Sex Determination & Sex Linkage IGCSE Biology

Sex Determination and Sex-Linked Inheritance

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Sex is determined by sex chromosomes: XX produces a female and XY produces a male. Sex-linked genes are carried on the X chromosome. Males are more affected by recessive sex-linked conditions because they have only one X chromosome.

Sex determination and sex linkage are Supplement topics that require genetic diagrams with X and Y chromosomes. The critical exam skill is explaining why males are more commonly affected by recessive sex-linked conditions such as red-green colour blindness and haemophilia.

Sex determination by chromosomes

In humans, sex is determined by the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The remaining 22 pairs are autosomes.

All eggs carry an X chromosome. Sperm can carry either an X or a Y chromosome. If a sperm carrying X fertilises the egg, the offspring is XX (female). If a sperm carrying Y fertilises the egg, the offspring is XY (male). The probability of being male or female is 1:1 or 50:50.

Genetic diagram for sex determination

A Punnett square for sex determination uses the parental genotypes XX (mother) and XY (father). The gametes are X from the mother and X or Y from the father.

	X (from father)	Y (from father)
X (from mother)	XX (female)	XY (male)
X (from mother)	XX (female)	XY (male)

This gives an expected ratio of 1 female: 1 male. The father's sperm determines the sex of the offspring because the mother always passes on an X chromosome.

Sex-linked inheritance

A sex-linked gene is a gene located on the X chromosome (there are very few genes on the much shorter Y chromosome). The alleles are written as superscripts on the X chromosome, e.g. X^B for normal colour vision and X^b for colour blindness.

Females have two X chromosomes, so they can be X^BX^B (normal), X^BX^b (carrier, normal vision) or X^bX^b (colour blind). Males have only one X chromosome, so they are either X^BY (normal) or X^bY (colour blind). A single recessive allele on the X chromosome is expressed in males because there is no second X with a dominant allele to mask it.

Why males are more commonly affected

Because males have only one X chromosome, a single copy of the recessive allele will produce the condition. They cannot be carriers. Females need two copies of the recessive allele (one on each X chromosome) to show the condition, which is much less likely.

A carrier female (X^BX^b) does not show the condition herself but can pass the recessive allele to her sons. If a carrier female has children with a normal male (X^BY), the expected offspring are: 1 normal female (X^BX^B): 1 carrier female (X^BX^b): 1 normal male (X^BY): 1 colour-blind male (X^bY). Half the sons are affected.

Try a focused question (3 marks)

A woman who is a carrier of colour blindness (X^B X^b) has children with a man who has normal colour vision (X^B Y). What is the probability that a son will be colour blind?

Show the answer and marking guidance

Gametes from the mother: X^B or X^b; gametes from the father: X^B or Y.
Sons receive Y from the father and X^B or X^b from the mother.
Half the sons will be X^b Y (colour blind), so the probability is 1 in 2 (50%).

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Writing the allele on the Y chromosome (the Y carries very few genes; the allele is on the X).
Saying females cannot be colour blind (they can if they are X^b X^b, which is rare).
Forgetting to show the X and Y chromosomes in the genetic diagram.
Confusing carrier females with affected females.