The Hardy–Weinberg law of population genetics is usually associated with the notion of random mating of parents. A numerical example for a triallelic autosomal locus shows that an uncountable set of mating combinations can maintain Hardy–Weinberg proportions. Therefore, one cannot infer random mating in a population from the observation of Hardy–Weinberg equilibrium. The mating system which ensures that the genotypic distribution of offspring is the same as that of the parents is specified.