Inbreeding Depression - Mechanisms

Mechanisms

Breeding between closely related individuals, called inbreeding, may on one hand result in more recessive deleterious traits manifesting themselves, because the genomes of pair-mates are more similar: recessive traits can only occur in offspring if present in both parents' genomes, and 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).

An example of inbreeding depression is shown to the right. In this case, a is a recessive allele which has negative effects. In order for the a phenotype to become active, the gene must end up as aa because in the geneotype Aa, the A takes dominance over the a and the a does not have any effect. Due to evolutionary causes, recessive genes are, more often than not, detrimental phenotypes by causing the organism to be less fit to its natural environment.

Another mechanism responsible for inbreeding depression is overdominance of heterozygous alleles. This can lead to reduced fitness of a population with many homozygous genotypes, even if they are not deleterious. Here, even the dominant alleles result in reduced fitness if present homozygously (see also hybrid vigour).

Currently, it is not known which of the two mechanisms is more prevalent in nature. For practical applications, e.g. in livestock breeding, the former is probably more significant – it may yield completely unviable offspring (meaning outright failure of a pedigree), while the latter can only result in relatively reduced fitness.