Sex determinationSpec 3.15
In short
Sex is determined by one pair of sex chromosomes: females are XX and males are XY. All eggs carry an X chromosome, while half of sperm carry an X and half carry a Y. The sperm that fertilises the egg decides the sex, so 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.
Punnett square for sex determination | 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.
Common mistake:The father's sperm decides the sex of the baby, because it carries either an X or a Y. Every egg carries an X.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026
Frequently asked questions
What is a genotype and phenotype?
A genotype is the alleles an organism has for a gene, such as Tt. A phenotype is the characteristics that are expressed and can be observed, such as being tall. A heterozygous plant, Tt, has the phenotype of the dominant allele, so it is tall even though it also carries the allele for short.
What is a Punnett square used for in genetics?
A Punnett square is used to show the possible offspring of a genetic cross. The gametes from each parent are written along two sides, then combined in the boxes to give the possible genotypes. Counting the boxes gives the ratio, probability or percentage of each genotype and phenotype in the offspring.
What is the difference between homozygous and heterozygous?
Homozygous means having two identical alleles of a gene, such as TT or tt. Heterozygous means having two different alleles of a gene, such as Tt. A heterozygous organism shows the dominant characteristic, because a dominant allele is expressed in the phenotype even when only one copy is present.
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