Question

Galactosemia is a recessive human disease that is treatable by restricting lactose and glucose in the diet. Susan Smithers and her husband are both heterozygous for the galactosemia gene.

a. Susan is pregnant with twins. If she has fraternal (nonidentical) twins, what is the probability both of the twins will be girls who have galactosemia? b. If the twins are identical, what is the probability that both will be girls and have galactosemia?

Answer 1

The probability of both fraternal twins being girls with galactosemia is 1/8, while the probability for identical twins is 1/16.

Step-by-step explanation:

If Susan Smithers and her husband are both heterozygous for the galactosemia gene, there is a 1/4 probability that each of their children will inherit two copies of the galactosemia gene and have galactosemia. Since Susan is pregnant with twins, we can calculate the probability of both twins being girls with galactosemia separately for fraternal and identical twins.

a. For fraternal twins, the probability of both twins being girls with galactosemia is 1/4 * 1/2 = 1/8. This is because each twin has a 1/4 probability of having galactosemia, and there is a 1/2 probability of both twins being girls.

b. For identical twins, since they are derived from a single fertilized egg, the probability of both twins being girls with galactosemia is the same as for a single child. Therefore, the probability is 1/4 * 1/4 = 1/16.