this is in continuation to the last post

RULES FOR THE INHERITANCE OF TRAITS: Mendel’s Contribution

In human beings, both the parents contribute equal amount of genetic material to the child. This means that each character can be influenced by both paternal and maternal DNA (Deoxyribose Nucleic Acid). So, there are two versions for each character in a child. Which of these two versions will be expressed in the child? Gregor Johann Mendel was an Austrian monk. He worked out main rules of such inheritance by carrying out various crosses which are described below one by one. He selected garden pea (Pisum sativum) for carrying out his experiment. He studied on 7 different contrasting characters of garden pea. The two main crosses are:

a) Monohybrid cross- it involved only one pair of contrasting character. (Plant height)

b) Dihybrid cross- it involved two pairs of contrasting character. (Seed colour, Seed shape)

1. Principle/Rule of Paired Factor

Each factor (and its two forms called traits) is controlled by a pair of factors. The factors may be similar or dissimilar.

                    Character           Traits         Factors      Similar factors               dissimilar factors    

                                                                      in an organism              in an organism

                    -- Tallness           (Tall)                TT                

   Height ---                                                                                            Tt

                    -- Dwarfness      (Dwarf)              tt

·         Paired condition of factors is present in the parents like TT, tt or Tt.

·         Paired condition is broken at the time of gamete formation. Each gamete gets only one factor of that character, Either T or t.

Parent           Tt      X       Tt

               Gamete     T      t        T      t

·         Paired condition in restored after fertilization. Fertilization is random, it means, any male gamete can fuse with any female gamete.

2. Principle of Dominance

When two dissimilar factors of a character are present in an organism, only one expresses itself and is called dominant factor while other remains unexpressed and is called recessive factor.

For example, if one plant has two dissimilar factors for height, i.e., Tt, then out of these two factors T expresses itself and is called Dominant factor while t remains unexpressed and is called Recessive factor. There is no difference in the height between two plants with TT or Tt factors. No medium-height is reported in Tt plants, although t is present.

              

               Whether the tall plants F1 generation exactly the same as the tall plants of the parent generation. Mendel tested this by getting both the parental plants and these F1 tall plants to reproduce by self pollination. The progeny of the tall plants, of course, are all tall. However, F2 or second generation progeny of the F1 tall plants are not all tall. Instead three quarters of them are tall and one quarter are short, i.e., in the phenotypic ratio of 3:1.

                It is clearly indicated by the above observations that both the tallness (T) and shortness (t) traits were inherited in the F1 plants, but only the tallness trait was expressed. Thus, two copies of the trait are inherited in each sexually reproducing organism.

3. Principle of Segregation (Purity of Gametes)

Two factors of a character are separated at the time of gamete formation and each gamete gets only one factor for the character. It means gametes are always pure for their character. These gametes are (t) and (t). Gamete with capital T is pure for tallness and gamete with small t is pure for shortness/ dwarfness.

4. Principle of Independent assortment

This principle is explained by a Dihybrid cross. One of the Dihybrid crosses done by Mendel was the cross between a homozygous pea plant bearing round and yellow seeds (RRYY) and homozygous wrinkled green seeded pea plant. In F1 generation, hybrids with round yellow seeds were produced which on self pollination produce four types of plants in the ratio of 9:3:3:1

(9 ROUND YELLOW: 3 ROUND GREEN: 3 WRINKLED YELLOW: 1 WRINKLED GREEN).

From the above results, Mendel formulated the principle of independent assortment which can be defined as the inheritance of two or more pairs of contrasting traits in such a way that one pair of contrasting traits is independent of the other pair of contrasting traits. In other words, round/wrinkled traits and yellow/green traits are independently inherited.

For his contribution in the field of genetics he is considered as the

 “FATHER OF GENETICS”.