heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class heredity and evolution class

1. 4. Heredity and Evolution
By:- Mr. Rajesh

2. Topics in the chapter
 1. Introduction
 2. Types of Variation:- a) Somatic Variation b) Gametic Variation
 3. Accumulation of Variation during reproduction
a) Variation in asexual reproduction
b)Variation in sexual Reproduction
c) Importance of variation.
 4. Mendel and his work on inheritance.
a) Seven pairs of contrasting characters
b) Mendel’s Experimental Material
 Monohybrid cross
 Dihybrid Cross
 Sex determination

3. Some important terms
 Genetics:- Genetics refers to the study of heredity and variation
 Heredity:- The transmission of characters from one generation to next
generation is called heredity.
 Variations:- The difference in the character/traits between the parent
and offspring is called variation

4. Types of Variation:-
1) Somatic Variation:-
a) It takes place in the body cells
b) It is neither inherited nor transmitted.
c) It is also known as acquired traits.
examples :- Cutting of tails in dogs, Boring of pinna etc.
2) Gametic Variation:-
a) It takes place in the gamete or reproductive cells.
b) Inherited as well as transmitted
c) Also known as inherited traits
d) Ex:- Human heights, Skin colour etc.

5. Accumulation of variation during reproduction
 Variation occurs during reproduction whether organism multiplies sexually
or asexually.
1. Variation in asexual reproduction
a) Variations are fewer
b) Occurs due to small inaccuracies in DNA copying
2. Variation in sexual reproduction
a) Variations are large
b) Occurs due to crossing over, separation of chromosomes

6. Importance of Variations:-
 1. It forms the basis of heredity
 2. It causes adaptation due to which organism can easily adjust to its
changing environment
 3. Accumulation of variations forms the basis of evolution.

7. Some important terms
 1. Chromosomes:- Chromosomes are long thread like structures present in the
nucleus of a cell which contains hereditary information of the cell in the form
of genes.
 2. DNA ( Deoxyribo Nucleic acid):- It is a chemical in the chromosome which
carries the traits in the coded form
 3. Genes:- Gene is the part of a chromosome which controls specific biological
function.
 4. Contrasting Characters:- It refers to a pair of visible characters.
Ex:- Tall- dwarf, White – Violet, Round – wrinkled, Green- yellow seeds etc.

8. Some important terms
 5. Dominant Trait:- The character which expresses itself in a generation is
called dominant trait .
 6. Recessive Traits:- The character which does not expresses itself but is
present in a generation is called dominant trait .
 7. Homozygous:- The condition in which both the gene of same types are
present. Example :- If an organism has both the genes for tallness it is
expressed as “TT” and genes for dwarfness is written as “tt”
 8. Heterozygous:- The condition in which both the genes of different
types are present.
 9. Genotypes:- It is a genetic makeup of an individual . Example:- A pure
tall plant is expressed as “TT” and hybrid tall is “Tt”

9. Some important terms
 10. Phenotypes:- It is external appearance of the organism .
Example:- A plant having “Tt” composition will appear tall
although it has genes for dwarfness.
 11. Characters:- It is a distinct feature of an organism like flower
colour, plant’s height etc.
 12. Trait:- It is the expression of a character.
Ex: - Tallness for Height
 13. Alleles:- Genes occurring in homologous chromosomes at some
locus
 14. F1 Generation:- It is a generation of hybrid that is formed from a
cross between two genetically different pure breeding individuals.

10. Mendel and his work on inheritance
 Mendel: Known as father of genetics
 Mendel suggested pea plant for his experimental material
 Pisum Sativum:- Experimental material
a) It occupies small space
b) It has short life span and annual nature.
c) Presence of easily detectable contrasting traits.
d) Formation of large number of seeds.
e) The plant require little care.

11. Seven Characters with contrasting traits
selected by Mendel:-
S.NO Character Dominant Trait Recessive Trait
1 Height Tall (T) Dwarf (t)
2 Seed Shape Round (R) Wrinkled (r)
3 Seed Colour Yellow (Y) Green (g)
4 Flower Colour Violet/Purple (P) White (p)
5 Pod Colour Green (G) Yellow (g)
6 Shape of Ripe Pod Inflated (I) Constricted (i)
7 Flower Position Axial (A) Terminal (a)

12. Mendel’s Law of Inheritance
 1. Law of Unit Factor:-
It states that “ Each genetic character is represented or controlled by a pair of
unit factor called as alleles”.
 2. Law of dominance:- It states that only dominant allele or factor expresses its
effect in the hybrid though it has received the allele for recessive traits as well
Pure tall x Pure dwarf
Parent TT tt
Gametes T T t t
F 1 Generation Pure Tall
Pure dwarf
T T
t Tt Tt
t Tt Tt

13. Mendel’s Law of Inheritance
 3. Law of Segregation/Law pf purity of gametes:-
As per the law of segregation , the two alleles of a character separate out
during gametogenesis and pass to different gametes. A gamete comes to
have only a single allele of a character.
For Monohybrid Cross:-
a) Phenotypic ratio :- 3:1 ( tall: dwarf)
b) Genotypic Ratio :- 1:2:1 ( Pure tall: Hybrid tall: Pure dwarf)

14. Mendel’s Law of Inheritance
Pure tall x Pure dwarf
Parent TT t t
Gametes T T t t
F 1 Generation Tt
F 2 Generation
T t
T TT Tt
t Tt tt

15. Mendel’s Law of Inheritance
 4. Law of independent Assortment:- The two factors or allele of a character separate out
independent of factors of another character at the time of gametogenesis
 For dihybrid cross
Tall rounded= 9
Tall wrinkled = 3
Dwarf round = 3
Dwarf Wrinkled= 1
Therefore Phenotypic ratio = 9:3:3:1

16. Mendel’s Law of Inheritance
Pure tall and Pure Round x Pure dwarf and Pure Wrinkle
Parents TTRR ttrr
Gametes TR TR tr tr
F 1 Generation TtRr ( Hybrid tall and Round)
F 2 Generation
TR Tr tR tr
TR TTRR TTRr TtRR TtRr
Tr TTRr TTrr TtRr Ttrr
tR TtRR TtRr ttRR ttRr
tr TtRr Ttrr ttRr ttrr

17. Sex Determination
 Human beings possess 23 pairs of chromosomes.
 22 pairs are autosomes.
 One pair of chromosomes are sex chromosomes or allosomes.
 The sex chromosomes are similar are in female .They are XX
chromosomes.
 The sex chromosomes are different in male. They are XY chromosomes.
 When X chromosome of female fuses with X chromosome of male ,the
resultant is XX, a girl child .
 When X chromosome of female fuses with Y chromosome of male ,the
resultant is XY, a boy child.

18. Sex Determination