3.1.8 Sex Determination

• 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 (pair 23 in humans) XX
• Males have the sex chromosomes (pair 23 in humans) XY
  • Note that the rule XX for females and XY for males applies to mammals, but not to all species

• All other chromosomes (pairs 1 - 22 in humans) are autosomes and have no influence on determining the sex of offspring
• Because only a father can pass on a Y chromosome, he is responsible for determining the sex of the child
  • Due to meiosis, half of his sperm cells will carry his X chromosome, half his Y chromosome
  • The chromosome carried by the sperm that fertilizes the egg will determine the sex of the child
  • His daughters receive a copy of his X chromosome
  • His sons receive a copy of his Y chromosome

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

Genes carried by X and Y chromosomes

• The X chromosome is larger than the Y, and has its centromere more central than on the Y chromosome
  • Fewer genes are coded for on the Y chromosome as a result
    • The X carries around 16 × more genes than the Y chromosome
  • Non-sex phenotypic traits, including certain blood clotting factors, are coded for on the X chromosome but not on the Y

• The Y chromosome carries genes that code for male characteristics
• One of these genes is the SRY gene which is involved in
  • Development of testes in male embryos
  • Production of testosterone
• Females don't receive these genes, so instead, ovaries develop and female sex hormones are expressed