What assumptions must be met for a population to be in genetic equilibrium. Explain the importance of genetic equilibrium.

The Hardy–Weinberg statement

• For two alleles, p and q, random mating produces genotypes p², 2pq, and q², a relation named for G. H. Hardy and Wilhelm Weinberg in 1908.
• If the assumptions hold, those frequencies recur, and allele frequencies do not drift by the mating system itself.

Assumptions

• The population is indefinitely large, so chance genetic drift is negligible.
• Mating is random with respect to the locus, with no inbreeding or assortative mating that would raise homozygotes.
• There is no mutation at rates that would shift p and q in the study window.
• There is no migration that imports a different allele frequency.
• There is no selection for or against genotypes at that locus.
• Generations are discrete in the simple teaching model, and the locus is autosomal in the basic two-allele form; sex-linked and multiple-allele extensions need extra algebra.

Importance

• Equilibrium is the null hypothesis of population genetics: departure diagnoses drift, inbreeding, migration, or selection, which is why R. C. Lewontin and anthropological genetics use it on blood groups and now on SNPs.
• Forensic match probabilities and genetic counselling use Hardy–Weinberg expectations to translate allele frequencies into genotype frequencies.
• Sickle-cell heterozygote advantage in malaria belts is taught as a selection break of equilibrium, not as a failure of Mendelism.
• Inbreeding in small tribal isolates raises homozygosity above 2pq expectations, which is an anthropological use of the same test.
• Conservation genetics of small populations uses the opposite of the infinite-population assumption to explain loss of diversity.

What it is not

• No real human population meets every assumption forever.
• The model is still necessary as a measuring stick.