Sex linkage

CIE A-Level Biology· 17.4 (e)· 35 min read

1. Key Definitions and Types of Sex Linkage★★☆☆☆⏱ 10 min

📘 Definition

Sex linkage

Alleles written as superscripts on the X chromosome

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.

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?

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

2. Inheritance of X-Linked Recessive Traits★★★☆☆⏱ 15 min

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.

📘 Definition

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 $X^{H} X^{h}$ for haemophilia is a carrier with normal blood clotting.

📐 Worked Example

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

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

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.

3. Identifying Sex Linkage in Pedigree Diagrams★★★☆☆⏱ 15 min

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?

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Step 1: Eliminate autosomal dominant: Affected individuals need at least one affected parent, which contradicts unaffected parents having affected children.
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Step 2: Eliminate autosomal recessive: Autosomal recessive traits show roughly equal frequency in males and females, which does not match the pattern.
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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.

4. Common Pitfalls

Wrong move:

Assuming all traits more common in males are automatically sex-linked

Why:

Environmental factors or sex-influenced autosomal traits can also cause higher incidence in males

Correct move:

Check all pedigree clues (affected daughter has affected father, unaffected parents have affected sons) before confirming sex linkage

Wrong move:

Adding an allele superscript to the Y chromosome for X-linked traits

Why:

The Y chromosome does not carry the gene for X-linked traits, so this shows incorrect understanding

Correct move:

Write affected males as $X^{h} Y$ , not $X^{h} Y^{h}$

Wrong move:

Miscalculating probability: using all offspring when the question asks for probability for a son/daughter

Why:

For example, 1/4 probability for all children becomes 1/2 for sons, which is the correct answer when sex is specified

Correct move:

Always read the question carefully and adjust your probability to match the specified group of offspring

Wrong move:

Claiming females cannot get X-linked recessive disorders

Why:

Females can inherit two copies of the recessive allele if their father is affected and their mother is a carrier

Correct move:

State that X-linked recessive disorders are far less common in females, not impossible

5. Quick Reference 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)

6. Frequently Asked

Can females ever express X-linked recessive disorders?

Yes, if their father is affected (can only pass an affected X chromosome) and their mother is either a carrier or affected, they will inherit two copies of the recessive allele and express the disorder.

Why are Y-linked traits never passed from father to daughter?

Daughters inherit the father's X chromosome, not his Y chromosome, so they cannot receive any Y-linked alleles from their father.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 2
X-linked pedigree analysis question
2021 · 1
Haemophilia inheritance probability
2023 · 4
Colour blindness genetic cross question

Going deeper

SyllabusCIE 9700 2025-2027 SyllabusCheck section 17.4 for full requirements

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.