X-linked recessive inheritance probability for male offspring

Steps:

• Night-blindness is X-linked recessive; males express the trait if they inherit the allele from carrier mother.
• Probability unborn child is male: 1/2.
• If male, probability of inheriting recessive allele from heterozygous mother: 1/2 (mother passes X with allele half the time).
• If mother’s carrier status is uncertain (e.g., 1/2 chance based on pedigree), multiply by 1/2: overall 1/2 × 1/2 × 1/2 = 1/8.

Why C is correct:

• 1/8 matches the product of independent probabilities: sex (1/2), maternal carrier status (1/2), and allele transmission (1/2) per Mendel's law of segregation.

Why the others are wrong:

• A: Ignores genotype probability, assumes only 1/2 male chance.
• B: Assumes certain carrier mother, omitting status uncertainty (1/2 × 1/2 = 1/4).
• D: Overestimates by including female carriers or unaffected outcomes.

Final answer: C