X-linked recessive

Pedigree analysis of an X-linked recessive

    Marriage between a male with an X-linked recessive trait (aY) and an unaffected woman (AA) produces children with one of two genotypes. All of the sons will are AY, with the Y chromosome from the father and an A allele from the mother. All of the daughters are heterozygous Aa carriers (shown as a circle & dot), with the a allele from the father and an A allele from the mother. They do not show the trait, but can pass it along to their sons. AY and aY males are both referred to as hemizygotes, as they have only one allele per locus.

    When an Aa carrier woman marries an unaffected man (AY), four genotypes are produced, in equal proportions. Half of the sons will show the trait (aY) and half will not (AY), half the daughters will be carriers like their mother (Aa) and half will not (AA).

    Note the pattern of "criss-cross inheritance," where an affected male has an unaffected daughter, who in turn has an affected son. The trait "skips a generation."

    Homework: (1) Identify a cousin marriage that could produce a daughter with an aa genotype.
(2) First, white-out the dots in the carrier females, then write out the genotypes of every individual in the tree, as inferred from the distribution of affected and unaffected males.

Figure ©2002 by Griffiths et al;. all text material ©2012 by Steven M. Carr