Genetic distance between populations : Part 3: Wahlund's principle as related to genetic distance and an application.

An excess proportion of homozygous carriers of a gene arises on bulking of reproductively isolated subpopulations. This surplus of homozygotes in the mixture, measured relative to the panmictic proportion, is caused by variation of its frequencies in the respective subpopulations. It is shown that the result known as Wahlund's principle can be expressed as a simple function of genic distances among the subpopulations. If used with caution it can be employed to recognize mixtures of seed lots. The effect of bulking can be readily discriminated from that of inbreeding. It may also be distinguished from the effect of assortative mating by analysing such distances at several gene loci. Various effects of selection may disturb inference on whether a given lot is a mixture. Hence application should be confined to the dormant seed. Isozyme loci are most suitable for such checks since complete dominance occurs only rarely. Some advantages peculiar to conifer seed are discussed.