Sex Chromosomes (Edexcel IGCSE Biology)

Spec point — 4.26 · understand how the sex of a person is controlled by one pair of chromosomes, XX in a female and XY in a male

Sex 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

The sex chromosomes that a biological female and male possess.

Inheritance of Sex

Spec point — 4.27 · describe the determination of the sex of offspring at fertilisation, using a genetic diagram

• Sex is determined by the two sex chromosomes, X and Y

• Males carry chromosomes XY and females XX

• Due to this it is the male who determines the sex of offspring as

  • The mother can only pass on an X chromosome
  • The father can pass on either an X or a Y chromosome

• Therefore the offspring can inherit:

  • An X from the mother and an X from the father: this child will be a female
  • An X from the mother and a Y from the father: this child will be a male

Sperm cells determine the sex of offspring

Genetic diagrams showing determination 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 showing the inheritance of sex