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).

Answer: 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.

Answer: probability that both twins are affected is 0.0625 while the probability that both twins are unaffected is 0.5625

Explanation:

If both parents are heterozygous for albinism, it means they both have one recessive allele (Aa) each.

The possible genotype outcome of one child will be

AA Aa Aa aa

Therefore the possiblility one child will inherit both recessive allele (aa) is 1/4 and the probability that one child is unaffected is 3/4

If the had a dizygoyic twins this means the two children have different DNA.

So also the possibility of the second child to inherit both recessive allele (aa) is 1/4

The probability that both the twins are affected is

1/4 * 1/4 = 1/16 = 0.0625

The probability that both are unaffected is

3/4 * 3/4 = 9/16 = 0.5625