An X-linked ichthyosis is a recessive form of a family of skin diseases caused by a hereditary deficiency of the steroid sulfatase (STS) enzyme. A woman heterozygous for this mutation mates with the phenotypically normal man and produces an XXY son who suffers from the disorder. What kind of nondisjunction can explain this result?

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Quizplus · Accepted Answer

In this case, the mother is heterozygous for the mutation, meaning that she has one normal X chromosome and one X chromosome with the mutation. During meiosis II, there was likely a failure of separation of the X chromosomes, resulting in two X chromosomes being present in one daughter cell and no X chromosomes being present in the other daughter cell. The XXY son was then produced through fertilization of the daughter cell with two X chromosomes by a Y-bearing sperm from the father.

An X-Linked Ichthyosis Is a Recessive Form of a Family