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Not all genes are inherited equally. Those on the sex chromosomes show distinctive inheritance patterns because males and females carry different combinations of sex chromosomes. Sex-linked inheritance is particularly important because several well-known human genetic disorders, including haemophilia, red-green colour blindness and Duchenne muscular dystrophy, are X-linked recessive conditions that affect males much more frequently than females. OCR A-Level Biology A specification module 6.1.2(c) requires you to understand sex linkage and be able to analyse sex-linked pedigrees and crosses.
Key Definitions:
- Sex linkage — the inheritance of a gene carried on a sex chromosome.
- X-linked — carried on the X chromosome.
- Y-linked — carried on the Y chromosome.
- Autosome — any chromosome that is not a sex chromosome.
- Carrier (heterozygote) — a female heterozygous for a recessive X-linked allele who does not usually show the trait but can pass it on.
- Hemizygous — having only one copy of a gene (e.g. males for all X-linked genes).
Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes (the 23rd pair).
The Y chromosome is much smaller than the X and carries fewer than 100 genes. The X chromosome carries over 800. Therefore:
Men pass their X to daughters and their Y to sons. Women pass one of their two Xs to every child regardless of sex. This asymmetry produces the characteristic sex-linked inheritance pattern.
Most sex-linked diseases are X-linked recessive. A male with the allele is affected; a female needs two copies to be affected, and a female with one copy is a carrier (usually unaffected but able to pass it on).
Let Xᴬ be the normal allele and Xᵃ the recessive disease allele.
| Xᴬ | Y | |
|---|---|---|
| Xᴬ | XᴬXᴬ | XᴬY |
| Xᵃ | XᴬXᵃ | XᵃY |
Offspring: 1 unaffected daughter : 1 carrier daughter : 1 unaffected son : 1 affected son. Among sons alone, half are affected. Among daughters alone, half are carriers.
| Xᴬ | Xᴬ | |
|---|---|---|
| Xᵃ | XᴬXᵃ | XᴬXᵃ |
| Y | XᴬY | XᴬY |
All daughters are carriers; all sons are unaffected.
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