Hardy Weinberg Equilibrium, Gene and Genotypic frequencies

The Hardy Weinberg law states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences.

<> In case of single locus with two alleles denoted “A” and “a” with frequencies f(A) = p and f(a) = q respectively. <> The expected frequencies are f(AA) = p2 for AA homozygotes, f(aa) = q2 for the aa homozygotes and, <> f(Aa) = 2pq for the heterozygotes. <> p2, q2, 2pq are called Hardy Weinberg proportions. <> Sum of all genotype frequencies is the square of the sum of p and q. <> (p+q)2 = p2 + q2 + 2pq <> Solution of this equation is q = 1-p

Derivation A locus in the population has two alleles “A” and ‘a” occurring with initial frequency f’(A)=p and f’(a)=q respectively. The allele frequencies at each generation are obtained by pooling together the alleles from each genotype of the same generation according to the homozygote and heterozygote genotypes, which are 1 and ½ respectively. f”(A) = f”(AA) + 1/2f”(Aa) f”(a) = f”(aa) = 1/2f”(Aa)

Equation (1) and (2) gives out <> f1(A) = f1(AA) = 1/2f1(Aa) = p2 + pq = p(p+q) = p = f’(A) <> f1(a) = f1(aa) + 1/2f1(Aa) = q2 + pq = q(p+q) = q = f’(a)

Limitation of Hardy Weinberg Law Mutations : These are permanent heritable changes in the genetic material of individuals. Gene mutations produce new alleles. Recombinations : During meiosis, crossing over causes reshuffling of gene combinations which provides new combinations of existing genes and alleles. Gene Migration : If the migrating individuals breeds within the new population the immigrants will add new alleles to the local gene pool of the host population. Genetic Drift : The random change in the allele frequency occurring by chance alone are called genetic drift.

Gene Frequency It is relative frequency of an allele at a particular locus in a population, expressed as a fraction or percentage.

It is the frequency of a genotype – Genotypic Frequency It is the frequency of a genotype – Homozygous recessive, homozygous dominant, or heterozygous in a population.

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