Sex Determination and Sex-Linked Inheritance — AQA GCSE Biology
The sex of a person is determined by a pair of chromosomes, and some characteristics are inherited along with them.
Sex determination
Humans have 23 pairs of chromosomes in their body cells. 22 pairs control general characteristics; one pair determines sex:
- Females are XX.
- Males are XY.
A Punnett square of the sex chromosomes shows a 50% (1:1) chance of each sex:
| X | X | |
|---|---|---|
| X | XX | XX |
| Y | XY | XY |
The egg always provides an X; the sperm provides either an X or a Y, so it is effectively the sperm that determines the sex.
Sex-linked inheritance (Higher / Separate)
Some genes are carried on the sex chromosomes (often the X chromosome). Conditions caused by these genes are sex-linked and affect males and females differently, because males have only one X chromosome.
A male only needs one copy of a recessive sex-linked allele on his single X to show the condition, while a female would need two copies. This is why some recessive sex-linked conditions are more common in males.
Exam tips
- Females are XX, males XY; there is a 50:50 chance of each sex.
- The sperm determines sex (carries X or Y); eggs always carry X.
- For sex-linked conditions, males (one X) are more likely to be affected by recessive alleles.
- Use a Punnett square to show the probability of each sex or condition.