Types of selection

Selection against recessive phenotype:

• It acts on recessive phenotype say aa.
• It doesn’t affect the heterozygotes Aa.
• Since the deleterious effects of a in them are masked by A.
• “q” is the frequency of allele ‘a’.
• If value of q decreases the value of ∆q will also decline at increasing rate.
• So, selection against rare recessive alleles is virtually ineffective in eliminating them because these alleles escape the effects of selection by being present mainly in heterozygotes.

Selection against dominant phenotypes:

• Selection against a completely dominant alleles acts on both homozygous (AA) and heterozygous (Aa) dominant phenotypes.
• As a result frequency of A allele will decline in each generation.
• In due course of time, A will be eliminated and ‘a’ will become fixed in population.
• This happens because unlike the recessive allele ‘a’. the dominant allele A is not protected from selection in heterozygotes (Aa).

Selection in favor of heterozygotes:

• Selection for heterozygotes will retain both the allele A and a in the population and that these two alleles will be maintained in equilibrium.