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How does the probability of identical alleles compare for children of aunt-nephew unions and first-cousin unions?
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consanguinity
(Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with their child; (right) an inbreeding loop: horizontal bar at top of loop connects brother and sister, all other lines and bars as above
The probability of a child inheriting a pair of identical allelesâknown as the coefficient of inbreeding (F)âvaries based on the degree of consanguinity between the parents.
The likelihood of identical alleles is determined by the shared ancestry of the parents. For children of these unions, the probabilities are as follows:
- Aunt-nephew unions: The probability of a child inheriting a pair of identical alleles is 1 in 8 (or 0.125).1
- First-cousin unions: The probability of a child inheriting a pair of identical alleles is 1 in 16 (or 0.0625).12
As the degree of consanguinity becomes more remote, the likelihood of inheriting identical alleles derived from a common ancestor is halved with each subsequent degree. Consequently, the offspring of an aunt-nephew union face a higher probability of homozygosity by descent compared to the offspring of first cousins.12