The Law of Purity of Gametes states that ‘when a pair of contrasting factors or genes or allelomorphs are brought together in a heterozygote (hybrid) the two members of the allelic pair remain together without being contaminated and when gametes are formed from the hybrid the two separate out from each other and only one enters each gamete’.
Example: Pure tall plants are homozygous and, therefore, possess genes (factors) TT. Similarly, dwarf plants possess genes tt. Tallness and dwarfness are two independent but contrasting factors or determiners. Pure tall plants produce gametes all of which possess gene T and dwarf plants t type of gametes.
During cross-fertilization gametes with T and t unite to produce hybrids of F1 generation. These hybrids possess genotype Tt. It means that F1 plants through tall phenotypically, possess one gene for tallness and one gene for dwarfness.
Apparently, the tall and dwarf characters appear to have become contaminated developing only tall characters. But at the time of gamete formation, the genes T (for tallness) and t (for dwarfness) separate and are passed on to separate gametes. As a result, two types of gametes are produced from the heterozygote in equal numerosity. 50% of the gametes possess gene T and the other 50% possess gene t. Therefore, these gametes are either pure for tallness or for dwarfness. So, that’s why the law of segregation is also called as law of purity of gametes.