Population polymorphisms and their causes


 * A Population where the gene frequency of the most common allele is less than or equal to 0.99 (99%) is polymorphic for a given trait .
 * Of course, this stated value is not an objective limit, but was only determined by agreement.
 * It is most convenient to determine the degree of polymorphism using heterozygosity, which is defined as:


 * $$H = 1 - \sum_{i=1}^m x_i^2 $$
 * where m = number of alleles of the monitored gene and xi = gene frequency of the ith allele (C-H-W applies: x1 +x 2...+xm=1)
 * or verbally as the representation of individuals in a population who are heterozygous for a particular locus.


 * Example: In the population, the allele representation is p=0.5 and q=0.5.
 * $$H = 1 - \sum_{i=1}^m x_i^2 = 1 - 0,5^2 - 0,5^2 = 1 - 0,25 - 0,25 = 0,5$$
 * This is also the maximum that can be achieved. It is true that the larger m and the more unevenly distributed frequency x, the smaller H is.
 * The minimum would be for p → 1 and q → 0 (H ≈ 0), where the vast majority of homozygotes would be.


 * Heterozygosity can therefore serve us as a good measure between subpopulations of one population.

Stable polymorphism

 * gene frequencies do not change;
 * e.g. population in C-H-W equilibrium, or polymorphism maintained by the frequency of heterozygotes, or mutations and back mutations.

Transitive polymorphism

 * In a population, when due to selection one allele is gradually replaced by another, as is the case, for example, with selection against homozygotes.

Related Articles

 * Castle-Hardy-Weinberg Equilibrium
 * Nucleic acid polymorphisms
 * Selection
 * Population