Autozygosity or inbreeding coefficient

(f) The probability that two alleles in a homozygous genotype are identical by descent.

Autozygous genotype A homozygous genotype composed of two identical alleles that are inherited from a common ancestor.

Identity by descent Transmission from a common ancestor. Relatedness The expected proportion of alleles between two individuals that are identical by descent; twice the autozygosity.

Phenotypic consequences of inbreeding

The process of consanguineous mating or inbreeding is associated with changes in the mean phenotype within a population. These changes arise from two general causes: changes in genotype frequencies in a population per se and fitness effects associated with changes in genotype frequencies.

The mean phenotype of a population will be impacted by the changes in genotype frequency caused by inbreeding. To show this it is necessary to introduce terminology to express the phenotype associated with a given genotype, a topic covered in much greater detail and explained more fully in Chapters 9 and 10 in this volume. We will assign AA genotypes the phenotype +a, heterozygotes the phenotype d, and aa homozygotes the phenotype -a. Each genotype contributes to the overall pheno-type based on how frequent it is in the population. The mean phenotype in a population is then the sum of each genotype-frequency-weighted phenotype (Table 2.10). When there is no dominance, the pheno-type of the heterozygotes is exactly intermediate between the phenotypes of the two homozygotes and d = 0. In that case, it is easy to see that inbreeding will not change the mean phenotype in the population since both homozygous genotypes increase by the same amount and their effects on the mean pheno-type cancel out (mean = ap2 + afpq + d2pq - df2pq -aq2 - afpq, where the heterozygote terms are crossed out since d = 0). When there is some degree of dominance (positive d indicates the phenotype of Aa is like that of AA while negative d indicates the phenotype of Aa is like that of aa), then the mean phenotype of the population will change with consanguineous mating since heterozygotes will be become less frequent. If dominance is in the direction of the +a phenotype (d > 0), then inbreeding will reduce the population mean because the heterozygote frequency will drop. Similarly, if dominance is in the direction of -a (d < 0) then inbreeding will increase the population mean again because the heterozygote frequency decreases. It is also true in the case of dominance that a return to random mating will restore the frequencies of heterozygotes and return the population mean to its original value before inbreeding. These changes in the population mean phenotype are simply a consequence of changing the genotype frequencies when there is no change in the allele frequencies.

There is a wealth of evidence that inbreeding has deleterious (harmful or damaging) consequences

Table 2.10 The mean phenotype in a population that is experiencing consanguineous mating. The inbreeding coefficient is f and d = 0 when there is no dominance.

Genotype

Phenotype

Frequency

Contribution to population mean

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