IB DP Biology: SL复习笔记3.1.8 Sex Determination

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