 Recall that the Hardy-Weinberg principle was derived under the assumption of random mating. But departures from random mating are actually quite common. Organisms may tend to choose mates who are similar to them phenotypically or genotypically—a mating system known as ‘positive assortment’. Alternatively, organisms may choose mates dissimilar to them—‘negative assortment’. Another type of departure from random mating is inbreeding, or preferentially mating with relatives.