X-linked recessive inheritance and sex probability

Steps:

• Night-blindness is X-linked recessive; males express the trait if they inherit the recessive allele from their carrier mother.
• Probability individual 7 is male: 1/2 (0.500).
• Mother's genotype: heterozygous carrier with 1/2 probability of passing the recessive X allele to son.
• Additional factor: grandmother's status requires 1/4 probability mother is carrier, so overall P(affected male) = (1/2) × (1/2) × (1/4) = 1/8 (0.125).

Why D is correct:

• Multiplies independent probabilities: sex (1/2), maternal transmission (1/2), and carrier status (1/4) per X-linked recessive rules.

Why the others are wrong:

• A: Ignores inheritance; just P(male).
• B: No basis in standard genetic ratios.
• C: Assumes known carrier mother, omitting grandmaternal probability.

Final answer: D