Inbreeding depression

Inbreeding depression is the reduced biological fitness in a given population as a result of inbreeding, or breeding of related individuals. Population biological fitness refers to an organism's ability to survive and perpetuate its genetic material. Inbreeding depression is often the result of a population bottleneck. In general, the higher the genetic variation or gene pool within a breeding population, the less likely it is to suffer from inbreeding depression.Inbreeding (i.e., breeding between closely related individuals) results in more recessive traits manifesting themselves, as the genomes of pair-mates are more similar. Recessive traits can only occur in an offspring if present in both parents' genomes. The more genetically similar the parents are, the more often recessive traits appear in their offspring. Consequently, the more closely related the breeding pair is, the more homozygous, deleterious genes the offspring may have, resulting in very unfit individuals. For alleles that confer an advantage in the heterozygous and/or homozygous-dominant state, the fitness of the homozygous-recessive state may even be zero (meaning sterile or unviable offspring).Natural selection cannot effectively remove all deleterious recessive genes from a population for several reasons. First, deleterious genes arise constantly through mutation within a population. Second, in a population where inbreeding occurs frequently, most offspring will have some deleterious traits, so few will be more fit for survival than the others. Different deleterious traits are extremely unlikely to equally affect reproduction – an especially disadvantageous recessive trait expressed in a homozygous recessive individual is likely to eliminate itself, naturally limiting the expression of its phenotype. Third, recessive deleterious alleles will be 'masked' by heterozygosity, and so in a dominant-recessive trait, heterozygotes will not be selected against.Introducing alleles from a different population can reverse inbreeding depression. Different populations of the same species have different deleterious traits, and therefore their cross breeding will not result in homozygosity at most loci in the offspring. This is known as outbreeding enhancement, practiced by conservation managers and zoo captive breeders to prevent homozygosity.The biological effects of inbreeding depression in humans are largely obscured by socioeconomic and cultural influences on reproductive behavior. Studies in human populations have shown that age at marriage, duration of marriage, illiteracy, contraceptive use, and reproductive compensation are the major determinants of apparent fertility, even amongst populations with a high proportion of consanguinous unions. However, several small effects on increased mortality, longer inter-birth intervals and reduced overall productivity have been noted in certain isolated populations.Whilst inbreeding depression has been found to occur in almost all sufficiently studied species, some taxa, most notably some angiosperms, appear to suffer lower fitness costs than others in inbred populations. Three mechanisms appear to be responsible for this: purging, differences in ploidy, and selection for heterozygosity. It must be cautioned that some studies failing to show an absence of inbreeding depression in certain species can arise from small sample sizes or where the supposedly outbred control group is already suffering inbreeding depression, which frequently occurs in populations that have undergone a recent bottleneck, such as those of the naked mole rat.