soma is ‘Mortal’ because it eventually dies and disintegrates, the germ is “immortal”, because it continues forever.
Genetics – Nirmaan TYCRP
 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009 2
MENDELISM :
Experiments performed by Mendel on genetics and description of mechanisms of hereditery processes and
formulation of principles are known as Mendelism.
Gregor Johann Mendel (1822 – 1884) : Mendel was born on july 22, 1822 at Heinzendorf in Austria a Silesia
village. Mendel worked in Augustinian Monastery as monk at Brunn city, Austria.
In 1856-57, he started his historical experiments of heredity on pea (pisum sativum) plant. His experimental work
continued on pea plant till 1865 (19th century)
The results of his experiments were published in the science journal, “Nature For schender varien” in 1866.
This journal was in Germen language. Title was “verschue uber Pflangen Hybridan”.
This journal was published by ‘Natural History society of Bruno’.
A paper of Mendel by the name of ‘‘Experiment in plant Hybridization’’ published in this journal. Mendel was unable
to get any popularity. No one understood of him. He died in 1884 without getting any credit of his work due to kidney
disease (Bright disease)
After 16 years of Mendel’s death in 1900, Mendel’s postulates were rediscovered. Rediscovery by three scientists
independently.
? Carl Correns : Germany (Experiment on Maize)
? Hugo deVries (Holland) (Experiment on Evening Primerose)
He republished the Mendel’s results in 1901 in Flora magazine
? Erich von Tschermak Seysenegg - (Austria) (Experiment on different flowering plants)
The credit of rediscovery of Mendelism goes to three scientists.
Correns given two laws fo Mendelism
Law of heredity / Inheritance / Mendel
? Mendel Results Remain Hidden due to :
? At that time Darwin’s book ‘‘Origin of Species’’ published. Scientists were busy in discussion with this book.
? Medel’s ideas were ahead of that time.
? Mendel used higher statistical calculation in his experiments so the results were complicated to understand.
? Mendel also performed his experiments on Hawkeweed (hieracium) and beans (Lablab) plant on suggestion of Karl
Nageli but mendel did not get succeed in Heriacium, Parthenogenesis is present in it.
? Reasons for Mendel’s success :
? Mendel studied the inheretance of one or two characters at a time unlike his predecessors who had considered
many characters at a time (Kolreuter-Tobacco plant, John Goss &  Knight-Pea plant).
? Selection of Material (Pea plant - Pisum sativum)
? Selection of garden pea plant is suitable for studies. Which have following advantage.
? Pea plant is annual plant with short life cycle of 2-3 months so large number of offsprings can be analysed within a
short period of time.
? It has many contrasting traits.
? Natural self pollination is present in pea plant so purity of character is maintained in it.
? Cross pollination can be performed in it artificially so hybridization can be made possible.
? Pea plant easy to cultivate.
? Pea seeds are large. In addition to pea, Mendel worked on rajama.
? Mendel quantitatively analyse the inheritance of qualitative characters.
? He maintained the statistical records of all the experiments.
Genetics – Nirmaan TYCRP
 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009
4
? Mendel’s work : Mendel studied 7 characters or 7 pairs of contrasting traits.
S.No.
Character (Chromosomal 
position)
Dominant Recessive Ratio
1 Length of plant (4) 787 (tall) 277 (dwarf) 2.84:1
2 Colour of flower (1) 705 (violet) 224 (white) 3.15:1
3 Pod or flower position (4) 651 (axial) 207 (terminal) 3.14:1
4 Shape of pod (4) 882 (inflated) 299 (constricted) 2.94:1
5 Colour of pod (5) 428 (green) 152 (yellow) 2.82:1
6 Shape of seed (7) 5,474 (round) 1850 (wrinkled) 2.96:1
7 Colour of cotyledon (1) 6,022 (yellow) 2,001 (green) 3.01:1
2.98:(=3:1)
Actual data obtained by Mendel in F
2
 progenies in garden pea
Average of all traits studied
? Technique of Mendel
He developed a technique Emasculation and Bagging for hybridization in plants.
Flowers of pea plant are bisexual. In this method one considered as male and another as female.
The plant used as female, stamens of this plant are removed at juvenile stage, this is called Emasculation.
Emasculation is done to prevent self pollination.
Emasculated flowers covered by bags, this is called bagging.
Bagging is only used to prevent undersirable cross pollination.
Mature pollen grains are collected from male plants and spread over emasculated flower.
Seeds are formed in the female flower after pollination.
The plants that are obtained from these seeds are called First Filial generation or F
1
 generation according to mendel.
Mendel was great plant breader (true breader)
? SOME DEFINED TERMS :
? Factors : Unit of heredity which is responsible for inheritance and appearance of characters. These factors were
referred as genes by Johannsen (1909). Mendel used term “element” for factor.
? Morgan first use symbol to represent the factor. Dominant factor are represented by capital letter while recessive
factor by small letter
? Allele : Alternative forms of a gene which are located on same position [locus] on the homologous chromosome is
called Allele. Term allele was coined by Bateson.
T T T t t t
? Homozygous : A zygote is formed by fusion of two gametes having identicle factors is called homozygote and
organism developed from this zygote is called homozygous. Ex. TT, RR, tt
? Heterozygous : A zygote is formed by fusion of two different types of gamete carrying different factors is called
Heterozygous (Tt) (Rr) and individual developed from such zygote is called hetrozygous. The term homozygous and
heterozygous are coined by Bateson.
? Hemizygous : If individual contains only one gene of a pair then individual said to be hemizygous. Male individual is
always hemizygous for sex linked gene.
? Phenotype : It is the external and morphological appearance of an organism for a particular character.
Page 5


Genetics – Nirmaan TYCRP
 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009 0
Genetics term was given by W.Bateson. (1905) (Father of Modern Genetics).
Genetics = Branch of biology which deals with the study of heredity and variation
? Heredity – Transmission of genetic characters from generation to generation.
? Variation – Individuals of same species have some difference, these are called variation.
Variation
Germinal (Blastogenic) variation
transfer from generation to generation (Inheritable)
Two types
Somatic variation
(Non inheritable)
Continuous variation Discontinuous variation
(due to crossing over) (due to mutation)
Substantive Meristic Substantive Meristic
? INHERITANCE : HEREDITY AND VARIATIONS
Heredity : It is the transmission of genetic characters from parents to the offsprings. It deals with the phenomenon of
‘‘like begets like’’ e.g., human babies are like human beings in overall characteristics.
Variations are common in sexually reproducing organisms. Variations are of following two types :
? Somatogenic : These are acquired variations and are non-inheritable in nature. The ability of an organism to alter its
phenotype in response to environment is called phenotypic plasticity.
? Blastogenic Variations : These are germinal variations and are hereditary in nature. They are again of two types:
? Continuous variations : These are the fluctuating variations and can not give rise to new species. These are
further of two types
? Substantive :  Variation in size, shape and colour of organism.
? Meristic : Variation in number of parts e.g. number of grains in an ear of wheat
? Discontinuous Variations : Also known as mutations, sports or saltations. These variation are responsible for
formation of new species and organism thus formed is called mutant.
Types of discontinuous variations
? Substantive variations : These influence shape, colour, size etc., e.g., hairless cat, short legged ancon
sheep.
? Meristic variation : These affect number of parts e.g. polydactyly in humans.
Variations are significance in evolution as they make the organism better suited to modifying environmental
conditions, produce new trait in organism and provide raw material for evolution.
History :
? Muller : Proposed the term ‘‘Cytogenetics’’ (Cytology + Genetics)
Father fo Actinobiology
Actinobiology - study the effect of radiation of living organisms.
GENETICS
GENETICS
Genetics – Nirmaan TYCRP
 97/1, 3F, Adhchini, Sri Aurobindo Marg, Near NCERT, New Delhi |  011-32044009
2
? Morgan : Father of Experimental genetics
He experiment on Drosophila melanogaster & proposed various concepts
Gene theory : According to gene theory; genes are linearly located on chromosome.
Linkage term, Theory of sex linkage, Crossing over term, Criss - cross inheritance.
EARLY SPECULATIONS (PREMENDELIAN) 
(1) Vapour Theory (2) Preformation Theory
(3) Encasement Theory (4) Theory of Epigenesis
(5) Pangenesis Theory (6) Weismann Theory of Germ plasm
(7) Mendelism
(1) Vapour Theory :
? Proposed by a Greek philosopher Pythagoras in 500 B.C.
? Each organ of an animal body emitted some kind of vapour and that a new individual was formed by combination of
the vapour from different organs.
(2) Preformation Theory (1632 – 1723) :
? Dutch scientist ‘Swammerdam’ propounded the preformation theory. This theory holds that the sex cells (sperm
and ova) had the miniature copy of adults and the development of embryo was actually only the enlargement of
parts that were already present in the sperm or egg.
? This minature form of the animal present in the gamete was called ‘Homunculus’.
? This theory was supported by Hartsoeker.
? Those who considered homunculus to be present in sperms were called Spermist.
? Those who considered homunculus to be present in ova (eggs) were called Ovist.
? Antony Van Leeuwenhoek was the first to observe human sperms.
(3) Encasement Theory :
‘Charles Bonnet’ and his supporters presumed that every female contains within her body minature prototypes of all the
creatures which will every descend from her, one generation within the other some what like a series of chinese
bones. (a box inside a box and so on).
(4) Theory of Epigenesis :
? Wolff proposed that germ cells contain definite but undifferentiated substances which, after fertilization becomes
organized into various complex body organs (Differentiated) which form the adult.
This idea was referred to as epigensis.
(5) Pangenesis Theory :
? Charles Darwin propounded pangenesis theory.
? According to this theory every cell, tissue and organ of animal body produces many minute particles known as
pangenes or gemmules.
? These gemmules are discharged in the blood stream and are deposited in the reproductive organs. These reproductive
cells contain these pangenes and a child develops as a result of blending of the pangenes from two parents.
? Thus the individual would represent the mixture of both of the parents.
(6) Theory of Germplasm :
? ‘August Weismann’ (1889) suggested the theory of continuity of germplasm.
? They referred to the reproductive cells as germplasm and to rest of the body as somatoplasm.
? The germplasm forms the bridge of life and passes from one generation to the next.
? Soma is ‘Mortal’ because it eventually dies and disintegrates, the germ is “immortal”, because it continues forever.
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MENDELISM :
Experiments performed by Mendel on genetics and description of mechanisms of hereditery processes and
formulation of principles are known as Mendelism.
Gregor Johann Mendel (1822 – 1884) : Mendel was born on july 22, 1822 at Heinzendorf in Austria a Silesia
village. Mendel worked in Augustinian Monastery as monk at Brunn city, Austria.
In 1856-57, he started his historical experiments of heredity on pea (pisum sativum) plant. His experimental work
continued on pea plant till 1865 (19th century)
The results of his experiments were published in the science journal, “Nature For schender varien” in 1866.
This journal was in Germen language. Title was “verschue uber Pflangen Hybridan”.
This journal was published by ‘Natural History society of Bruno’.
A paper of Mendel by the name of ‘‘Experiment in plant Hybridization’’ published in this journal. Mendel was unable
to get any popularity. No one understood of him. He died in 1884 without getting any credit of his work due to kidney
disease (Bright disease)
After 16 years of Mendel’s death in 1900, Mendel’s postulates were rediscovered. Rediscovery by three scientists
independently.
? Carl Correns : Germany (Experiment on Maize)
? Hugo deVries (Holland) (Experiment on Evening Primerose)
He republished the Mendel’s results in 1901 in Flora magazine
? Erich von Tschermak Seysenegg - (Austria) (Experiment on different flowering plants)
The credit of rediscovery of Mendelism goes to three scientists.
Correns given two laws fo Mendelism
Law of heredity / Inheritance / Mendel
? Mendel Results Remain Hidden due to :
? At that time Darwin’s book ‘‘Origin of Species’’ published. Scientists were busy in discussion with this book.
? Medel’s ideas were ahead of that time.
? Mendel used higher statistical calculation in his experiments so the results were complicated to understand.
? Mendel also performed his experiments on Hawkeweed (hieracium) and beans (Lablab) plant on suggestion of Karl
Nageli but mendel did not get succeed in Heriacium, Parthenogenesis is present in it.
? Reasons for Mendel’s success :
? Mendel studied the inheretance of one or two characters at a time unlike his predecessors who had considered
many characters at a time (Kolreuter-Tobacco plant, John Goss &  Knight-Pea plant).
? Selection of Material (Pea plant - Pisum sativum)
? Selection of garden pea plant is suitable for studies. Which have following advantage.
? Pea plant is annual plant with short life cycle of 2-3 months so large number of offsprings can be analysed within a
short period of time.
? It has many contrasting traits.
? Natural self pollination is present in pea plant so purity of character is maintained in it.
? Cross pollination can be performed in it artificially so hybridization can be made possible.
? Pea plant easy to cultivate.
? Pea seeds are large. In addition to pea, Mendel worked on rajama.
? Mendel quantitatively analyse the inheritance of qualitative characters.
? He maintained the statistical records of all the experiments.
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? Mendel’s work : Mendel studied 7 characters or 7 pairs of contrasting traits.
S.No.
Character (Chromosomal 
position)
Dominant Recessive Ratio
1 Length of plant (4) 787 (tall) 277 (dwarf) 2.84:1
2 Colour of flower (1) 705 (violet) 224 (white) 3.15:1
3 Pod or flower position (4) 651 (axial) 207 (terminal) 3.14:1
4 Shape of pod (4) 882 (inflated) 299 (constricted) 2.94:1
5 Colour of pod (5) 428 (green) 152 (yellow) 2.82:1
6 Shape of seed (7) 5,474 (round) 1850 (wrinkled) 2.96:1
7 Colour of cotyledon (1) 6,022 (yellow) 2,001 (green) 3.01:1
2.98:(=3:1)
Actual data obtained by Mendel in F
2
 progenies in garden pea
Average of all traits studied
? Technique of Mendel
He developed a technique Emasculation and Bagging for hybridization in plants.
Flowers of pea plant are bisexual. In this method one considered as male and another as female.
The plant used as female, stamens of this plant are removed at juvenile stage, this is called Emasculation.
Emasculation is done to prevent self pollination.
Emasculated flowers covered by bags, this is called bagging.
Bagging is only used to prevent undersirable cross pollination.
Mature pollen grains are collected from male plants and spread over emasculated flower.
Seeds are formed in the female flower after pollination.
The plants that are obtained from these seeds are called First Filial generation or F
1
 generation according to mendel.
Mendel was great plant breader (true breader)
? SOME DEFINED TERMS :
? Factors : Unit of heredity which is responsible for inheritance and appearance of characters. These factors were
referred as genes by Johannsen (1909). Mendel used term “element” for factor.
? Morgan first use symbol to represent the factor. Dominant factor are represented by capital letter while recessive
factor by small letter
? Allele : Alternative forms of a gene which are located on same position [locus] on the homologous chromosome is
called Allele. Term allele was coined by Bateson.
T T T t t t
? Homozygous : A zygote is formed by fusion of two gametes having identicle factors is called homozygote and
organism developed from this zygote is called homozygous. Ex. TT, RR, tt
? Heterozygous : A zygote is formed by fusion of two different types of gamete carrying different factors is called
Heterozygous (Tt) (Rr) and individual developed from such zygote is called hetrozygous. The term homozygous and
heterozygous are coined by Bateson.
? Hemizygous : If individual contains only one gene of a pair then individual said to be hemizygous. Male individual is
always hemizygous for sex linked gene.
? Phenotype : It is the external and morphological appearance of an organism for a particular character.
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? Genotype : The genetic constitution or genetic make-up of an organism for a particular character. Genotype &
phenotype terms were coined by Johannsen.
? Phenocopy : If different genotypes are placed in different environmental conditions if they produce same phenotype.
These individual are said to be Phenocopy of each other.
MONOHYBRID CROSS
When we consider the inheritance of one character at a time by a cross is called monohybrid cross.
First of all, Mendel selected tall and dwarf plants
Tall
(Pure)
Dwarf
(Pure)
All tall (impure)
F -Generation
1
Self pollination
Dwarf Tall (Phenotypic ratio or basic ratio or Mendelian ratio)
3 1
F -Generation
2
Dwarf (pure)
1 Tall (Pure)
(Selfing)
All Tall F -Generation
3
2 Tall (impure)
(Selfing)
3 Tall : 1 dwarf
Self Pollination
(Selfing)
Conclusions (results) of Monohybrid Cross
I
st
 Conclusion :
According to Mendel each genetic character is controlled by a pair of unit factor. It is known as conclusion of paired
factor or unit factor.
II
nd
 Conclusion :
This conclusion is based on F
1
 - generation. When two different unit factors are present in single individual, then only
one unit factor is able to express itself and known as dominant unit factor. Another unit factor fails to express is the
recessive factor. In the presence of dominant unit factor recessive unit factor can not express and it is known as
conclusion of dominance.
Tall Dwarf
TT tt
Tt F -Generation
1
All tall
III
rd
 Conclusion :
During gamete formation; the unit factors of a pair segregate randomly and transfer inside different gamete. Each gamete
receives only one factor of a pair; so gametes are pure for a particular trait. It is known as conclusion of purity of gametes
or segregation.
Tt
Tt Tt
gametogenesis
gamete gamete
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