The most intense form of inbreeding is the self-fertilisation in an hermaphrodite, in which the proportion of heterozygotes is halved in each generation. Aa x Aa (100%) gives 1 AA: 2 Aa; 1 aa (50%), and so on. A similar effect takes twice as long with brother-sister mating, and longer still with mating in very small isolated groups. The latter situation is famous because Sewall Wright did his calculations for just such groups, and he called the process genetic drift. Genetic drift and inbreeding are closely related concepts.