The Hardy-Weinberg law makes several fundamental assumptions that are not always true of actual human populations:
1. The population is characterized by random mating, with little if any stratification, assortative mating, or inbreeding.
2. The locus under consideration exhibits a constant mutation rate, and mutant alleles lost by death are replaced by new mutations.
3. There is no selection for or against a particular phenotype; all genotypes at a locus are equally viable.
4. The population is sufficiently large that there has been no random fluctuation of frequencies resulting from transmission of any one genotype simply by chance.
5. There has been no charge in the population structure by migration, which can gradually change gene frequencies by increasing or decreasing the number of increasing or decreasing the number of individuals with a particular genotype.
If in a given population one or more of these assumptions does not hold true, the genotypes in that population may not be in Hardy-Weinberg equilibrium.