Question
A colour-blind man (X^cY) marries a carrier woman (X^CX^c). What is the probability that their son will be colour-blind?
This exact cross appeared in NEET 2023 and trips up students who mix up “probability among sons” vs “probability among all children.”
Solution — Step by Step
Father is colour-blind, so he carries the recessive allele on his only X chromosome: X^cY.
Mother is a carrier — she has one normal allele and one colour-blind allele: X^CX^c. She has normal vision herself because X^C is dominant.
Father produces two types of gametes: X^c and Y (in equal proportion).
Mother produces two types: X^C and X^c (again, equal proportion, since she’s heterozygous).
| X^C | X^c | |
|---|---|---|
| X^c | X^CX^c (carrier daughter) | X^cX^c (colour-blind daughter) |
| Y | X^CY (normal son) | X^cY (colour-blind son) |
Four equally likely outcomes: carrier daughter, colour-blind daughter, normal son, colour-blind son.
There is 1 colour-blind son out of 4 total children — so the probability of a colour-blind son is 1/4 = 25%.
If the question asks “what fraction of sons will be colour-blind?” — that’s 1 out of 2 sons = 1/2 = 50%.
Read the question stem carefully. NEET 2023 asked for probability among all offspring, so the answer is 25%.
Why This Works
Colour blindness follows X-linked recessive inheritance. The gene for colour vision sits on the X chromosome, and the colour-blind allele (X^c) is recessive to the normal allele (X^C).
Sons only get one X chromosome — from their mother. So a son’s colour vision is entirely decided by whichever X the mother passes. Since this mother is X^CX^c, there’s a 50% chance she passes X^c to any son. Every son who receives X^c is colour-blind (because there’s no second X to mask it).
Daughters get one X from each parent. Even if they inherit X^c from mother, father’s X^c gives them X^cX^c (colour-blind) — which is actually a notable outcome in this cross. This cross is special because even daughters can be colour-blind, unlike in the classic “normal father × carrier mother” cross.
Alternative Method — Probability Multiplication
Instead of a Punnett square, use the multiplication rule directly.
Probability child is a son = 1/2
Probability son gets X^c from mother = 1/2 (she’s heterozygous)
Probability of colour-blind son = 1/2 × 1/2 = 1/4
This method is faster in MCQs. Once you know the mother is a heterozygous carrier, P(colour-blind son) = P(son) × P(gets X^c from mother). For a carrier mother, that’s always 1/2 × 1/2 = 1/4.
Common Mistake
Students often write the father’s genotype as X^CY (normal father) when the problem says he’s colour-blind. Here, the father is X^cY — not normal. This changes the cross completely. In the standard NCERT cross (normal father × carrier mother), daughters cannot be colour-blind. But here, with a colour-blind father × carrier mother, daughters can be colour-blind (X^cX^c). If you use the wrong paternal genotype, you’ll miss this and also get the wrong daughter ratios.
Always underline the father’s phenotype in the question before writing any genotype. One missed word changes every ratio in the Punnett square.