So far, we have considered only a single deleterious allele at a single locus. However, extrapolating from lower organisms and plants (Lynch et al. 1995), about 100 deleterious alleles are present in individuals of higher organisms when we look across all genetic loci (see Lynch and Gabriel 1990). The problem is therefore not trivial when all of the loci are considered. Most of these deleterious mutations produce only a small reduction in fitness of about 2%, when the alleles are made homozygous. If all of the loci in an individual are made homozygous through mating between relatives, the reduction in fitness would be on the order of 200%, enough to "kill" the individual two times over. This, essentially, is inbreeding depression.
Outbreeding depression
This phenomenon can occur in two ways. One way is by the "swamping" of locally adapted genes in a wild population by straying from, for example, a hatchery population. In this case, adaptive gene complexes in wild populations are simply being displaced by the immigration of genes that are adapted to the hatchery environment or to some other locality. For example, selection in one population might produce a large body size, whereas in another population small body size might be more advantageous. Gene flow between these populations may lead to individuals with intermediate body sizes, which may not be adaptive in either population. A second way outbreeding depression can occur is by the breakdown of biochemical or physiological compatibilities between genes in the different populations. Within local, isolated populations, alleles are selected for their positive, overall effects on the local genetic background. Due to nonadditive gene action, the same genes may have rather different average effects in different genetic backgrounds--hence, the potential evolution of locally coadapted gene complexes. Offspring between parents from two different populations may have phenotypes that are not good for any environment. It is important to keep in mind that these two mechanisms of outbreeding depression can be operating at the same time. However, determining which mechanism is more important in a particular population is very difficult.