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Inheritance of biological gender

Human body cells have 23 pairs of chromosomes in the nucleus. Of these, 22 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, XY
• females have two X chromosomes, XX

These diagrams are known as human karyotypes, and show all the chromosomes aligned in pairs.

Chromosomes from a male

The blue box shows the two sex chromosomes – these are different sizes, an X (the large chromosome) and a Y (the small chromosome).

Chromosomes from a female

The pink box shows the two sex chromosomes – these are the same size, two large X chromosomes.

Genetic diagram

A genetic diagram, like a Punnett square, shows how alleles may combine in zygotes. The diagram below shows how biological gender (sex) is inherited.

Mother/female alleles – XX

Father/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 genotype)

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).