Waardenburg syndrome is an autosomal dominant condition that

[Hijeck]

Waardenburg syndrome is an autosomal dominant condition that accounts for 1.4% of cases of congenital deafness. In addition to deafness, patients with this condition have an atypical facial appearance, including lateral displacement of the inner canthi, hypertelorism ,white forelock and white patches of skin on the ventral midline . A mother has Waardenburg syndrome, her husband is unaffected, and they plan to have a family with three children. What is the probability that one of the three children will be affected?

A. 1/8
B. 1/4
C. 3/4
D. 1/3
E. 1/2

[Hijeck]

Answer is C; 3/4

For each pregnancy, the probability that the child will be affected is one-half. Therefore, the probability that all three children will be affected is the product of three independent events (1/2 × 1/2 × 1/2 = 1/.

The probability that all three children will be unaffected is the same. When evaluating the prob- ability that one of the three children will be affected, there are three of eight possible birth orders that have one affected child. For two of three children to be affected, there are also three of eight possible birth orders.