Suppose that a husband and wife are both heterozygous for a recessive allele for albinism. If they have dizygotic (two-egg) twins, what is the probability that both the twins will have the same phenotype for pigmentation? (i. e. they are either both unaffected or both affected).

The probability of both twins being unaffected is 9/16 = 0.5625.

The probability of both twins being affected is 1/16 = 0.0625

Explanation: Since the two parents are heterozygous for a recessive allele for albinism, therefore their genotype is Aa.

A cross between them will produce the following possible genotypes: AA, Aa, Aa and aa.

_ A a

A AA Aa

a Aa aa

The probability of one child being unaffected is 3/4. The probability of both twins being unaffected is 3/4 x 3/4 = 9/16

=0.5625

The probability of one child being affected is 1/4

The probability of both twins being affected is 1/4 x 1/4= 1/16

=0.0625.

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