ABSTRACT
One of the more important requirements for the Hardy-Weinberg equilibrium is that mating in the population is random, in the sense that every genotype has equal probability to mate with any other. If mating is not random, prediction of genotype frequencies in the offspring is possible only if the mating system can be specified. If a population is subdivided, an overall deficiency in heterozygotes and excess of homozygotes is expected due to variation in allele frequencies among subunits. Outbreeding mating systems reduce the probability of union of two gametes identical by descent. Such mating systems are not as common in nature as inbreeding. The extreme case of outbreeding is found in systems of self-sterility or self-incompatibility. The majority of such cases in multicellular organisms are found in the vegetable kingdom. Outbreeding has been considered favorable, but crossing of widely-distinct varieties may also bring about deleterious effects and loss of fitness.
