When a colour-blind man marries a carrier woman. What is the probability of their offspring being colour blind?

Analyze the question Colour-blindness is an X-linked recessive trait. We need the probability that an offspring (any child, boy or girl) is colour-blind when the father is colour-blind and the mother is a carrier.

Concept

• Genes for colour vision are on the X chromosome.
• Let X^C = normal allele, X^c = colour-blind allele.
• Colour-blind man: X^c Y
• Carrier woman: X^C X^c (normal-looking but carries one recessive allele)

Punnett square

| | X^C | X^c | |---|---|---| | X^c | X^C X^c (carrier girl) | X^c X^c (colour-blind girl) | | Y | X^C Y (normal boy) | X^c Y (colour-blind boy) |

Results

• X^C X^c: carrier daughter (normal vision)
• X^c X^c: colour-blind daughter
• X^C Y: normal son
• X^c Y: colour-blind son

Two out of four offspring are colour-blind, so the probability is 2/4 = 50%.

Answer: B (50%)

Exam tip For X-linked problems, always write the genotypes with X and Y explicitly and draw the Punnett square. If the question asks about "offspring" in general, count all four boxes. If it specifies sons or daughters only, count just those. Here, 50% of sons and 50% of daughters are colour-blind, so the overall result is 50% too.