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."
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