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6.9 Sex determination

How sex is decided by the sex chromosomes (XX female, XY male), a genetic diagram showing why there is a 50% chance of a boy or a girl, and why the father's sperm decides the sex of the child.

AQA Edexcel OCR 1 min read

Sex chromosomes

Key terms

  • Sex chromosomes: the pair of chromosomes that decides sex: XX in females and XY in males.

Human body cells have 23 pairs of chromosomes. 22 pairs control characteristics only. One pair carries the genes that decide sex.

  • In females, the sex chromosomes are the same: XX.
  • In males, they are different: XY. The Y chromosome is much smaller.

Gametes

  • All eggs carry an X chromosome.
  • Half of the sperm carry an X and half carry a Y.

So it is the sperm that decides the sex of a child.

The genetic diagram

A Punnett square of XX × XY giving a 50% chance of a boy or a girl, and drawings of the XX and XY sex chromosomes
A Punnett square of XX × XY giving a 50% chance of a boy or a girl, and drawings of the XX and XY sex chromosomes

The cross gives 2 XX : 2 XY, a 1 : 1 ratio. So each time a child is conceived there is a 50% chance it is a boy and a 50% chance it is a girl, whatever the sex of any brothers or sisters.

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