# Sex linkage

> CIE A-Level Biology · Unit 17: Inheritance
> Source: https://www.owlsprep.com/study/cie-9700-u17-sex-linkage/

This sub-topic covers the inheritance of genes located on the X and Y sex chromosomes, focusing on X-linked recessive traits, pedigree interpretation, and solving genetic probability problems for sex-linked traits for CIE A-Level Biology.

**Prerequisites:** [Mendelian inheritance and genetic crosses](https://www.owlsprep.com/study/cie-9700-u17-mendelian-inheritance/); [Sex determination in humans](https://www.owlsprep.com/study/cie-9700-u16-chromosome-theory-of-inheritance/)

## Learning objectives

- Define sex linkage and distinguish between X-linked and Y-linked inheritance
- Predict inheritance patterns for X-linked recessive traits
- Interpret pedigree diagrams to identify sex-linked traits
- Explain the higher incidence of X-linked recessive disorders in males

## Key Definitions and Types of Sex Linkage

**Sex linkage** — The pattern of inheritance shown by genes located on the sex chromosomes (X or Y). Phenotypic ratios differ between male and female offspring due to the non-homologous nature of X and Y chromosomes.

*Notation:* Alleles written as superscripts on the X chromosome

*Example:* Red-green colour blindness in humans is a well-studied sex-linked trait.

Most sex-linked traits are **X-linked**, because the X chromosome is much larger than the Y chromosome and carries thousands of genes that are not present on the Y. Y-linked traits are rare, and almost always related to male sexual development, passed only from father to son.

**Worked example:** A couple with no symptoms of haemophilia have a son with haemophilia. Why is haemophilia most likely an X-linked recessive trait?

1. Step 1: Rule out Y-linked inheritance: If haemophilia were Y-linked, the father would have haemophilia, but the father is unaffected.
2. Step 2: Rule out autosomal dominant: If it were dominant, at least one parent would be affected, which they are not.
3. Step 3: Check X-linked recessive: The mother is a carrier (genotype $X^HX^h$) and the father is unaffected ($X^HY$). The son inherits the Y chromosome from his father and the $X^h$ chromosome from his mother, giving him genotype $X^hY$ and causing haemophilia.
4. Step 4: Conclusion: This inheritance pattern matches X-linked recessive inheritance.

> **Exam tip:** Nearly all sex-linked questions in CIE exams refer to X-linked recessive traits, unless explicitly stated otherwise.

## Inheritance of X-Linked Recessive Traits

Males are hemizygous for all X-linked genes, meaning they only have one copy of any X-linked gene. Any recessive allele on the X chromosome will therefore be expressed in males, because there is no second allele to mask its effect. Females need two copies of the recessive allele to express the trait, so they can be asymptomatic carriers.

**Carrier** — A heterozygous female that carries one copy of the recessive harmful allele, but does not express the disorder because the dominant normal allele masks the recessive phenotype.

*Example:* A woman with genotype $X^HX^h$ for haemophilia is a carrier with normal blood clotting.

**Worked example:** A carrier woman for red-green colour blindness ($X^CX^c$) has children with a man with normal vision ($X^CY$). What is the probability that: (1) any son is colour blind, (2) any child is colour blind?

1. Step 1: List the gametes produced by each parent: Mother produces $X^C$ and $X^c$; Father produces $X^C$ and Y.
2. Step 2: Combine gametes to get all possible offspring genotypes:
3. 1. $X^CX^C$: normal daughter, 2. $X^CX^c$: normal carrier daughter, 3. $X^CY$: normal son, 4. $X^cY$: colour blind son
4. Step 3: Calculate probability (1): For a son: 1 out of 2 sons is colour blind, so probability = 1/2 (50%).
5. Step 4: Calculate probability (2): For any child: 1 out of 4 total offspring is colour blind, so probability = 1/4 (25%).

> **info**
>
> Common X-linked recessive traits tested in CIE exams: red-green colour blindness, haemophilia A, Duchenne muscular dystrophy.

> **Exam tip:** Always write alleles as superscripts on the X chromosome to gain full marks for notation in CIE exams. The Y chromosome never gets an allele, as it does not carry the gene.

## Identifying Sex Linkage in Pedigree Diagrams

CIE exams regularly ask you to identify if a trait is sex-linked from a pedigree diagram. There are three key clues that indicate an X-linked recessive trait: more affected males than females, affected daughters always have an affected father, and unaffected parents can produce an affected son.

**Worked example:** A rare genetic disorder shows: more affected males than females, affected daughters always have affected fathers, and unaffected parents can have affected sons. Is this disorder X-linked recessive?

1. Step 1: Eliminate autosomal dominant: Affected individuals need at least one affected parent, which contradicts unaffected parents having affected children.
2. Step 2: Eliminate autosomal recessive: Autosomal recessive traits show roughly equal frequency in males and females, which does not match the pattern.
3. Step 3: Confirm X-linked recessive: All three clues match the expected pattern for X-linked recessive inheritance. Affected daughters must inherit one X from their father, so if she has two recessive alleles, her father must have the affected allele and be affected.

**Exam command terms**

- **Explain why X-linked recessive traits are more common in males** — You must explicitly link the pattern to male XY karyotype and hemizygosity to get full marks *(Answers must mention males have only one X chromosome, so recessive alleles are always expressed, not just that males need one copy.)*

- **Calculate the probability that the child will be affected** — You must show your working (usually a Punnett square) and read the question to check if it asks for probability for all children or for a specific sex

## Common pitfalls

- **Wrong:** Assuming all traits more common in males are automatically sex-linked
  - Why it fails: Environmental factors or sex-influenced autosomal traits can also cause higher incidence in males
  - Correct: Check all pedigree clues (affected daughter has affected father, unaffected parents have affected sons) before confirming sex linkage
- **Wrong:** Adding an allele superscript to the Y chromosome for X-linked traits
  - Why it fails: The Y chromosome does not carry the gene for X-linked traits, so this shows incorrect understanding
  - Correct: Write affected males as $X^hY$, not $X^hY^h$
- **Wrong:** Miscalculating probability: using all offspring when the question asks for probability for a son/daughter
  - Why it fails: For example, 1/4 probability for all children becomes 1/2 for sons, which is the correct answer when sex is specified
  - Correct: Always read the question carefully and adjust your probability to match the specified group of offspring
- **Wrong:** Claiming females cannot get X-linked recessive disorders
  - Why it fails: Females can inherit two copies of the recessive allele if their father is affected and their mother is a carrier
  - Correct: State that X-linked recessive disorders are far less common in females, not impossible

## Cheatsheet

| Pedigree Clue | Indicates X-linked recessive? |
| --- | --- |
| More affected males than females | Yes |
| Affected daughter has an affected father | Yes |
| Unaffected parents have an affected son | Yes |
| Affected male passes trait to all sons | No (Y-linked) |
| Equal frequency in males and females | No (autosomal) |
| All affected individuals have an affected parent | No (indicates dominant) |

## What's next

Sex linkage is a core non-Mendelian inheritance pattern that builds on basic Mendelian genetics, and is a frequent topic for both multiple choice and extended response questions in CIE 9700 exams. Mastering sex linkage and pedigree interpretation will help you tackle other types of non-Mendelian inheritance and genetic disorder questions. It is also a common topic for synoptic questions linking inheritance to evolution and population genetics.

- [Linkage and Crossing Over](https://www.owlsprep.com/study/cie-9700-u17-linkage-and-crossing-over/)
- [Polygenic Inheritance](https://www.owlsprep.com/study/cie-9700-u17-polygenic-inheritance/)
- [Selection and Evolution](https://www.owlsprep.com/study/cie-9700-u18-overview/)

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