A woman is a carrier of the haemophilia allele, and her husband is normal…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A woman is a carrier of the haemophilia allele, and her husband is normal. They have a large family.
- What per cent of their sons are haemophilic?
- What per cent of their daughters are carriers?
- What per cent of all their children are haemophilic sons?
Need a reminder? The key ideaShow
An X-linked recessive allele (h) shows in every man who has it, and in a woman only if she has two copies. A carrier woman is . Carrier mother × normal father: one quarter of all the children are haemophilic sons. Haemophilic father × normal mother: all daughters are carriers and all sons are normal.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Mother , father .
Plan. Make the four boxes: , , and .
Step 1. (a) The sons are and , one box each. Half the sons are haemophilic: .
Step 2. (b) The daughters are and . Half the daughters are carriers: .
Step 3. (c) The haemophilic sons are one box in four: of all the children.
Answer: of the sons; of the daughters; of all children.
Check. No daughter is haemophilic, because the father gives every daughter a normal .
Watch out. Do not mix the two bases. of the sons is of all the children.
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