Factors affecting equilibrium frequencies

a) Migration:

• Migration may introduce new alleles into the population or may change the frequencies of existing alleles.
• The amount of change in gene frequency q will primarily depend upon two factors; first, the ratio of migrant individuals to those of the original population and second, the magnitude of difference between the values of q in the population and in the migrants.
• In plant breeding programmes, migration is represented by intervarietal crosses, polycrosses, etc., wherein the breeder brings together into a single population two or more separate populations.

b) Mutation:

• Mutation may produce a new allele not present in the population or may change the frequencies of existing alleles.
• However, since the mutation rate is generally very low, i.e., approximately 10-6, the effects of mutation on gene frequency would be detectable only after a large number of generations.
• Therefore, in breeding populations such effects may be ignored.

c) Random drift:

• Random drift or genetic drift is a random change in gene frequency due to sampling error.
• Random drift occurs in small populations because sampling error is greater in a smaller population than in a larger one.
• Ultimately, the frequency of one of the alleles becomes zero and that of the other allele becomes one.
• Breeding populations are generally small; hence a certain amount of genetic drift is bound to occur in them.
• The breeder cannot do anything to prevent this genetic drift, except to use very large populations, which is often not practicable.
• Alternatively, he may resort to phenotypic disassortative mating, which would again require time, labor and money.

d) Inbreeding:

• Mating between individuals sharing a common parent in their ancestry is known as inbreeding.
• Inbreeding reduces the proportion of heterozygotes or heterozygosity and increases the frequency of homozygotes or Homozygosity.
• The rate of decrease in heterozygosity is equal to ½ N (N=number of plants in the population) per generation in monoecious or hermaphrodite species.
• In dioecious species and in monoecious species where self-pollination is prevented, the decrease in heterozygosity is somewhat lower; it is equal to

 ½(N=1) per generation.

• Thus, in small populations, even with strict random mating or even with strict cross-pollination the frequency of homozygotes increases, while that of heterozygotes decreases due to inbreeding.

e) Selection:

• Differential reproduction rates of various genetopes is known as selection.
• It allows the selected genotypes to reproduce, while the undesirable genotypes are eliminated.
• Thus, the breeder is able to improve the various characteristics by selecting for the desirable types.
• In a random mating population, if plants with AA or aa genotypes are selected, the frequency of A allele in the selected population would be 1 or 0, respectively.
• Selection is expected to change gene frequencies rather than to eliminate one or the other allele.