Consider the inheritance of hemophilia in Queen Victoria's descendants. The pedigree below shows the family of Victoria's son Leopold. Leopold's daughter, Alice, had one hemophilic son (Rupert) and two other children: boy and a girl. What is the probability that Alice's daughter was a carrier AND her son was not hemophilic?

Question

BlackTom AI · Accepted Answer

The topic concerns X-linked recessive inheritance as seen with hemophilia. First, observe that Alice has a hemophilic son (Rupert). For a son to be affected by an X-linked recessive condition, his mother must be a carrier (or affected), since sons inherit the X chromosome from their mother. Therefore, given Rupert is hemophilic, Alice must be a carrier (XhX) with certainty.

Now consider the two other children of Alice: one son and one daughter. For a carrier mother (XhX):
- Each son has a 1/2 chance to be affected (XhY) and a 1/2 chance to be unaffected (XY).
- Each daughter has a 1/2 chance to be a carrier (XhX) and a 1/2 chance to be non-carrier (XX).

We are asked for the probability that Alice's daughter was a carrier AND her son was not hemophilic. Since the events for the daughter being a carrier and the son being unaffected are independent given that Alice is a carrier, multiply the individual probabilities:
- Probability the daughter is a carrier: 1/2
- Probability the son is not hemophilic: 1/2
Combined probability = (1/2) × (1/2) = 1/4.

Option analysis:
- 1/2: Too large; this would imply either the daughter being a carrier or the son not hemophilic with probability 1, which is not the case under a carrier mother.
- 1/4: This matches the joint probability of both independent events (daughter carrier and son unaffected).
- 1/8: Incorrect; would correspond to multiplying by an additional factor or miscounting dependencies.
- 1/16: Incorrect; would correspond to two successive halving beyond the correct independent probabilities.

BIOL 1009 (001-006/050-055) Midterm Exam II

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