According to Mendel, the individual contains two alleles for each character. Every organism receives on allele from male parent and another allele from female parent. The alleles are represented by english letters.
A. Genes for tall and Dwarf:
The gene that controls tallness is ‘T. The gene that controls dwarfness is ‘t’.
B. Genotypes of parents:
The genotype of homozygous tall is ‘TT. The genotype of pure dwarf plant is ‘tt’.
C. Gametes of ‘P’ generation:
Pure tall (TT) produces 1 00% one type of gametes. Every gamete carries ‘T’ to the progeny.
Pure dwarf pisum plant tt also produces 1 00% one type of gametes. Each gamete carries ‘t’ to the progeny.
D. Formation of monohybrids:
‘T’ type of gametes fuse with ‘t’ type of gametes. Hence all ‘F1’ individuals are ‘Tt’. As ‘T’ allele is dominant over’t’ allele, all ‘F1’ individuals are phenotypically tall. These are monohybrids. Hybrids are always heterozygous. 
E. Formation of gametes from ‘F1’ or hybrid tall (Tt): The two alleles ‘T’ and ‘t’ of the hybrid tall seperate during gamete formation. If ‘T’ enters into one gamete, automatically t’
enters into another gamete. This tells us that the monohybrid produces two types of gametes. They are 50% ‘T’ type and 50% ‘t’ type.
F. Formation of F individuals:
Male parent produces two types of Pollengrains, ‘T’ and ‘t’. Like wise female parent also produces two types of ovules ‘T’ and ‘t’. When ‘T’ and ‘t’ types of ovules are fertilized by ‘T’ and ‘t’ types of pollen grains, the ‘F2’ generation produced is of 1/4 n 2/4 Tt and 1/4 if (1/4 pure tall, 2/4 heterozygous tall, 1/4 homozygous dwarf).
So, monohybrid ‘F2’ genohypic ratio is I : 2: 1. As the ‘t’ ovule is fertilized by ‘t’ pollen grain, dwarf ‘if’ individual is produced. This indicates that ‘t’ is seperated from ‘T’ in ‘F1’during
gametogenesis. This seperation is called law of segregation. So, the formation of 1/4 recessive progeny in ‘F2’ is explained by law of segregation.
Mendel did 7 monohybrid crosses and observed the inheritance of ‘7’ characters. Each character appears in two morphological forms. He got the same ratios 3 : 1 phenotypic and 1 : 2 : 1 genotypic ratios for all the ‘7’ characters.
