Breeding of Vegetable, Spice and Tuber crops (2+1)

Effects of self-pollination on genotype

• Self-pollination increases homozygosity with a corresponding decrease in heterozygosity.
  

• Inbreeding also increases homozygosity and reduces heterozygosity. Inbreeding is mating between individuals related by descent, that is, having a common parent or parents in their ancestry.
  
• Some examples of inbreeding are, sib mating (brother-sister mating), half-sib mating (brother-stepsister mating), etc.
  
• Self-pollination is the most intense form of inbreeding, since in this case the same individual functions as the male as well as the female parent.
• The effect of self-fertilization on homozygosity and heterozygosity may be illustrated by an example. Suppose an individual is heterozygous for a single gene (Aa) and the successive generations derived from it are subjected to self-pollination,every generation of self pollination will reduce the frequency of heterozygote Aa to 5 per cent of that in the previous generation.
  
• There is a corresponding increase in the frequency of the two homozygotes AA and aa. As a result, after 10 generations of selfing, virtually all the plants in the population would be only homozygous and heterozygosity would be 0.097 per cent, which is negligible.
  
• It is assumed here that the three genotypes AA, Aa and aa have equal survival. If there is unequal survival, it may increase or decrease the rate at which homozygosity is achieved.
  
• If Aa is favoured, the rate of increase in homozygosity would be lower than expected. But if Aa is selected against, homozygosity would increase at a faster rate than expected.
• When a number of genes are segregating together, each gene would become homozygous at the same rate as Aa. Thus the number of genes segregating does not affect the percentage of homozygosity.
  
• The term homozygosity denotes the frequency of genes in homozygous condition in the population. Similarly, linkage between genes does not affect the percentage of homozygosity in the population.
• Another way of visualizing the effect of self-pollination is to consider the frequency of plants, which are homozygous for all the genes.
  
• In case of a single gene, the frequency of completely homozygous plants in a generation is the same as the proportion of homozygosity increases at a much faster rate than that of completely homozygous plants.
  
• The proportion of completely homozygous plants is given by the following formula.

Proportion of completely
homozygous plants = [(2m -1)/2m] n

• Where, m is the number of generations of self pollination and n is the number of genes segregating.
  
• The proportion of completely homozygous plants under self-pollination is depicted.
  
• It is clear that as the number of genes increases, the proportion of completely homozygous plants after a given number of generation of selfing decreases. But the effect of selfing is so strong that even if 100 genes are segregating, more than 95 per cent of the population would be completely homozygous after only 12 generations.
• Linkage would increase the proportion of homozygous plants since it would reduce the number of genes segregating independently. Unequal survival of different genotypes would have the same effect as in the case of homozygosity.
• Thus continued self-pollination has two main effects on the population: first, all the plants in the population become completely homozygous, and second, the population of a self-pollinated species represents a mixture of several homozygous genotypes