Genetics: Principles of Inheritance and variation | IBPS PO Prelims & Mains Preparation - Bank Exams PDF Download

 Page 1


 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 1 
 
Chapter- 5   Principles of Inheritance and variation 
 
Genetics –  
- The branch of biology that deals with the study of heredity and variations is known as genetics. 
- The term genetics was coined by William Batson in 1906. 
-  Gregor Johann Mendel is known as father of genetics.  
- Archibald Garrod is considered as father of human genetics. 
 
Heredity – Heredity is transmission of genetic characters from parents to the offsprings. 
 
Basis of Heredity – 
 
1. Physical basis – Mendel announced in 1866 that heredity was controlled by particles called 
germinal units or factors. 
- These factors were present in all the cells, somatic as well as germinal and the gametes carried 
the factors to the next generation. Thus the factors or genes are physical basis (carriers) of 
heredity.  
 
2. Chemical basis – The genes are definite segments of chromosomes. The DNA part of gene 
functions as the genetic material and forms the chemical basis of heredity. 
 
Gregor Johann Mendel (1822-1884) –  
- Mendel was born in a peasant family of Moravia. 
- Mendel conducted breeding experiments on garden 
peas (Pisum sativum) for 7 years (1856-1863) and 
proposed the laws of inheritance in living organisms. 
- His laws form the basis of science of genetics even 
today and he is remembered as father of genetics.  
- The results of this experiments were published in the 
science journal “Nature for schender varein” in 1866 
This journal was in germen language title was 
“Verschue uber plangent  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 
 
Contrasting traits studied by Mendel in Pea – 
 
  Contrasting traits 
 Characters Dominant Recessive 
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 
 
Mendel’s experimental plant –  
Mendel selected garden pea plant as his experimental material because – 
1) It is an annual plant with short life cycle which made it possible to study several generations 
with a short period. 
2) It has perfect bisexual flowers which are pre dominantly self-pollinating. 
Page 2


 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 1 
 
Chapter- 5   Principles of Inheritance and variation 
 
Genetics –  
- The branch of biology that deals with the study of heredity and variations is known as genetics. 
- The term genetics was coined by William Batson in 1906. 
-  Gregor Johann Mendel is known as father of genetics.  
- Archibald Garrod is considered as father of human genetics. 
 
Heredity – Heredity is transmission of genetic characters from parents to the offsprings. 
 
Basis of Heredity – 
 
1. Physical basis – Mendel announced in 1866 that heredity was controlled by particles called 
germinal units or factors. 
- These factors were present in all the cells, somatic as well as germinal and the gametes carried 
the factors to the next generation. Thus the factors or genes are physical basis (carriers) of 
heredity.  
 
2. Chemical basis – The genes are definite segments of chromosomes. The DNA part of gene 
functions as the genetic material and forms the chemical basis of heredity. 
 
Gregor Johann Mendel (1822-1884) –  
- Mendel was born in a peasant family of Moravia. 
- Mendel conducted breeding experiments on garden 
peas (Pisum sativum) for 7 years (1856-1863) and 
proposed the laws of inheritance in living organisms. 
- His laws form the basis of science of genetics even 
today and he is remembered as father of genetics.  
- The results of this experiments were published in the 
science journal “Nature for schender varein” in 1866 
This journal was in germen language title was 
“Verschue uber plangent  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 
 
Contrasting traits studied by Mendel in Pea – 
 
  Contrasting traits 
 Characters Dominant Recessive 
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 
 
Mendel’s experimental plant –  
Mendel selected garden pea plant as his experimental material because – 
1) It is an annual plant with short life cycle which made it possible to study several generations 
with a short period. 
2) It has perfect bisexual flowers which are pre dominantly self-pollinating. 
 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 2 
3) Artificial cross pollination (hybridization). 
4) Could be easily achieved by removing of stamen (emasculation).  
 
Special point: 
S. Blixt concluded that the genes studied by Mendel are located on four different pairs of 
chromosomes. 
? Cotyledon/seed colour and Flower colour/Seed coat 
colour- Chromosome no. 1 
? Pod form, Flower position and Stem length- Ch no. 4 
? Pod colour- Ch no. 5 
? Seed form- Ch no. 7 
 
Mendel’s experimental technique –  
Mendel’s experiments were performed in three stages– 
1. Selection of pure or true breeding parents. 
2. Hybridisation and obtaining of F 1 generation of plants. 
3. Self-pollination of hybrid plants and rising of 
subsequent generations like F 2, F 3, F 4, etc. 
 
Some terms – 
? Unit factor – A unit of inheritance which passes from 
one generation to the next and controls the 
expression of a character in the organisms. 
 
? Alleles/Allelomorphs –The two Mendelian factors 
which occur on the same locus in the two homologous 
chromosomes of an individual and control the 
expression of a character. Term by Bateson. 
 
? Pure line – Term was given by Johannsen. It is strain 
of genetically pure true breeding individuals which have been derived from a single self 
fertilized homozygous ancestor or identical homozygous ancestors. 
 
? Dominant factor – One of a pair of  alleles which can express itself whether present in 
homozygous or heterozygous whether present in homozygous or heterozygous state. 
 
? Recessive factor – The factor of an allelic pair which is unable to express its affect in the 
presence of its contrasting factor in a heterozygous state. 
 
? Homozygous – An individual which contains identical alleles of a gene of a character on it 
homologous chromosomes. Homozygous and heterozygous term by Bateson. 
 
? Heterozygous – An individual which contains two contrasting factors of a character or two 
different alleles of a gene on its homologous chromosomes. 
 
? F 1 or first filial generation – Hybrids produced from a cross between the genetically different 
individuals. 
 
? F 2 or 2
nd
 filial generation – Individual arises as a result of inbreeding or interbreeding 
amongst individuals of F 1 generation. 
 
? Genotype – The genetic constitution of an individual with regard to 1 or more character 
irrespective of whether genes are expressed or not. Genotype and phenotype term by 
Johannsen. 
 
Page 3


 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 1 
 
Chapter- 5   Principles of Inheritance and variation 
 
Genetics –  
- The branch of biology that deals with the study of heredity and variations is known as genetics. 
- The term genetics was coined by William Batson in 1906. 
-  Gregor Johann Mendel is known as father of genetics.  
- Archibald Garrod is considered as father of human genetics. 
 
Heredity – Heredity is transmission of genetic characters from parents to the offsprings. 
 
Basis of Heredity – 
 
1. Physical basis – Mendel announced in 1866 that heredity was controlled by particles called 
germinal units or factors. 
- These factors were present in all the cells, somatic as well as germinal and the gametes carried 
the factors to the next generation. Thus the factors or genes are physical basis (carriers) of 
heredity.  
 
2. Chemical basis – The genes are definite segments of chromosomes. The DNA part of gene 
functions as the genetic material and forms the chemical basis of heredity. 
 
Gregor Johann Mendel (1822-1884) –  
- Mendel was born in a peasant family of Moravia. 
- Mendel conducted breeding experiments on garden 
peas (Pisum sativum) for 7 years (1856-1863) and 
proposed the laws of inheritance in living organisms. 
- His laws form the basis of science of genetics even 
today and he is remembered as father of genetics.  
- The results of this experiments were published in the 
science journal “Nature for schender varein” in 1866 
This journal was in germen language title was 
“Verschue uber plangent  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 
 
Contrasting traits studied by Mendel in Pea – 
 
  Contrasting traits 
 Characters Dominant Recessive 
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 
 
Mendel’s experimental plant –  
Mendel selected garden pea plant as his experimental material because – 
1) It is an annual plant with short life cycle which made it possible to study several generations 
with a short period. 
2) It has perfect bisexual flowers which are pre dominantly self-pollinating. 
 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 2 
3) Artificial cross pollination (hybridization). 
4) Could be easily achieved by removing of stamen (emasculation).  
 
Special point: 
S. Blixt concluded that the genes studied by Mendel are located on four different pairs of 
chromosomes. 
? Cotyledon/seed colour and Flower colour/Seed coat 
colour- Chromosome no. 1 
? Pod form, Flower position and Stem length- Ch no. 4 
? Pod colour- Ch no. 5 
? Seed form- Ch no. 7 
 
Mendel’s experimental technique –  
Mendel’s experiments were performed in three stages– 
1. Selection of pure or true breeding parents. 
2. Hybridisation and obtaining of F 1 generation of plants. 
3. Self-pollination of hybrid plants and rising of 
subsequent generations like F 2, F 3, F 4, etc. 
 
Some terms – 
? Unit factor – A unit of inheritance which passes from 
one generation to the next and controls the 
expression of a character in the organisms. 
 
? Alleles/Allelomorphs –The two Mendelian factors 
which occur on the same locus in the two homologous 
chromosomes of an individual and control the 
expression of a character. Term by Bateson. 
 
? Pure line – Term was given by Johannsen. It is strain 
of genetically pure true breeding individuals which have been derived from a single self 
fertilized homozygous ancestor or identical homozygous ancestors. 
 
? Dominant factor – One of a pair of  alleles which can express itself whether present in 
homozygous or heterozygous whether present in homozygous or heterozygous state. 
 
? Recessive factor – The factor of an allelic pair which is unable to express its affect in the 
presence of its contrasting factor in a heterozygous state. 
 
? Homozygous – An individual which contains identical alleles of a gene of a character on it 
homologous chromosomes. Homozygous and heterozygous term by Bateson. 
 
? Heterozygous – An individual which contains two contrasting factors of a character or two 
different alleles of a gene on its homologous chromosomes. 
 
? F 1 or first filial generation – Hybrids produced from a cross between the genetically different 
individuals. 
 
? F 2 or 2
nd
 filial generation – Individual arises as a result of inbreeding or interbreeding 
amongst individuals of F 1 generation. 
 
? Genotype – The genetic constitution of an individual with regard to 1 or more character 
irrespective of whether genes are expressed or not. Genotype and phenotype term by 
Johannsen. 
 
 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 3 
? Phenotype – Measurable distinctive structural and functional characteristic of an individual 
with regard to one or more characters. 
 
? Emasculation – Removal of stamens before the maturation of pollen grains. 
 
Inheritance of one gene (Mendel’s Monohybrid Cross) –  
- A cross between two organisms of a species which is made to study the inheritance of a single 
pair of allele or factors of character. 
- Mendel cross tall and dwarf pea plants to study the inheritance of one gene. Mendel observed 
that all the F 1 progeny plants were tall. 
- Mendel then self pollinated the tall F 1 plants and found that in the F 2 generation some of the 
offsprings were dwarf. 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
Phenotypic ratio - Tall : Dwarf       
           3   :   1 
Genotypic ratio - Pure tall : hybrid tall : Dwarf      
          1      : 2          :      1 
 
- 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 5.2).  
- 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 5.3). 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 was 1/4th 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 5.3). 
Similar results were obtained with the other traits that he studied: only one of the parental traits  
Page 4


 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 1 
 
Chapter- 5   Principles of Inheritance and variation 
 
Genetics –  
- The branch of biology that deals with the study of heredity and variations is known as genetics. 
- The term genetics was coined by William Batson in 1906. 
-  Gregor Johann Mendel is known as father of genetics.  
- Archibald Garrod is considered as father of human genetics. 
 
Heredity – Heredity is transmission of genetic characters from parents to the offsprings. 
 
Basis of Heredity – 
 
1. Physical basis – Mendel announced in 1866 that heredity was controlled by particles called 
germinal units or factors. 
- These factors were present in all the cells, somatic as well as germinal and the gametes carried 
the factors to the next generation. Thus the factors or genes are physical basis (carriers) of 
heredity.  
 
2. Chemical basis – The genes are definite segments of chromosomes. The DNA part of gene 
functions as the genetic material and forms the chemical basis of heredity. 
 
Gregor Johann Mendel (1822-1884) –  
- Mendel was born in a peasant family of Moravia. 
- Mendel conducted breeding experiments on garden 
peas (Pisum sativum) for 7 years (1856-1863) and 
proposed the laws of inheritance in living organisms. 
- His laws form the basis of science of genetics even 
today and he is remembered as father of genetics.  
- The results of this experiments were published in the 
science journal “Nature for schender varein” in 1866 
This journal was in germen language title was 
“Verschue uber plangent  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 
 
Contrasting traits studied by Mendel in Pea – 
 
  Contrasting traits 
 Characters Dominant Recessive 
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 
 
Mendel’s experimental plant –  
Mendel selected garden pea plant as his experimental material because – 
1) It is an annual plant with short life cycle which made it possible to study several generations 
with a short period. 
2) It has perfect bisexual flowers which are pre dominantly self-pollinating. 
 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 2 
3) Artificial cross pollination (hybridization). 
4) Could be easily achieved by removing of stamen (emasculation).  
 
Special point: 
S. Blixt concluded that the genes studied by Mendel are located on four different pairs of 
chromosomes. 
? Cotyledon/seed colour and Flower colour/Seed coat 
colour- Chromosome no. 1 
? Pod form, Flower position and Stem length- Ch no. 4 
? Pod colour- Ch no. 5 
? Seed form- Ch no. 7 
 
Mendel’s experimental technique –  
Mendel’s experiments were performed in three stages– 
1. Selection of pure or true breeding parents. 
2. Hybridisation and obtaining of F 1 generation of plants. 
3. Self-pollination of hybrid plants and rising of 
subsequent generations like F 2, F 3, F 4, etc. 
 
Some terms – 
? Unit factor – A unit of inheritance which passes from 
one generation to the next and controls the 
expression of a character in the organisms. 
 
? Alleles/Allelomorphs –The two Mendelian factors 
which occur on the same locus in the two homologous 
chromosomes of an individual and control the 
expression of a character. Term by Bateson. 
 
? Pure line – Term was given by Johannsen. It is strain 
of genetically pure true breeding individuals which have been derived from a single self 
fertilized homozygous ancestor or identical homozygous ancestors. 
 
? Dominant factor – One of a pair of  alleles which can express itself whether present in 
homozygous or heterozygous whether present in homozygous or heterozygous state. 
 
? Recessive factor – The factor of an allelic pair which is unable to express its affect in the 
presence of its contrasting factor in a heterozygous state. 
 
? Homozygous – An individual which contains identical alleles of a gene of a character on it 
homologous chromosomes. Homozygous and heterozygous term by Bateson. 
 
? Heterozygous – An individual which contains two contrasting factors of a character or two 
different alleles of a gene on its homologous chromosomes. 
 
? F 1 or first filial generation – Hybrids produced from a cross between the genetically different 
individuals. 
 
? F 2 or 2
nd
 filial generation – Individual arises as a result of inbreeding or interbreeding 
amongst individuals of F 1 generation. 
 
? Genotype – The genetic constitution of an individual with regard to 1 or more character 
irrespective of whether genes are expressed or not. Genotype and phenotype term by 
Johannsen. 
 
 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 3 
? Phenotype – Measurable distinctive structural and functional characteristic of an individual 
with regard to one or more characters. 
 
? Emasculation – Removal of stamens before the maturation of pollen grains. 
 
Inheritance of one gene (Mendel’s Monohybrid Cross) –  
- A cross between two organisms of a species which is made to study the inheritance of a single 
pair of allele or factors of character. 
- Mendel cross tall and dwarf pea plants to study the inheritance of one gene. Mendel observed 
that all the F 1 progeny plants were tall. 
- Mendel then self pollinated the tall F 1 plants and found that in the F 2 generation some of the 
offsprings were dwarf. 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
Phenotypic ratio - Tall : Dwarf       
           3   :   1 
Genotypic ratio - Pure tall : hybrid tall : Dwarf      
          1      : 2          :      1 
 
- 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 5.2).  
- 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 5.3). 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 was 1/4th 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 5.3). 
Similar results were obtained with the other traits that he studied: only one of the parental traits  
 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 4 
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 5.2 Steps in 
making a cross in pea Based on these observations, Mendel proposed that something was being 
stably passed down, unchanged, from parent to offspring through the gametes, over successive 
generations.  
- He called these things as ‘factors’. Nowadays, we call them as genes. Genes, therefore, are the 
units of inheritance. They contain the information that is required to express a particular trait, in 
an organism.  
- Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly 
different forms of the same gene.  
- If we use alphabetical symbols for each gene, then the capital letter is used for the trait 
expressed at the F 1 stage and the small alphabet for the other trait. For example, in case of the 
character of height, T is used for the Tall trait and t for the ‘dwarf’, and T and t are alleles of each 
other. Hence, in plants the pair of alleles for height would be TT, Tt or tt.  
- Mendel also proposed that in a true breeding, tall or dwarf pea variety the allelic pair of genes 
for height are identical or homozygous, TT and tt, respectively. TT and tt are called the genotype 
of the plant while the descriptive terms tall and dwarf are the phenotype. 
- As Mendel found the phenotype of the F 1 heterozygote Tt to be exactly like the TT parent in 
appearance, he proposed that in a pair of dissimilar factors, one dominates the other (as in the 
F 1) and hence is called the dominant factor while the other factor is recessive.  
- In this case T (for tallness) is dominant over t (for dwarfness), that is recessive. He observed 
identical behaviour for all the other characters/trait-pairs that he studied. It is convenient (and 
logical) to use the capital and lower case of an alphabetical symbol to remember this concept of 
dominance and recessiveness. (Do not use T for tall and d for dwarf because you will find it 
difficult to remember whether T and d are alleles of the same gene/character or not).  
- Alleles can be similar as in the case of homozygotes TT and tt or can be dissimilar as in the case 
of the heterozygote Tt. Since Figure 5.3 Diagrammatic representation of monohybrid cross the 
Tt plant is heterozygous for genes controlling one character (height), it is a monohybrid and the 
cross between TT and tt is a monohybrid cross.  
- From the observation that the recessive parental trait is expressed without any blending in the 
F 2 generation, we can infer that, when the tall and dwarf plant produce gametes, by the process 
of meiosis, the alleles of the parental pair separate or segregate from each other and only one 
allele is transmitted to a gamete.  
- This segregation of alleles is a random process and so there is a 50 per cent chance of a gamete 
containing either allele, as has been verified by the results of the crossings. In this way the 
gametes of the tall TT plants have the allele T and the gametes of the dwarf tt plants have the 
allele t.  
- During fertilisation the two alleles, T from one parent say, through the pollen, and t from the 
other parent, then through the egg, are united to produce zygotes that have one T allele and one 
t allele. In other words the hybrids have Tt.  
- Since these hybrids contain alleles which express contrasting traits, the plants are heterozygous. 
The production of gametes by the parents, the formation of the zygotes, the F 1 and F 2 plants can 
be understood from a diagram called Punnett Square as shown in Figure 5.4. It was developed  
Page 5


 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 1 
 
Chapter- 5   Principles of Inheritance and variation 
 
Genetics –  
- The branch of biology that deals with the study of heredity and variations is known as genetics. 
- The term genetics was coined by William Batson in 1906. 
-  Gregor Johann Mendel is known as father of genetics.  
- Archibald Garrod is considered as father of human genetics. 
 
Heredity – Heredity is transmission of genetic characters from parents to the offsprings. 
 
Basis of Heredity – 
 
1. Physical basis – Mendel announced in 1866 that heredity was controlled by particles called 
germinal units or factors. 
- These factors were present in all the cells, somatic as well as germinal and the gametes carried 
the factors to the next generation. Thus the factors or genes are physical basis (carriers) of 
heredity.  
 
2. Chemical basis – The genes are definite segments of chromosomes. The DNA part of gene 
functions as the genetic material and forms the chemical basis of heredity. 
 
Gregor Johann Mendel (1822-1884) –  
- Mendel was born in a peasant family of Moravia. 
- Mendel conducted breeding experiments on garden 
peas (Pisum sativum) for 7 years (1856-1863) and 
proposed the laws of inheritance in living organisms. 
- His laws form the basis of science of genetics even 
today and he is remembered as father of genetics.  
- The results of this experiments were published in the 
science journal “Nature for schender varein” in 1866 
This journal was in germen language title was 
“Verschue uber plangent  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 
 
Contrasting traits studied by Mendel in Pea – 
 
  Contrasting traits 
 Characters Dominant Recessive 
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 
 
Mendel’s experimental plant –  
Mendel selected garden pea plant as his experimental material because – 
1) It is an annual plant with short life cycle which made it possible to study several generations 
with a short period. 
2) It has perfect bisexual flowers which are pre dominantly self-pollinating. 
 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 2 
3) Artificial cross pollination (hybridization). 
4) Could be easily achieved by removing of stamen (emasculation).  
 
Special point: 
S. Blixt concluded that the genes studied by Mendel are located on four different pairs of 
chromosomes. 
? Cotyledon/seed colour and Flower colour/Seed coat 
colour- Chromosome no. 1 
? Pod form, Flower position and Stem length- Ch no. 4 
? Pod colour- Ch no. 5 
? Seed form- Ch no. 7 
 
Mendel’s experimental technique –  
Mendel’s experiments were performed in three stages– 
1. Selection of pure or true breeding parents. 
2. Hybridisation and obtaining of F 1 generation of plants. 
3. Self-pollination of hybrid plants and rising of 
subsequent generations like F 2, F 3, F 4, etc. 
 
Some terms – 
? Unit factor – A unit of inheritance which passes from 
one generation to the next and controls the 
expression of a character in the organisms. 
 
? Alleles/Allelomorphs –The two Mendelian factors 
which occur on the same locus in the two homologous 
chromosomes of an individual and control the 
expression of a character. Term by Bateson. 
 
? Pure line – Term was given by Johannsen. It is strain 
of genetically pure true breeding individuals which have been derived from a single self 
fertilized homozygous ancestor or identical homozygous ancestors. 
 
? Dominant factor – One of a pair of  alleles which can express itself whether present in 
homozygous or heterozygous whether present in homozygous or heterozygous state. 
 
? Recessive factor – The factor of an allelic pair which is unable to express its affect in the 
presence of its contrasting factor in a heterozygous state. 
 
? Homozygous – An individual which contains identical alleles of a gene of a character on it 
homologous chromosomes. Homozygous and heterozygous term by Bateson. 
 
? Heterozygous – An individual which contains two contrasting factors of a character or two 
different alleles of a gene on its homologous chromosomes. 
 
? F 1 or first filial generation – Hybrids produced from a cross between the genetically different 
individuals. 
 
? F 2 or 2
nd
 filial generation – Individual arises as a result of inbreeding or interbreeding 
amongst individuals of F 1 generation. 
 
? Genotype – The genetic constitution of an individual with regard to 1 or more character 
irrespective of whether genes are expressed or not. Genotype and phenotype term by 
Johannsen. 
 
 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 3 
? Phenotype – Measurable distinctive structural and functional characteristic of an individual 
with regard to one or more characters. 
 
? Emasculation – Removal of stamens before the maturation of pollen grains. 
 
Inheritance of one gene (Mendel’s Monohybrid Cross) –  
- A cross between two organisms of a species which is made to study the inheritance of a single 
pair of allele or factors of character. 
- Mendel cross tall and dwarf pea plants to study the inheritance of one gene. Mendel observed 
that all the F 1 progeny plants were tall. 
- Mendel then self pollinated the tall F 1 plants and found that in the F 2 generation some of the 
offsprings were dwarf. 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
Phenotypic ratio - Tall : Dwarf       
           3   :   1 
Genotypic ratio - Pure tall : hybrid tall : Dwarf      
          1      : 2          :      1 
 
- 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 5.2).  
- 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 5.3). 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 was 1/4th 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 5.3). 
Similar results were obtained with the other traits that he studied: only one of the parental traits  
 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 4 
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 5.2 Steps in 
making a cross in pea Based on these observations, Mendel proposed that something was being 
stably passed down, unchanged, from parent to offspring through the gametes, over successive 
generations.  
- He called these things as ‘factors’. Nowadays, we call them as genes. Genes, therefore, are the 
units of inheritance. They contain the information that is required to express a particular trait, in 
an organism.  
- Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly 
different forms of the same gene.  
- If we use alphabetical symbols for each gene, then the capital letter is used for the trait 
expressed at the F 1 stage and the small alphabet for the other trait. For example, in case of the 
character of height, T is used for the Tall trait and t for the ‘dwarf’, and T and t are alleles of each 
other. Hence, in plants the pair of alleles for height would be TT, Tt or tt.  
- Mendel also proposed that in a true breeding, tall or dwarf pea variety the allelic pair of genes 
for height are identical or homozygous, TT and tt, respectively. TT and tt are called the genotype 
of the plant while the descriptive terms tall and dwarf are the phenotype. 
- As Mendel found the phenotype of the F 1 heterozygote Tt to be exactly like the TT parent in 
appearance, he proposed that in a pair of dissimilar factors, one dominates the other (as in the 
F 1) and hence is called the dominant factor while the other factor is recessive.  
- In this case T (for tallness) is dominant over t (for dwarfness), that is recessive. He observed 
identical behaviour for all the other characters/trait-pairs that he studied. It is convenient (and 
logical) to use the capital and lower case of an alphabetical symbol to remember this concept of 
dominance and recessiveness. (Do not use T for tall and d for dwarf because you will find it 
difficult to remember whether T and d are alleles of the same gene/character or not).  
- Alleles can be similar as in the case of homozygotes TT and tt or can be dissimilar as in the case 
of the heterozygote Tt. Since Figure 5.3 Diagrammatic representation of monohybrid cross the 
Tt plant is heterozygous for genes controlling one character (height), it is a monohybrid and the 
cross between TT and tt is a monohybrid cross.  
- From the observation that the recessive parental trait is expressed without any blending in the 
F 2 generation, we can infer that, when the tall and dwarf plant produce gametes, by the process 
of meiosis, the alleles of the parental pair separate or segregate from each other and only one 
allele is transmitted to a gamete.  
- This segregation of alleles is a random process and so there is a 50 per cent chance of a gamete 
containing either allele, as has been verified by the results of the crossings. In this way the 
gametes of the tall TT plants have the allele T and the gametes of the dwarf tt plants have the 
allele t.  
- During fertilisation the two alleles, T from one parent say, through the pollen, and t from the 
other parent, then through the egg, are united to produce zygotes that have one T allele and one 
t allele. In other words the hybrids have Tt.  
- Since these hybrids contain alleles which express contrasting traits, the plants are heterozygous. 
The production of gametes by the parents, the formation of the zygotes, the F 1 and F 2 plants can 
be understood from a diagram called Punnett Square as shown in Figure 5.4. It was developed  
 
Dr. Ramakant Mishra  Mob.:- 9827442330, 8871886728 
      
 
  Unit- VII 5 
by a British geneticist, Reginald C. Punnett. It 
is a graphical representation to calculate the 
probability of all possible genotypes of 
offspring in a genetic cross.  
- The possible gametes are written on two 
sides, usually the top row and left columns. All 
possible combinations are represented in 
boxes below in the squares, which generates a 
square output form.  
- The Punnett Square shows the parental tall TT 
(male) and dwarf tt (female) plants, the 
gametes produced by them and, the F1 Tt 
progeny. The F 1 plants of genotype Tt are self-
pollinated.  
- The symbols & and % are used to denote the 
female (eggs) and male (pollen) of the F 1 
generation, respectively.  
- The F 1 plant of the genotype Tt when self-
pollinated, produces gametes of the genotype 
T and t in equal proportion.  
- When fertilisation takes place, the pollen 
grains of genotype T have a 50 per cent chance 
to pollinate eggs of the genotype T, as well as 
of genotype t. Also pollen grains of genotype t 
have a 50 per cent chance of pollinating eggs of genotype T, as well as of Figure 5.4  
- A Punnett square used to understand a typical monohybrid cross conducted by Mendel between 
true-breeding tall plants and true-breeding dwarf plants genotype t. As a result of random 
fertilisation, the resultant zygotes can be of the genotypes TT, Tt or tt.  
- From the Punnet square it is easily seen that 1/4th of the random fertilisations lead to TT, 1/2 
lead to Tt and 1/4th to tt. Though the F 1 have a genotype of Tt, but the phenotypic character 
seen is ‘tall’. At F 2, 3/4
th
 of the plants are tall, where some of them are TT while others are Tt. 
Externally it is not possible to distinguish between the plants with the genotypes TT and Tt.  
- Hence, within the genopytic pair Tt only one character ‘T’ tall is expressed. Hence the character 
T or ‘tall’ is said to dominate over the other allele t or ‘dwarf’ character.   
- It is thus due to this dominance of one character over the other that all the F 1 are tall (though the 
genotype is Tt) and in the F 2 3/4
th
 of the plants are tall (though genotypically 1/2 are Tt and 
only 1/4th are TT). This leads to a phenotypic ratio of 3/4
th
 tall : (1/4 TT + 1/2 Tt) and 1/4th tt, 
i.e., a 3:1 ratio, but a genotypic ratio of 1:2:1. The 1/4 : 1/2 : 1/4 ratio ofTT: Tt: tt is 
mathematically condensable to the form of the binomial expression (ax +by) 2, that has the 
gametes bearing genes T or t in equal frequency of ½. The expression is expanded as- 
(½ T + ½ t)2= (½ T + ½ t) X (½ T + ½ t) = ¼ TT + ½ Tt + ¼ tt 
 
Test Cross –  
- Cross to know whether an individual is homozygous or heterozygous for dominant characters so 
individual is crossed with the recessive parent.