The Inheritance of Sex (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note

XX & XY Chromosomes

• Sex (male or female) is determined by an entire chromosome pair (as opposed to most other characteristics that are just determined by one or several genes)

• Biological females have the sex chromosomes XX

• Biological males have the sex chromosomes XY

• Only a biological male can pass on a Y chromosome, he is therefore responsible for determining the sex of the child

• He does this because

  • He produces (during ejaculation) 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 (also an X chromosome), the fetus will be female (XX)

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

    • If one of these fertilises the egg (X chromosome) this will result in a male (XY) fetus 

Sperm cells determine the sex of offspring

Inheritance of sex

• 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

Punnett square shows the inheritance of sex