When a recessive trait is rare, persons from outside the family are usually homozygous for the normal allele. Thus, when an affected person mates with someone outside the family (aa X AA), usually none of the children will display the trait, although all will be carriers (i.e., heterozygous). A recessive trait is more likely to appear in a pedigree when two people within the same family mate, because there is a greater chance of both parents carrying the same recessive allele. Mating between closely related people is called consanguinity. In the pedigree shown in Figure 6.4, persons III-3 and III-4 are first cousins, and both are heterozygous for the recessive allele; when they mate, /4 of their children are expected to have the recessive trait.


Autosomal recessive traits appear with equal frequency in males and females. Affected children are commonly born to unaffected parents, and the trait tends to skip generations. Recessive traits appear more frequently among the offspring of consanguine matings.

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