Sex determination

Inheritance of biological sex

Human body cells have 23 pairs of chromosomes in the nucleus. Twenty two pairs are known as autosomes - they control characteristics, but one pair carries genes that determine sex:

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

Chromosomes from a male

Chromosomes from a male, since it contains a Y chromosome

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

Chromosomes from a female

Females have two X chromosomes

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

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.Mother's / female alleles – XX and father's / male alleles - XY

All eggs are X . Half of sperm are X and the other half are Y. So 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).

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