Sex-linked inheritance
- ICSE Class 10, Unit 4 Genetics
Sex-linked inheritance is the passing on of a trait whose gene lies on a sex chromosome, as in the X-linked disorders haemophilia and colour blindness.
The genes for haemophilia, where blood clots very slowly, and for colour blindness sit on the X chromosome. The faulty allele is recessive. The Y chromosome has no matching allele.
A woman has two X chromosomes. If one X has the faulty allele, the normal allele on the other X hides it. She is a carrier. A man has only one X. If it has the faulty allele, nothing hides it, and he has the disorder.
A small example
A carrier mother (XᶜX) and a father with normal vision (XY) have children. Every son gets his Y from the father and one X from the mother:
- Sons: XᶜY is colour blind; XY is normal. So 1 son in 2, or 50%.
- Daughters: XᶜX is a carrier; XX is normal. No daughter is colour blind.
Where marks go
Read who the question counts: all the children, or the sons only. Out of all children, the chance is 1 in 4; out of sons, it is 1 in 2.
The ICSE 2027 specimen paper asks why haemophilia is uncommon in females, for 2 marks. Its draft marking scheme wants both halves. Females have two X chromosomes, so a dominant allele hides the recessive one. Males have one X, so they show the disease.
Sources
Facts last checked against these sources on 30 September 2026.
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