The genetic basis of inbreeding depression

Data on the effects of inbreeding on fitness components are reviewed in the light of population genetic models of the possible genetic causes of inbre eding depression. Deleterious mutations probably play a major role in causi ng inbreeding depression. Putting together the different kinds of quantitat ive genetic data, it is difficult to account for the very large effects of inbreeding on fitness in Drosophila and outcrossing plants without a signif icant contribution from variability maintained by selection. Overdominant e ffects of alleles on fitness components seem not to be important in most ca ses. Recessive or partially recessive deleterious effects of alleles, some maintained by mutation pressure and some by balancing selection, thus seem to be the most important source of inbreeding depression. Possible experime ntal approaches to resolving outstanding questions are discussed.