John, a male hemophiliac, marries Jane, a normal woman, and together they have four children, two boys (Mark and Mike) and two girls (Molly and Mary). None of the children display the symptoms of hemophilia. Mark, Mike, Molly, and Mary all marry normal individuals and have children. None of Mark's or Mike's children, male or female, display symptoms of hemophilia, but the sons of Molly and Mary all display symptoms of hemophilia while the daughters of Molly and Mary do not. If one of Mike's daughters marries a normal man, what is the probability that one of their children will display symptoms of hemophilia?

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Solution

The correct option is B 0%
Haemophilia is a sex-linked recessive disorder. Gene responsible for it present on X chromosome and only expresses in a dominant state. Out of four children of John (haemophiliac male) and Jane (normal woman), both the girls Molly and Mary become a carrier for haemophilia as they got haemophiliac X chromosome from John, which they pass on to their children resulting sons with haemophilia expressed and daughters are either normal or carrier. The boys Mark and Mike are normal as they don't get haemophiliac X chromosome from father and when they married with normal individuals, all children would be normal. Thus, the correct answer is '0%.'

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