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A boy with Klinefelter Syndrome (XXY) has colorblindness, which is an X-linked recessive trait. His mother...

A boy with Klinefelter Syndrome (XXY) has colorblindness, which is an X-linked recessive trait. His mother and father both have normal vision.

a) Did nondisjunction occur during the formation of the egg or the sperm in this situation?

b) How do you know?

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Answer #1

a) Non-disjunction occurred in the father. i.e in the formation of sperm.

b) We know that both the parents are carriers of the colourblindness as they are both unaffected.

Hence, the mother would be XcX

and the father would be XcY

The boy is colourblind that means that he is inheriting one X chromosome with the colour blindness allele from each parent i.e Xc

As he's a boy he is also inheriting a Y chromosome from the father.

This means the father is donating two chromosomes i.e Xc   and Y. Hence the non-disjunction occurred in the father.

The final cross would be,

Xc X x XcY Parents

Xc    XcY   Gamates

XcXcY Progeny

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