Sex-linked genetic disordersSpec 3.18B
In short
A sex-linked genetic disorder is caused by an allele on a sex chromosome, usually a recessive allele on the X chromosome, as in haemophilia. Males have only one X chromosome, so one copy of the recessive allele causes the disorder. Females need two copies, and with one copy they are carriers. So sex-linked disorders are more common in males.
A sex-linked genetic disorder is caused by an allele on a sex chromosome. Most are caused by a recessive allele on the X chromosome. Haemophilia is an example.
- Males (XY) have only one X chromosome, so one copy of the recessive allele is enough to cause the disorder. The Y chromosome does not carry a matching allele.
- Females (XX) have two X chromosomes. They need two copies of the recessive allele to have the disorder. With one copy they are carriers.
- This is why sex-linked disorders are more common in males.
| XH (egg) | Xh (egg) | |
|---|---|---|
| XH (sperm) | XHXH unaffected female | XHXh carrier female |
| Y (sperm) | XHY unaffected male | XhY affected male |
Here XH is an X chromosome carrying the dominant normal allele (H) and Xh is an X chromosome carrying the recessive allele for haemophilia (h). The probability that a son has haemophilia is 50%. No daughters are affected, but half are carriers. An affected father passes his X to every daughter and his Y to every son, so his sons do not inherit the disorder from him.
Write the allele next to the X chromosome, for example XhY (exam papers print the h small and raised). Show the Y with no allele.
Quick check
What is an allele?
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A different version of the same gene.
What is the difference between genotype and phenotype?
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Genotype is the alleles an organism has. Phenotype is the observable characteristic that is expressed.
Which sex chromosomes do human females and males have?
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Females XX, males XY.
Two parents without a disorder have an affected child. Is the allele dominant or recessive?
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Recessive, and both parents are carriers.
What is codominance?
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Both alleles are expressed in the phenotype when both are present, as with IA and IB giving blood group AB.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026