Mendel crossed varieties of edible peas which showed clearcut differences in morphological characters such as colour of flowers (red vs. white), shape of pod (inflated vs.constricted), colour of pod (green vs. yellow), texture of seed (round vs. wrinkled), colour ofcotyledons (yellow vs. green), flower position (axial vs. terminal) and height of plant (tall vs.dwarf). He was dusting the pollen of one variety on the pistil of the other. To prevent self pollination of the female parent, he removed its stamens before the flowers had opened and
shed the pollen. After making the cross he would enclose the flowers in bags to protect them from insects and foreign pollen.
Mendel’s first experiments explain how a single gene segregates in inheritance. When Mendel crossed a true breeding tall plant (female parent) with a true breeding plant of the dwarf variety (male parent), he got tall plants like one parent in the first filial generation designated Fv He used the term “dominant” for the tall character which dominated in the F1 generation, and “recessive” for the character of dwarfness which remained hidden (latent) in the Fl generation. Self fertilisation of the Fx hybrids produced the second filial generation F2 consisting of a total of 1064 plants of which 787 were tall and 277 were dwarf. That is tall and dwarf plants appeared in F2 in the proportion of 2.84 : 1 which is roughly equal to 3 : 1. When he performed the reciprocal cross by reversing the sexes of the parents, the same results were obtained showing thereby that it did not matter which plant was used as male or as female parent. Similarly, Mendel crossed pea plants differing in other characters such as colour of flowers (red flowered versus white flowered), texture of seed (round versus wrinkled), colour of cotyledons (yellow versus green). Such a cross which involves only one character from each parent is called a monohybrid cross. In each case Mendel found one parental character dominating in th eF1 hybrid, and after self fertilisation inF2 generation both parental characters appeared in the proportion of three-fourths to one-fourth. He performed each experiment on
several thousand plants and counted all the plants in F2 progeny which gave an average ratio of 3 : 1.
Segregation of Genes: From his experiments Mendel concluded that each parent contributes one factor for a character to the F1 hybrid. In this way the Fx hybrid has two factors for each character. When the Fl hybrid forms gametes the two factors separate from each other. There is no mixing up of factors thus emphasising the purity of gametes. The phenomenon of separation became Mendel’s First Principle and was later termed as the Law
of Segregation. This is explained diagrammatically as follows:
P: Tall (true breeding) x Dwarf (true beeding)
Factors: (TT) (tt)
Gametes: T t
Fl hybrid: Tall x Self i.e. Tall (Tt)
Gametes: T, t T, t
F2 : 3 tall, 1 dwarf
(TT,Tt, tT), (tt)