# Sex-linked inheritance

Genes on the X chromosome follow a different pattern in males (XY) and females (XX). A male has one X chromosome, so a recessive allele on it shows in him with no second copy to hide it.

Biology (university) · Part 07: Genetics and inheritance · Card 5 of 8 · Last updated 2026-10-19

## Analogy

A single-copy document with no backup.

## Example

**Carrier mother**

Mother X^H X^h (carrier).
Father X^H Y.
Sons: 1 in 2 affected.
Daughters: none affected,
1 in 2 carriers.

## In practice

Red-green colour blindness and haemophilia are X-linked recessive. They are more common in males.

## Practice

### Flashcards

- **Why are X-linked recessive traits more common in males?** Males have one X chromosome, so one recessive allele is enough to show the trait.

### Interview questions

**Why are X-linked recessive disorders more common in males? Use a cross to support your answer.**

- Males have one X
- Recessive allele shows without second copy
- Carrier mother passes to half of sons
- Daughters usually carriers

## Questions people ask

### Why can a father not pass an X-linked trait to his son?

A son receives the Y chromosome from his father and his X from his mother. So the father's X-linked alleles can never go to his sons. A father with an X-linked condition will pass his X to all his daughters, who will become carriers.

### Can females have X-linked recessive disorders?

Yes, but it is rarer. A female needs two copies of the recessive allele, one from each parent, so her father must be affected and her mother must be an affected person or a carrier. Also, random inactivation of one X in each cell can occasionally cause mild symptoms in carriers.

## Related concepts

- [Allele](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-1)
- [Law of segregation](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-2)
- [Law of independent assortment](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-3)
- [Incomplete dominance and codominance](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-4)

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