Question Detail
Question
Haemophilia is caused by a sex-linked, recessive allele. Two parents have a haemophilic son, a normal son and a haemophiliac daughter. What are the most likely genotypes of the mother and father respectively?
Correct Answer
Option A is the correct answer.
Detailed Explanation
For a daughter to be haemophiliac (XhXh), she must inherit an Xh from both parents. This means the father must be haemophiliac (XhY) and the mother must carry at least one Xh. The mother also has a normal son (XHY) and a haemophiliac son (XhY), confirming she is a carrier (XHXh). Therefore, the parents are XHXh (carrier mother) and XhY (haemophiliac father).
Hint
Work backwards from the affected daughter—she must get a recessive allele from each parent.