• 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