Inheritance of sex

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Inheritance of sexSpec 17.1.4

In short

Sex is inherited through one pair of sex chromosomes: females are XX and males are XY. Every egg carries an X chromosome, but half the sperm carry X and half carry Y. So the sex of a baby depends on which sperm fertilises the egg, giving a 1 : 1 chance of a boy or a girl.

Human body cells contain one pair of sex chromosomes, which decide whether a person is male or female. There are two types, called X and Y.

Females have two X chromosomes (XX).
Males have one X chromosome and one Y chromosome (XY).

Gametes (sex cells) carry only one sex chromosome. All eggs carry an X chromosome. Half of the sperm carry an X chromosome and half carry a Y chromosome. The sex of a baby depends on which sperm fertilises the egg.

Genetic diagram: inheritance of sex
	Mother (female)	Father (male)
Parental genotypes	XX	XY
Gametes	X and X (every egg carries X)	X and Y (half the sperm carry X, half carry Y)

Punnett square: eggs across the top, sperm down the side
Sperm ↓ Eggs →	X	X
X	XX (girl)	XX (girl)
Y	XY (boy)	XY (boy)

Offspring: 2 XX (girls) : 2 XY (boys). The chance of a baby being a boy is the same as the chance of it being a girl: 1 : 1, or 50% each.

Genetic diagram for inheritance of sex: a mother (XX) and a father (XY) produce eggs that all carry X and sperm that carry X or Y, giving offspring XX, XX (girls) and XY, XY (boys) in a ratio of 1 : 1. — Every egg carries X; half the sperm carry X and half carry Y, so the chance of a girl or a boy is 1 : 1.

Exam tip:

The father decides the sex of the baby, because only his gametes can carry either X or Y. State this in your answer and show the gametes in a genetic diagram.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

What is the difference between a gene and an allele?

A gene is a length of DNA that codes for a protein, while an allele is an alternative form of a gene. For example, the gene for flower colour may have one allele for red flowers and another for white flowers. Different alleles have slightly different DNA, so they code for slightly different proteins.

How is the sex of a baby determined?

The sex of a baby is determined by which sperm fertilises the egg. Females are XX, so every egg carries an X chromosome. Males are XY, so half the sperm carry X and half carry Y. An X sperm gives a girl (XX) and a Y sperm gives a boy (XY), a 1 : 1 chance.

How do genes control the making of proteins?

Supplement Genes control protein production through their sequence of bases, which determines the sequence of amino acids in a protein. The gene stays in the nucleus, and a copy called mRNA is made. The mRNA moves to the cytoplasm and passes through a ribosome, which assembles amino acids into the protein in the correct order.

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