Sex-determination system
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals, this is often accompanied by chromosomal differences. In other cases, sex is determined by environmental variables (such as temperature) or social variables (the size of an organism relative to other members of its population). The details of some sex-determination systems are not yet fully understood.

# Chromosomal determination

## XX/XY sex chromosomes
The XX/XY sex-determination system is one of the most familiar sex-determination systems and is found in human beings and most other mammals. In the XY sex-determination system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). Some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as the fruit fly) use the presence of two X chromosomes to determine femaleness.

## XX/X0 sex determination
In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (X0).  The 0 denotes the absence of a second sex chromosome.  This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches (order Blattodea).
The nematode C. elegans is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite.

## ZW sex chromosomes
The ZW sex-determination system is found in 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).

## Haplodiploidy
Haplodiploidy is found in insects belonging to Hymenoptera, such as ants and bees. Unfertilized eggs develop into haploid individuals, which are the males. Diploid individuals are generally female but may be sterile males. Thus, if a queen bee mates with one drone, her daughters share 34 of their genes with each other, not 12 as in the XY and ZW systems. This is believed to be significant for the development of eusociality, as it increases the significance of kin selection.
This is common also in wasps that are parasitic and in the male greenflies.

# Non-genetic sex-determination systems
Many other exotic sex-determination systems exist. In some species of reptiles, including alligators, some turtles, and the tuatara, sex is determined by the temperature at which the egg is incubated. Other species, such as some snails, practice sex change: adults start out male, then become female. In tropical clown fish, the dominant individual in a group becomes female while the other ones are male.
Some species have no sex-determination system. Earthworms and some snails are hermaphrodites; a few species of lizard, fish, and insect are all female and reproduce by parthenogenesis.
In some arthropods, sex is determined by infection, as when Bacteria of the genus Wolbachia alter their sexuality; some species consist entirely of ZZ individuals, with sex determined by the presence of Wolbachia.
Other unusual systems :
- Swordtail fish
- The Chironomus midge species
- The Platypus lacks the mammalian sex-determining gene SRY, meaning that the process of sex determination in the Platypus remains unknown.