Inbreeding depression
This is the exposure of the individuals in a population to the effects of deleterious recessive genes through matings between close relatives. For a given locus, some alleles will confer more fitness on an individual than other alleles. Within the "other" class of alleles are rare deleterious recessive alleles, which when appearing as a homozygous genotype in an individual because of mating between relatives, greatly reduces the fitness of the individuals carrying them. Deleterious alleles arise constantly through mutation, so they are always present in a population at low frequencies. Suppose we have two alleles, A and a, where A is a normal allele and a is a deleterious allele. The homozygous genotype aa of the deleterious allele is rare in a large population, because with random mating the expected frequency of a homozygote is the square of the allelic frequency p2, and for a low-frequency allele this is a small value. AA individuals are the most fit of the three possible genotypes. Aa individuals have the same fitness as AA individuals if the A allele is dominant over the a alleles, or they may have some intermediate level of fitness if the effects of the alleles are more additive. Lastly, aa individuals show some recessive deleterious trait that reduces their fitness.
In a large population where the a allele occurs at a low frequency, the a allele appears chiefly in the heterozygous state Aa, and heterozygous individuals will almost always mate AA individuals. The offspring of an AA X Aa mating will be AA or Aa, and the effects of the recessive deleterious allele are masked. On the other hand, if mating occurs between relatives in which both relatives have a copy of the deleterious allele in the heterozygous state, an Aa X Aa mating, one-fourth of the offspring of the mating are expected to have the deleterious aa genotype. Mating between relatives "unmasks" the effects of recessive deleterious alleles that would otherwise occur only in heterozygous individuals.
This is the exposure of the individuals in a population to the effects of deleterious recessive genes through matings between close relatives. For a given locus, some alleles will confer more fitness on an individual than other alleles. Within the "other" class of alleles are rare deleterious recessive alleles, which when appearing as a homozygous genotype in an individual because of mating between relatives, greatly reduces the fitness of the individuals carrying them. Deleterious alleles arise constantly through mutation, so they are always present in a population at low frequencies. Suppose we have two alleles, A and a, where A is a normal allele and a is a deleterious allele. The homozygous genotype aa of the deleterious allele is rare in a large population, because with random mating the expected frequency of a homozygote is the square of the allelic frequency p2, and for a low-frequency allele this is a small value. AA individuals are the most fit of the three possible genotypes. Aa individuals have the same fitness as AA individuals if the A allele is dominant over the a alleles, or they may have some intermediate level of fitness if the effects of the alleles are more additive. Lastly, aa individuals show some recessive deleterious trait that reduces their fitness.
In a large population where the a allele occurs at a low frequency, the a allele appears chiefly in the heterozygous state Aa, and heterozygous individuals will almost always mate AA individuals. The offspring of an AA X Aa mating will be AA or Aa, and the effects of the recessive deleterious allele are masked. On the other hand, if mating occurs between relatives in which both relatives have a copy of the deleterious allele in the heterozygous state, an Aa X Aa mating, one-fourth of the offspring of the mating are expected to have the deleterious aa genotype. Mating between relatives "unmasks" the effects of recessive deleterious alleles that would otherwise occur only in heterozygous individuals.
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