Sex determination

Sex determination

  • Women have two X chromosomes and men have one X and one Y. The 23rd pair of chromosomes is called the sex chromosomes.
  • When making sperm, the X and Y chromosomes are drawn apart in the first division in meiosis. There’s a 50% chance each sperm cell gets an X-chromosome and a 50% chance it gets a Y-chromosome. All egg cells have one X chromosome.
  • Punnett squares can be used to calculate the probability of the genders of people.


Sex-linked inheritance

  • Most of our chromosomes come in matching pairs. The two chromosomes in a pair have the same genes in the same places but may have different alleles.
  • However, the X chromosome is much larger than the Y chromosome. As well as this, there are more genes on the X chromosome than on the Y chromosome. This means that males will only have one copy of most of the genes on the X chromosome.
  • One gene found only on the X chromosome codes for a substance that clots blood. The normal allele, H, allows the blood to clot. The recessive allele, h, prevents normal clotting and causes a disease called haemophilia.
  • Haemophilia is an example of a sex-linked genetic disorder.

  • The probability of the inherited alleles can be found the same way as with normal genes, except we must show which chromosomes the genes are on.
  • Each time this couple have a child, the probability that it will be a boy with haemophilia is 1 in 4. This can be written 1:3 or as 25%.
  • The gene for red-green colour blindness is also on the X chromosome. The normal allele of this gene gives normal colour vision, but there is a recessive allele that means a person cannot distinguish red from green.