Human body cells have 23 pairs of chromosomes in the nucleus. Twenty two pairs control most of the characteristics. Pair 23 are the sex chromosomes. They carry genes that determine an embryo's sex - whether offspring are male or female:
These photographs 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 Y chromosome carries a gene called SRY. It causes the testes to develop. They produce male sex hormones which cause male characteristics to develop. If they are not present female characteristics develop.
The red box shows the two sex chromosomes - these are the same size, both two X larger chromosomes.
Mothers/female alleles - XX and the fathers/male alleles - XY
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).