# Sex chromosomes

The pair of chromosomes that decides whether a person is female (XX) or male (XY).

Biology (school) · Part 05: Genes and inheritance · Card 10 of 11 · Last updated 2026-10-19

## Analogy

A coin toss at conception, because the father's sperm carries X or Y.

## In practice

Every egg carries an X. A sperm carrying X gives a girl. A sperm carrying Y gives a boy.

## Practice

### Interview questions

**Describe how the sex of a baby is determined.**

- Females are XX and males are XY
- Eggs always carry X
- Sperm carry X or Y with equal chance
- The sperm that fertilises the egg decides the sex

## Questions people ask

### Which parent decides the sex of a baby?

The father's sperm decides. Every egg carries an X chromosome. A sperm carries either an X or a Y. If an X sperm fertilises the egg, the baby is female, XX. If a Y sperm fertilises it, the baby is male, XY. The chance is about half each.

### Why are some conditions more common in males?

Some genes, like those for red-green colour blindness, are on the X chromosome. Males have only one X, so a single faulty recessive allele will show. Females have two X chromosomes and the second may hide the faulty one, so they are more often just carriers.

## Related concepts

- [Dominant and recessive alleles](https://studyvisually.com/learn/science/biology-school/biology-school-part-5/biology-school-part-5-concept-5)
- [Genotype and phenotype](https://studyvisually.com/learn/science/biology-school/biology-school-part-5/biology-school-part-5-concept-6)
- [Punnett square](https://studyvisually.com/learn/science/biology-school/biology-school-part-5/biology-school-part-5-concept-7)
- [Inherited disorder](https://studyvisually.com/learn/science/biology-school/biology-school-part-5/biology-school-part-5-concept-11)

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