CIE IGCSE Biology: 复习笔记:17.1.2 The Inheritance of Sex

CIE IGCSE Biology: 复习笔记:17.1.2 The Inheritance of Sex

XX & XY 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
    • 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 fetus if one of these fertilises the egg


Sex-chromosomes_1Sperm 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


Inheritance-of-sexPunnett square showing the inheritance of sex