# Explain how sex-linked inheritance can result in a phenotypic ratio that differs between males and females in the offspring of a carrier mother and an unaffected father, using a named X-linked recessive trait as an example.

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

## Mark scheme (4 marks)

1. Sex-linked genes are located on the X chromosome (X-linked), and males are hemizygous — they carry only one allele for the trait.
2. A carrier mother is heterozygous (X^A X^a) and an unaffected father is X^A Y; correct genotypes stated or implied for both parents using a named trait (e.g. colour blindness, haemophilia).
3. Among female offspring, half are unaffected homozygous (X^A X^A) and half are unaffected carriers (X^A X^a) — no females are affected.
4. Among male offspring, half are unaffected (X^A Y) and half are affected (X^a Y), giving a 1:1 ratio of affected to unaffected males — creating a different phenotypic ratio between the sexes.

## Key terms

- [sex-linked inheritance](https://www.gradenine.co.uk/glossary/sex-linked-inheritance)
- [carrier](https://www.gradenine.co.uk/glossary/carrier)
- [X-linked recessive](https://www.gradenine.co.uk/glossary/x-linked-recessive)
- [phenotypic ratio](https://www.gradenine.co.uk/glossary/phenotypic-ratio)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-sex-linked-inheritance-can-result-05adfa02) · Published by Druglandscape Ltd.