Why are genes sex-linked?

• Genes are sex-linked because:

  • That gene is on the X chromosome
• Its on the X chromosome because:

  • Its bigger than the Y chromosome
• • A man only needs 1 recessive allele
  • A woman needs 2 recessive alleles
• What alleles are in red-green colour blind people:

  • Colour blind man = Xb Y
  • Colour blind woman = Xb Xb
• What happen if the mum has 2 recessive alleles?

  • She will pass it on to her sons
• Example of sex-linked disorder:

  Haemophilia:

  • No blood clotting factor VIII
  • So blood doesn't clot properly
  • Always found in males