Genetics use the terms underdominance or heterozygote disadvantage to describe the condition where organisms that are heterozygotes for a given trait are less fit than organisms that are homozygotes. In other words, given a single locus with two alleles, A , and a, underdominance occurs when the heterozygote, Aa, is less fit than either homozygote, AA or aa.
We can use polynomial functions to calculate the equilibrium allele frequency for a trait that exhibits underdominance.
Consider a single locus with two alleles A and a, with frequencies p and q, respectively. Suppose we assign fitness values 1 + s1 : 1 : 1 + s2 to AA: Aa: aa where s1 and s2 are positive. Using these fitness values, we can find the equilibrium allele frequencies by solving the equation,
Equilibrium allele frequencies are special values of p and q that satisfy the above equation and, once attained, result in no change in allele frequency in future generations. These equilibrium allele frequencies may or may not be attainable.
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Some herbicide resistance genes in plants exhibit underdominance in the absence of herbicides. This can be studied in the laboratory using Arabidopsis thaliana, pictured above. Photo credit: Keith Weller, courtesy of United States Department of Agriculture, Agricultural Research Service.
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