Mother is XX and is not color blind. Father is XY and is not color blind. Son is

Question

Mother is XX and is not color blind. Father is XY and is not color blind. Son is XXY and is color blind. Son’s maternal grandfather is XY and is color blind. There are several mechanisms that could explain the son’s color blindness phenotype. Can you propose a mechanism that would involve nondisjunction?

A. first division nondisjunction only in mother

B. second division nondisjunction only in mother

C. first division nondisjunction only in father

D. second division nondisjunction only in father

E. nondisjunction in both mother and father

Explanation / Answer

Father has normal color vision; the mother must be the carrier for color-blindness (but phenotypically normal). Son with color- has two copies of the color-blind X chromosome from his mother. The nondisjunction event most likely took place in meiosis II of the egg (which is when identical sister chromatids are separated).

Hence, the final answer is option B (second division nondisjunction only in mother)

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