A boy with Klinefelter Syndrome XXY has colorblindness which

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

Did nondisjunction occur during the formation of the egg or the sperm in this situation? How do you know?

Solution

Color blindness is an X-recessive trait, there is no way that the father can have color-blindness gene (let’s say X^c). Hence, the color blindness gene should come from mother (but only a single copy of gene is present as it is not expressed).

Genotype

Mother: XX^c

Father: XY

Child: X^cX^cY (for trait to be expressed, color blindness gene should be present on both the copies of X)

X^c can be inherited only from mother (as father doesn’t have it) and both the copies of X^c should come from her. The egg must have arisen from a nondisjunction in meiosis II. In meiosis I, the homologous X chromosomes separate, so one cell has X and the other has X^c. Failure of chromatids to separate in meiosis II would then result in an egg with two copies of X^c