Sex Determination

The human body has 23 pairs of chromosomes in its nucleus. 22 of these pairs, called autosomes, control our characteristics. The 23rd pair, known as the sex chromosomes, determine the sex of an individual.

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• In females, the sex chromosomes are the same – XX

• In males, the sex chromosomes are different – XY

Each parent donates 50% of the offspring’s alleles. The female always contributes an X chromosome, whereas the male has a 50/50 chance of contributing either an X or a Y chromosome.

Half of all sperm cells carry an X chromosome, and the other half carry a Y chromosome. In contrast, all egg cells carry an X chromosome.

This means that a child’s biological sex is determined by the chromosome that the father contributes.

Genetic Diagrams

We can see the inheritance of sex using a Punnett Square diagram:

There is a 50% chance that the offspring will be female (XX) and a 50% chance they will be male (XY). Thus, the average ratio of female to male offspring is 1:1.