Human body cells have 23 pairs of chromosomes in the nucleus. Twenty two pairs are known as autosomes - they control characteristics, but one pair carries genes that determine sex:
These diagrams are known as human karyotypes and show all the chromosomes aligned in pairs.
The blue box shows the two sex chromosomes – these are different sizes, an X (larger chromosome) and a Y (smaller one).
The red box shows the two sex chromosomes – these are the same size, so two X larger chromosomes.
A genetic diagram, like a Punnett square, shows how alleles may combine in zygotes. The diagram below shows how biological sex is inherited.
All eggs are X . Half of sperm are X and the other half are Y. So the two possible combinations are:
The ratio of female to male offspring is 1:1 – on average, half of the offspring will be girls and half will be boys. This can also be converted into a probability of 50% (XX) and 50% (XY).