Question
Explain sex-linked inheritance using colour blindness as an example. Why is colour blindness more common in males than females? Show the possible genotypes and phenotypes for a cross between a carrier female and a normal male.
(NEET 2023, similar pattern)
Solution — Step by Step
Colour blindness and haemophilia are both caused by recessive alleles on the X chromosome. Since males have only one X chromosome (XY), a single recessive allele is sufficient to express the trait. Females have two X chromosomes (XX), so they need two recessive alleles to be affected.
Let = normal vision allele (dominant), = colour blind allele (recessive).
| Genotype | Phenotype |
|---|---|
| Normal female | |
| Carrier female (normal vision, but carries the allele) | |
| Colour blind female | |
| Normal male | |
| Colour blind male |
Males cannot be “carriers” — they either have the trait or they do not, because they have only one X.
Parents: (carrier female) x (normal male)
Offspring:
- Daughters: 1 normal () : 1 carrier () — all daughters have normal vision
- Sons: 1 normal () : 1 colour blind () — 50% chance a son is colour blind
Overall: 50% of sons are colour blind, 0% of daughters are colour blind (but 50% of daughters are carriers).
Males need only one copy of to be colour blind (hemizygous condition). Females need two copies — one from each parent. For a female to be colour blind, her father MUST be colour blind () AND her mother must be at least a carrier ().
The probability is simply much lower for females. About 8% of males are colour blind, but only about 0.5% of females.
Why This Works
Sex-linked inheritance follows a distinctive pattern: affected fathers cannot pass the trait to sons (fathers give Y to sons, not X). The trait appears to “skip” generations — an affected grandfather passes the allele through his carrier daughter to his grandson. This criss-cross inheritance pattern is the signature of X-linked recessive traits.
The same logic applies to haemophilia. Queen Victoria was a carrier of haemophilia, and the trait spread through European royal families via her daughters (carriers) to her grandsons (affected).
NEET pedigree analysis tip: if a trait appears mostly in males and never passes directly from father to son, it is likely X-linked recessive. If affected fathers always have affected daughters, it is likely X-linked dominant. Learn to distinguish these patterns quickly.
Common Mistake
The most common error: writing that a colour-blind father can have colour-blind sons. For an X-linked trait, the father gives his Y chromosome (not X) to sons. A colour-blind father () passes only to his daughters (who become carriers if the mother is normal). His sons get the Y from him and an X from the mother — so the son’s vision depends entirely on the mother’s genotype.
Another frequent mistake: forgetting that carrier females exist. When a question says “a normal female whose father was colour blind,” this female MUST be a carrier () — she received from her father. Students sometimes write her genotype as , which is impossible.