1 2N

for generation t under the assumption that none of the alleles in the gamete pool in generation t - 1 are identical by descent. To make this more general, we could also account for the possibility that some of the gametes in the ancestral population of generation t - 1 have alleles that are identical by descent from past inbreeding or random sampling. To do this we need to reexamine the probability that alleles are allozygous. Although two distinct alleles in the gamete pool of Fig. 3.18 may be sampled to form what appears to be an allozygous homozygote, it is possible that these two alleles sampled are actually identical by descent. That would mean that the gamete pool would be inbred to some degree Ft-1 instead of containing only allozygous alleles. The fixation index then becomes

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