Sex determination in humans

Sexual reproduction

Two parents are needed in sexual reproduction.

During this process the nuclei of the male and female sex cells, called gametes, fuse to create a zygote. This process is known as fertilisation.

Each gamete contains half the number of chromosomes and is described as haploid.

When the male and female gametes combine, they restore the full complement of chromosomes - or diploid number - to produce a human embryo.

The gametes in animals are sperm and eggs.

The gametes in flowering plants are pollen and eggs.

The offspring produced in sexual reproduction are genetically different to each other and the parents. This process results in variation as it involves the mixing of genetic information.

Humans can only reproduce sexually.

The advantages of sexual reproduction are:

  • it produces variation in the offspring
  • the species can adapt to new environments due to variation, which gives them a survival advantage
  • a disease is less likely to affect all the individuals in a population
  • humans can speed up natural selection through selective breeding, which can increase food production

Sexual reproduction - conception

Sex determination in humans

Human body cells have 23 pairs of chromosomes in the nucleus.

22 pairs are known as autosomes, and control characteristics only, but one pair carries genes that determine sex - whether offspring are male or female:

  • males have two different sex chromosomes, X and Y, and are described as XY
  • females have two X chromosomes, and are described as XX

Chromosomes from a male

These diagrams are known as human karyotypes, and show all the chromosomes, cut out from a micrograph and aligned in pairs.

The blue box shows the two sex chromosomes - these are different sizes, and are 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 biological sex is inherited.

Sex determinationThe mother's/female chromosomes - XX and the father's/male chromosomes - XY

The two possible combinations are:

  • an X chromosome from the mother and an X chromosome from the father - producing a girl (XX)
  • an X chromosome from the mother and a Y chromosome from the father - producing a boy (XY)

The ratio of female to male offspring is 1:1, ie half of the offspring will be girls and half will be boys ie on average. This can also be converted into a probability of 50% (XX) and 50% (XY).