The Inheritance of Sex (CIE IGCSE Biology: Co-ordinated Sciences (Double Award))

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Cara Head

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Cara Head

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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) fetusĀ 

Sex chromosomes_1, downloadable IB Biology revision notesSperm 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

Inheritance of sex, IGCSE & GCSE Biology revision notesPunnett square shows the inheritance of sex

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding