Inbreeding coefficient and autozygosity in a pedigree

The effects of consanguineous mating can also be thought of as increasing the probability that two alleles at one locus in an individual are inherited from the same ancestor. Such a genotype would be homozygous and considered autozygous since the alleles were inherited from a common ancestor. If the two alleles are not inherited from the same ancestor in the recent past, we would call the genotype allozygous (alio- means other). You are probably already familiar with autozygosity, although you may not recognize it as such. Two times the probability of autozygosity (since diploid individuals have two alleles) is commonly expressed as the degree of relatedness among relatives. For example, full siblings (full brothers and sisters) are one-half related and first cousins are one-eighth related. Using a pedigree and tracing the probabilities of inheritance of an allele, the autozygosity and the basis of average relatedness can be seen.

Inbreeding in the autozygosity sense, often called the coefficient of inbreeding (f), can best be seen in a pedigree such as that shown in Fig. 2.14. Fig. 2.14a gives a hypothetical pedigree for four generations. The pedigree can be used to determine the probability that the fourth-generation progeny, labeled G, have autozygous genotypes due to individual A being a common ancestor of both their maternal and paternal parents. To make the process simpler, Fig. 2.14b strips away all of the external

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