Sex determination in humans

Inheritance of biological sex

Egg (23 chromosomes) combines with sperm (23 chromosomes).  Fertilisation occurs creating a zygote with 46 chromosomes in 23 pairs. This matures into embryo with 46 chromosomes in 23 pairs.

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:

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

These photographs show all the chromosomes aligned in pairs.

Chromosomes from a male

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

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.

Chromosomes from a female

Females have two X chromosomes

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

Diagram showing how genes are inherited

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 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).

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