Cbse guide: " Hereditary and evolution" class 10th

Topics to be covered

Introduction
What is Heredity?
Inherited traits
Mendel's experiments
Laws of Inheritance
Sex determination
Evolution
- Variations & its relation to Evolution
Acquired vs. Inherited traits
- Speciation
-Evolution by stages
Human evolution

What is Heredity?

Passing of traits from parents/ancestors to offspring
Genetics :- The branch of science which deal with the study of heredity

Heredity vs. Inheritance

Heredity

The process by which characters are transferred from one generation to the next generation is called inheritance / heredity
The differences in traits of individuals of a progeny from each other and from their parents are called variations
The branch of science which deals with inheritance and variation is called genetics

Inherited trait:

A trait that is genetically passed down from one generation to another

Inherited traits: Examples

Hair colour

. Eye colour

. Height

Shape of feet

. Ear lobes

How do the traits get inherited?

1. Mendel's Experiment

Gregor Johann Mendel (1822-1884) is known as 'Father of Genetics'.
Mendel performed his experiments with garden pea plant (Pisum sativum)
He conducted artificial pollination/cross-pollination experiments using several true-breeding varieties having contrasting traits
He observed one trait at a time
He hybridised plants with alternate forms of a single trait (Monohybrid cross). The seeds thus produced were grown to develop into plants of first filial generation (F₁)
Mendel then self-pollinated the F, plants to generate plants of second filial generation (F₂)
Later, Mendel also crossed pea plants that differed in two characters (Dihybrid cross)

2. Mendel's Experimental Plant

Mendel selected garden pea as his experimental material because of the following reasons.

(1) It is an annual plant with a short life-cycle. So, several generations can be studied within a short period.

(2) It has perfect bisexual flowers containing both male and female parts

(3) The flowers are predominantly self-pollmating It is easy to get pure line for several generations.

(4) It is easy to cross-pollmate them because pollens from one plant can be introduced to the stigma of another plant by removing the anthers

(5) Pea plant produces a large number of seeds in one generation

(6) Pea plants could easily be raised, maintained and handled

(7) A number of easily detectable contrasting characterstraits were available

3. Mendel's Observations

(1) F1 progenies always resembled one of the parents and trait of other parent was not seen

(2) F2 stage expressed both the parental traits in the proportion 3.1.

(3) The contrasting traits did not show any blending at either F1or F2 stage.

(4) In dihybrid cross, he got identical results as in monohybrid cross

(5) He found that the phenotypes in F2 generation appeared in the ratio 3:1

(6) He found that the genotypic ration in F2 generation appeared in the ratio 1:2:1

4. Mendel's Laws of Inheritance

■ Based on his hybridisation experiments, Mendel proposed the laws of inheritance.

(i) Law of dominance (First law)

■ This law states that when two alternative forms alleles are present in an organism, only one factor expresses itself in F1 progeny and is called dominant while the other that remains masked is called recessive.

(ii) Law of segregation (Second law)

■ This law states that alleles of a pair segregate from each other during gamete formation, such that a gamete receives only one of the two factors. They do not show any blending.

iii) Law of independent assortment

According to this law the two factors of each character separate out independent of the factors of other characters at the time of gamete formation and get randomly rearranged in the offsprings producing both parental and new combinations of characters.