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 depression seems to be present in most groups of organisms, but varies across mating systems.
Hermaphroditicspecies often exhibit lower degrees of inbreeding depression than
outcrossing species, as repeated generations of
selfing is thought to
purge deleterious
alleles from populations. For example, the outcrossing nematode (roundworm)
Caenorhabditis remanei has been demonstrated to suffer severely from inbreeding depression, unlike its hermaphroditic relative
C. elegans, which experiences
outbreeding depression.
[2]
Inbreeding (i.e., breeding between closely related individuals) results in more
recessive, deleterious traits manifesting themselves, because the
genomesof 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).
An example of inbreeding depression is shown to the right. In this case,
a is the recessive allele which has negative effects. In order for the
a phenotype to become active, the gene must end up as homozygous
aa because in the geneotype A
a, the A takes dominance over the
a and the
a does not have any effect. Due to their reduced phenotypic expression and their consequent reduced selection, 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 the fitness advantage of heterozygosity, which is known as
overdominance. This can lead to reduced fitness of a population with many homozygous genotypes, even if they are not deleterious or recessive. 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 thought to be more significant – it may yield completely unviable offspring (meaning outright failure of a
pedigree), while the latter can only result in relatively reduced fitness.
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