How Is Sex Determined Genetically?

In the usual GCSE model, genetic sex is determined by the sex chromosomes inherited at fertilisation. An egg contributes an $X$ chromosome, while a sperm contributes either $X$ or $Y$. An $X$-carrying sperm gives $XX$, usually female; a $Y$-carrying sperm gives $XY$, usually male.

Where do the chromosomes come from?

Ordinary human body cells contain $23$ pairs of chromosomes. One pair consists of sex chromosomes: typically $XX$ in females and $XY$ in males. The other $22$ pairs carry genes for other characteristics. A chromosome is a long DNA molecule carrying many genes.

Eggs and sperm are gametes, meaning reproductive cells. They are made by meiosis and contain one chromosome from each pair. Every egg carries an $X$ sex chromosome. A sperm can carry $X$ or $Y$. At fertilisation, the nuclei of the egg and sperm join, restoring $23$ pairs in the first cell of the new organism.

A simple genetic cross

Suppose an $X$-carrying egg meets a sperm. There are two possible sex-chromosome combinations:

• Egg $X$ with sperm $X$ gives $XX$, usually female.
• Egg $X$ with sperm $Y$ gives $XY$, usually male.

Because approximately half of sperm carry $X$ and half carry $Y$, each combination has an approximate $50\%$ chance at each fertilisation. This is a probability, not a guarantee. For example, two children in one family could both be $XX$, both be $XY$, or one of each.

How to show this in an exam

In a Punnett square, place the egg’s $X$ on one side and the sperm’s $X$ and $Y$ on the other. The outcomes are $XX$ and $XY$, giving the ratio $1:1$ and probabilities $\frac{1}{2}$ and $\frac{1}{2}$.

Exam mistake to avoid: The egg does not decide whether the combination is $XX$ or $XY$. It always contributes $X$; the chromosome carried by the fertilising sperm makes the difference.

This explanation applies to both tiers of AQA GCSE Biology; separate Biology is assumed.