Sex Determination (WJEC GCSE Biology): Revision Note

Sex Determination

• Sex is determined by an entire chromosome pair (as opposed to most other characteristics that are just determined by one or a number of genes)

• Females have the sex chromosomes XX

• Males have the sex chromosomes XY

• As only a father can pass on a Y chromosome, he is responsible for determining the sex of the child

• He does this because:

  • He produces (ejaculates) around 250 million sperm cells during sexual intercourse

  • Of those, half (125 million sperm) will be carrying his X chromosome

    • If one of these sperm fertilises the egg, the fetus will be female

  • The other 125 million of his sperm will be carrying his Y chromosome

    • Which will result in a male foetus if one of these fertilises the egg

Inheritance of sex diagram

Sperm cells determine the sex of offspring

• The inheritance of sex can be shown using a genetic diagram (known as a Punnett square), with the X and Y chromosomes taking the place of the alleles usually written in the boxes

Punnet square sex determination diagram

Punnett square showing the inheritance of sex