In every human body cell there are 23 pairs of chromosomes. 22 of these just control your characteristics, it's the final pair the 23rd pair that determines whether or not you are male or female.


These chromosomes are XX for female, and XY for male.


In the Male (sperm) - The X and Y chromosomes are pulled apart during the production of sperm (meiosis) during the first division. When the chromosomes divide again during the second division there is a 50% chance that the sperm will receive an X chromosome and a 50% chance that it will receive a Y chromosome:




In the Female (egg, ovum) - The same type of division takes place in the female, but this time because the chromosomes are all XX, each egg inherits a copy of the X chromosome:



When the sperm penetrates into the egg during fertilisation, the chromosomes from both sperm and egg combine. Whether or not you end up as male or female depends on the combination of chromosomes that you receive. This is known as "sex inheritance".


Sex inheritance is usually depicted in a diagram, known as a "punnet square". This simple grid-like diagram allows you to show the possible combinations of the gametes.