The frequency of an autosomal recessive disease in the offspring of a consanguineous mating can be calculated based on the frequency of the disease in the general population and the inbreeding coefficient for the mating.
For an autosomal recessive disease:
frequency of a defective gene =
= SQRT(frequency of disease in general population)
frequency of disease in a consanguineous mating =
= (frequency of disease in general population) + ((1 - (frequency of defective gene)) * (frequency of defective gene) * F)
where:
• F = inbreeding coefficient for consanguineous mating
• frequency of nondefective gene = 1 - (frequency of defective gene)
Consanguineous Mating |
Proportion of Shared Alleles |
F |
parent and offspring |
1/2 |
1/4 |
brother and sister |
1/2 |
1/4 |
half siblings |
1/4 |
1/8 |
uncle and niece |
1/4 |
1/8 |
aunt and nephew |
1/4 |
1/8 |
first cousins |
1/8 |
1/16 |
double first cousins |
1/4 |
1/8 |
half first cousins |
1/16 |
1/32 |
first cousins once removed |
1/16 |
1/32 |
second cousins |
1/32 |
1/64 |
second cousins once removed |
1/64 |
1/128 |
third cousins |
1/128 |
1/256 |
Table 15-5, page 352, Keats 1996
Specialty: Genetics
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