Boy or Girl? Sex inheritance in humans

• Sex determination in humans

  • Inherited through the X and Y chromosomes
• People always want to know if a new baby is a boy or a girl
• Sex determination in humans

  1. Egg fertilised by sperm

  2. Embryo gets half chromosomes from egg (mother)

  3. Embryo gets half chromosomes from sperm (father)
• Chromosomes


  Carry the genes, you get one copy from your mother and one from your father
• Chromosome pairs


  • 22 pairs are the same size and shape
  • 23rd pair are the sex chromosomes (X and Y)
• Female sex chromosomes


  Two X chromosomes (XX)
• Male sex chromosomes


  One X chromosome and one Y chromosome (XY)
• Fertilisation


  1. Sperm with X chromosome fertilises egg

  2. Results in XX (female)

  3. Sperm with Y chromosome fertilises egg

  4. Results in XY (male)
• For every pregnancy, the chances of a baby boy or girl being born are 50%