Читать книгу Fundamentals of Conservation Biology - Malcolm L. Hunter Jr. - Страница 108

Inbreeding refers to the mating between closely related individuals; such individuals are likely to share identical copies of some of their genes because they have ancestors in common. We measure inbreeding with the inbreeding coefficient, F, which is the probability that two copies of the same allele are identical by descent – in other words, derived from a common ancestor (Templeton and Read 1994). For example, in our bison example, if both MDH‐1 X alleles in the X/X homozygous buffalo calf were derived from its grandmother, those alleles would be considered identical by descent.

Among several methods to estimate F (Frankham et al. 2009), the simplest involves counting links in the pedigree chain: F = (½) ⁿ , where n is the number of individuals or links in the pedigree chain starting with one parent, going back to the common ancestor, and then going down the other branch to the other parent. Figure 5.12(a) shows the pedigree chain for the offspring (A) of a half‐sister (B) mating with her half‐brother (C) (i.e. B and C have the same mother, D, but different fathers). The inbreeding chain has three links – B, D, and C – and thus F is equal to (½)³ = 1/8 = 0.125. If B and C were full siblings (i.e. they had both the same mother D and the same father E) (Fig. 5.12b), then there would be two chains, one for each common ancestor (B, D, and C for the mother plus B, E, and C for the father). In this case the F values for each chain would be added: (½)³ + (½)³ = ¼ = 0.25.

Figure 5.12 Inbreeding pedigrees for matings between: (a) a half‐sister with her half‐brother, (b) full sister and brother, and (c) full cousins (different parents but identical grandparents). See the text for an explanation of (a) and (b).