Sex Determination in Humans

Sex Determination in Humans

  • Human beings typically have 23 pairs of chromosomes in each cell. Out of these, 22 pairs are called autosomes and they are the same in males and females. The remaining pair is the sex chromosomes which determine the gender of an individual.

  • Females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY).

  • Each child inherits one sex chromosome from each parent. The chromosome from the mother is always X, but the one from the father can be either X or Y.

  • If a sperm carrying an X chromosome fertilises an egg, the resulting combination of XX results in a female child. Conversely, if a sperm carrying a Y chromosome fertilises an egg, the combination of XY results in a male child.

  • Therefore, the sex of a child is determined by the father.

  • Unlike most traits, which involve genes on the autosomes and so are influenced by genes from both parents, much of sex determination is controlled by the Y chromosome if it’s present.

  • Disorder of sexual development can occur when there are additional, missing or rearranged sex chromosomes e.g. Turner Syndrome (females with only one X chromosome), Klinefelter Syndrome (males with an additional X chromosome—XXY), etc.

  • A gene called SRY on the Y chromosome needs to be activated for male development to take place. If this gene is not activated, even in the presence of a Y chromosome, female development will occur.

  • This sex-determination system is called XY because the presence or absence of the Y chromosome, not the number of X chromosomes, determines the sex.

  • Outside of sex determination, the X and Y chromosomes also carry genes for other traits, some related to sexual development and some for non-sexual traits.

  • X-linked recessive disorders, like colorblindness and haemophilia, occur more frequently in males than females because males have only one X chromosome, so a single recessive gene on that X chromosome will cause the disease.

  • Females can also acquire these X-linked recessive disorders when they inherit two recessive genes, one from each parent. However, it is less common due to the likelihood of inheriting at least one dominant gene. But females can be carriers of these diseases and pass them onto their children.