Question #6b0b0
1 Answer
Answer is (B) i.e. mutant allele is present on X chromosome and the allele is recessive. Would also request to read the comment section where I would add something.
Explanation:
The disease (absence of sweat glands in skin) shows criss cross inheritance, i.e. the disease skips one generation and appears in male grandchild (son of a daughter).
When disease is X linked recessive, the allele could be carried by apparently normal females. As the male child receives the allele from carrier mother through her X chromosome, and as there is no corresponding allele in Y to mask the effect of this allele, the disease condition gets expressed.
An affected female may appear only rarely when father is affected and mother a carrier.
In this question, P is an affected individual who has one daughter and one son. Daughter definitely received one X chromosome from father which had abnormal recessive allele, hence she is carrier.
(please see left half of the illustration)
She passed the abnormal allele along one of her X chromosome to her son, so he (one brother of individual Q) becomes affected.
(right half of the diagram)
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