Human body cells have 23 pairs of chromosomes in the nucleus. Twenty two pairs are known as autosomes, and control characteristics, but one pair carries genes that determine sex - whether offspring are male or female:
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, therefore an X (larger chromosome) and a Y (smaller one).
The red box shows the two sex chromosomes - these are the same size, both 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.
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).