The second child of the couple above is unaffected does not display the condition). What is the probability this child is a carrier for the ataxia condition?

Question

Quizplus · Accepted Answer

Since we know that the father must be a carrier (because he has one affected X chromosome and one normal X chromosome) and the mother is unaffected but could be a carrier (because she has one normal X chromosome and one unknown X chromosome), there are four possible outcomes for the second child's genotype: XA (affected female), Xa (carrier female), XA (unaffected male), or Xa (unaffected male). However, we know that the second child is unaffected, so we can eliminate XA from the possible genotypes. This leaves us with three possible genotypes: Xa (carrier female), XA (unaffected male), and Xa (unaffected male). Since each of these three genotypes has an equal chance of occurring, and two out of three of them result in the second child being a carrier, the probability that the second child is a carrier is 2/3. Therefore, the correct answer is C.

The Second Child of the Couple Above Is Unaffected Does