A genetic counsellor meets a woman whose brother has Duchenne muscular dystrophy, a condition caused by a recessive allele on the X chromosome. Her mother i...
Question 1 Report
A genetic counsellor meets a woman whose brother has Duchenne muscular dystrophy, a condition caused by a recessive allele on the X chromosome. Her mother is known to be a carrier and her father does not have the condition. The woman has no symptoms and wants to know the risk for a future child with a man who does not have the disease.
(a) Name the sex chromosomes in a normal male body cell. [1]
(b) Complete the possible genotypes of the woman, using XD for the normal allele and Xd for the disease allele. [2]
(c) Use genetic inheritance to calculate the probability that the woman is a carrier. [2]
(d) Explain why a son of a carrier woman and an unaffected man has a 50% chance of having the condition. [3]
(e) Suggest one reason why the counsellor may discuss genetic testing with the woman. [1]
Answer Details
(a) A normal male body cell has sex chromosomes XY. [1]
(b) As the woman has no symptoms but has an unaffected father, her possible genotypes are \(X^DX^D\) [1] or \(X^DX^d\). [1]
(c) Her father is \(X^DY\), so every daughter receives his normal \(X^D\). Her carrier mother can pass either \(X^D\) or \(X^d\) with equal probability. Thus the woman has a \(\frac12\), or 50%, probability of being a carrier. [2]
(d) A carrier mother has one normal X allele and one disease X allele. [1] A son receives the Y chromosome from his unaffected father. [1] He has a one in two chance of receiving \(X^d\) from his mother; if he does, he has no normal allele on a second X chromosome, so he has the condition. [1]
(e) Genetic testing may establish whether she carries \(X^d\), giving a more accurate estimate of risk for a future child. [1]
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