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

• 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

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

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