# Explain how sex linkage causes a difference in the phenotypic expression of X-linked recessive traits between males and females.

> IB DP Biology Higher Level (2023 syllabus) — D3.2 Inheritance · Explain · 4 marks

> In humans, the gene encoding red–green colour vision is located on the X chromosome. Red–green colour blindness is caused by a recessive allele of this gene.

## Mark scheme (4 marks)

1. Males are hemizygous for X-linked genes, possessing only one X chromosome (and one Y chromosome), so they have only one copy of the allele.
2. A single recessive allele on the X chromosome is sufficient to express the trait in males, because there is no second allele to mask it.
3. Females have two X chromosomes and therefore two alleles at the locus; a female must be homozygous recessive to express the trait.
4. Heterozygous females are carriers — they do not express the phenotype but can pass the recessive allele to offspring, explaining why the trait is more common in males than females in the population.

## Key terms

- [sex linkage](https://www.gradenine.co.uk/glossary/sex-linkage)
- [X-linked recessive](https://www.gradenine.co.uk/glossary/x-linked-recessive)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-sex-linkage-causes-a-290cdbdb) · Published by Druglandscape Ltd.