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Modern Genetic Analysis Chapter 17. Population and Evolutionary Genetics The Sources of Variation
Genetics In Process17-5: The effect of the mating of close relatives on homozygosity

The probability of a homozygous a /a offspring from a brothersister mating is

We assume that the chance that both grandparents are A / a is negligible. If p is very small, then q is nearly 1.0 and the chance of an affected offspring is close to p/4. For p = 1/1000, there is 1 chance in 4000 of an affected child, compared with the one-in-a-million chance from a random mating. In general, for full sibs, the ratio of risks will be

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