Sex determination

Inheritance of biological sex

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:

  • 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, therefore an X (larger chromosome) and a Y (smaller one).

Chromosomes from a male, since it contains a Y chromosome

Chromosomes from a female

The red box shows the two sex chromosomes - these are the same size, both two X larger chromosomes.

Females have two X chromosomes

Genetic diagram

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

Diagram showing how genes are inheritedMothers/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).