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. The X chromosome is the larger chromosome and the Y chromosome is the smaller one.
The red box shows the two sex chromosomes which are both the same size. They are two larger X chromosomes.
A genetic cross, like a Punnett square, shows how the alleles inherited from the parents may combine in a zygote. 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).