Sex determinationSpec 3.26, 3.27
In short
Sex determination in humans depends on the sex chromosomes: females are XX and males are XY. Every egg carries an X chromosome, while half of the sperm carry an X and half carry a Y. So the sperm decides the sex of the baby at fertilisation, and there is a 50% probability of a boy or a girl.
Humans have 23 pairs of chromosomes. One pair is the sex chromosomes. Females have two X chromosomes (XX). Males have one X and one Y chromosome (XY).
All eggs carry an X chromosome. Half of the sperm carry an X and half carry a Y. The sex of the offspring is decided at fertilisation by which sperm fertilises the egg.
| X (egg) | X (egg) | |
|---|---|---|
| X (sperm) | XX female | XX female |
| Y (sperm) | XY male | XY male |
Two outcomes are female and two are male, so the ratio is 1 : 1 and there is a 50% probability that a baby will be a boy or a girl.
The father's sperm decides the sex of the baby, because it carries either an X or a Y. Every egg carries an X.
Quick check
What is an allele?
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A different version (alternative form) of the same gene.
What is the difference between genotype and phenotype?
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Genotype is the alleles an organism has; phenotype is the characteristic that is expressed.
What does homozygous mean?
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Having two identical alleles of a gene.
Two parents without a disorder have an affected child. Is the disorder dominant or recessive?
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Recessive.
Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026