CBSE 2025, Set 1

Question

A man whose father was colour-blind marries a woman who had a colour-blind mother and normal father. What percentage of male children of this couple will be colour-blind? A). 25% B). 0% C). 50% D). 75%

Edvaya · Accepted Answer

The correct answer is 50%. **SOLUTION** **Step 1: Determine the genotype of the man.** His father was colour-blind ($X^c Y$), but since the man passes normal vision, his genotype is normal ($XY$). The Y chromosome comes from the father, and the X comes from his normal mother. **Step 2: Determine the genotype of the woman.** Her mother was colour-blind ($X^c X^c$), meaning the woman must have inherited one $X^c$ allele. Her father was normal ($XY$), so she inherited a normal $X$ allele from him. Thus, the woman is a carrier ($X^c X$). **Step 3: Evaluate the cross for male children.** Male children inherit the $Y$ chromosome from the father and one $X$ chromosome from the mother. The mother can pass either $X^c$ or $X$ with a 50% probability each. Therefore, the possible male offspring are: * $XY$ (Normal male) - 50% chance * $X^c Y$ (Colour-blind male) - 50% chance The percentage of male children that will be colour-blind is **50%**.

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Biology > Reproduction And Genetics > Principles Of Inheritance And Variation

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