The Inheritance of Sex (Cambridge International IGCSE Biology)

XX & XY chromosomes

Spec point — 17.1.2 · Describe the inheritance of sex in humans with reference to X and Y chromosomes

• 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, and of those:

  • Around half will be carrying an X chromosome, and if one of these sperm fertilises the egg, the fetus will be female
  • The other half will be carrying a Y chromosome and if one of these sperm fertilises the egg, the fetus will be male

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

Punnett square showing the inheritance of sex