Sex determination

Inheritance of biological sex

Human body cells have twenty-three pairs of 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:

males have two different sex chromosomes, X Y
females have two X chromosomes, XX

Chromosomes from a male

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.

Chromosomes from a female

The red box shows the two sex chromosomes which are both the same size. They are two larger X chromosomes.

Genetic crosses

A genetic cross, like a Punnett square, shows how the alleles inherited from the parents may combine in a . The diagram below shows how biological sex is inherited.

Mother has 2 x chromosomes, Father has an x and y chromosome. If the male x and one of the female x chromosomes combine they have a girl, if the x and y chromosomes combine they have a boy. — Figure caption,
Mothers/female alleles - XX and the fathers/male alleles - XY

The two possible combinations are:

an X chromosome from the mother and an X chromosome from the father - producing a girl (female phenotype from the XX genotype)
an X chromosome from the mother and a Y chromosome from the father - producing a boy (male phenotype from the XY phenotype)

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).