Autosome versus Heterosome

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•  Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex

In humans, sex is determined by a pair of chromosomes called the sex chromosomes (or heterosomes)

  • Females possess two copies of a large X chromosome (XX)
  • Males possess one copy of an X chromosome and one copy of a much shorter Y chromosome (XY)

The Y chromosome contains the genes for developing male sex characteristics (specifically the SRY gene)

  • In its absence of a Y chromosome, female sex organs will develop
  • The sex chromosomes are homologous in females (XX) but are not homologous in males (XY)

Hence the father is always responsible for determining the sex of offspring:

  • If the male sperm contains an X chromosome, the growing embryo will develop into a girl
  • If the male sperm contains a Y chromosome, the growing embryo will develop into a boy
  • In all cases the female egg will contain an X chromosome (as the mother is XX) 

The remaining chromosomes in the organism are called autosomes (they do not determine sex)

Sex Determination in Humans (the XY System)

XY Sex Determination