In humans:

Human sex refers to the processes by which an individual becomes either a male or female during development.

Complex mechanisms are responsible for male sex determination and differentiation. The steps of formation of the testes are dependent on a series of Y-linked, X-linked and autosomal genes actions and interactions

The combination of sex chromosomes that determine the sex of an offspring; in humans the sex chromosomes of a normal female are XX and the sex chromosomes of a normal male are XY.

The ZW sex-determination system is found in reptiles (including birds) and some insects and other organisms.

The ZW sex-determination system is reversed compared to the XY system: females have two different kinds of chromosomes (ZW), and males have two of the same kind of chromosomes (ZZ).

  • In the chicken, this was found to be dependent on the expression of DMRT1.In birds, the genes FET1 and ASW are found on the W chromosome for females, similar to how the Y chromosome contains SRY. However, not all species depend upon the W for their sex.
  • For example, there are moths and butterflies that are ZW, but some have been found female with ZO, as well as female with ZZW.
  • Also, while mammals inactivate one of their extra X chromosomes when female, it appears that in the case of Lepidoptera, the males produce double the normal amount of enzymes, due to having two Z’s.
  • Because the use of ZW sex determination is varied, it is still unknown how exactly most species determine their sex.
  • Despite the similarities between ZW and XY, the sex chromosomes do not line up correctly and evolved separately.

Drosophila (Fruit fly) have both X and Y chromosomes as well as autosomes. Unlike humans, the Y chromosome does not confer maleness, rather it encodes genes necessary for making sperm. Sex is instead determined by the ratio of autosomes to X chromosomes. Further, each cell “decides” whether to be male or female independently of the rest of the organism resulting in the occasional occurrence of gynandromorphs.

X Chromosomes Autosomes Ratio of X:A Sex
XXXX AAAA 1 Normal Female
XXX AAA 1 Normal Female
XX AA 1 Normal Female
X AA 0.50 Normal Male
XXX AA 1.50 Metafemale
XXXX AAA 1.33 Metafemale
XX AAA 0.66 Intersex
X AAA 0.33 Metamale


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