Sex-linked Inheritance — Genetics & Inheritance | IGCSE Biology

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 17% of your exam marks.

increasing

Very High

Increasing17%

Genetic crosses, Punnett squares, and dominant/recessive allele questions appear on almost every paper.

Some genes are carried on the sex chromosomes rather than on the autosomes. Genes on the sex chromosomes follow inheritance patterns that look different from the usual ones, because the two sexes have different numbers of X and Y chromosomes.

Why sex linkage matters

Most sex-linked genes are on the X chromosome, because the Y chromosome is so short and carries so few genes. A gene on the X chromosome is called X-linked.

A female (XX) has two copies of every X-linked gene. She inherits one X from each parent.
A male (XY) has only one copy of every X-linked gene, inherited from his mother. He has no second X to mask any recessive allele.

This means recessive X-linked conditions are far more common in males than females:

A female only shows the recessive condition if she inherits the recessive allele from both parents (X^r X^r).
A male shows the recessive condition if he inherits the recessive allele on his single X from his mother (X^r Y). He has no second X to override it.

Classic examples

Colour blindness (red-green colour blindness) is X-linked recessive. About 8% of males are colour blind, compared to about 0.5% of females.
Haemophilia is X-linked recessive. Affected males have blood that does not clot properly, so even a small cut can be life-threatening. The British Royal Family famously carried the haemophilia allele through Queen Victoria's descendants.
Duchenne muscular dystrophy is X-linked recessive. Affects mainly males.

Sex-linked inheritance in a Punnett square

For a recessive X-linked allele, use the notation X^R (capital R for the dominant normal allele) and X^r (lowercase r for the recessive disease allele). The Y chromosome has no allele at this position (it is not part of the gene), so it is written as just Y.

Example: a carrier mother (X^R X^r) and a normal father (X^R Y) have a child. What is the chance the child has the disease?

	X^R (father)	Y (father)
X^R (mother)	X^R X^R (normal female)	X^R Y (normal male)
X^r (mother)	X^R X^r (carrier female)	X^r Y (affected male)

Each child has:

25% chance of being a normal female (X^R X^R)
25% chance of being a carrier female (X^R X^r, looks normal but can pass the allele on)
25% chance of being a normal male (X^R Y)
25% chance of being an affected male (X^r Y)

So sons of a carrier mother have a 50% chance of being affected; daughters of a carrier mother have a 50% chance of being a carrier.

Why it skips generations

In a pedigree, X-linked recessive conditions often skip generations:

An affected grandfather (X^r Y) passes his X^r to all his daughters.
Each daughter is therefore a carrier (X^R X^r) but looks normal, so the condition seems to disappear.
The carrier daughters then pass X^r to half of their sons, who are affected.
So the condition reappears in the grandsons, having skipped the daughters' generation entirely.