Sex-determination systems

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.

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/XO sex determination
In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (XO). The O 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 (XO); with a pair of chromosomes (XX) it is a hermaphrodite.

WZ sex chromosomes
The WZ sex-determination system is found in birds and some insects and other organisms. In the WZ sex-determination system, the situation is reversed: females have two different kinds of chromosomes (WZ), and males have two of the same kind of chromosomes (ZZ).

It is unknown whether the presence of the W chromosome induces female features or the duplication of the Z chromosome induces male ones; Unlike mammals, no birds with a double W chromosome (WWZ) or a single Z (Z0) have been discovered. It's probable that either condition causes embryonic death, and both chromosomes are responsible for gender selection.

Chromosomes in the WZ region in birds are autosomal in mammals, and vice-versa; therefore, it's theorized that the WZ and XY couples come from different chromosomes of the common ancestor. A paper published in 2004 (Frank Grützner et al, Nature; doi:10.1038/nature03021) suggests that the two systems may be related. According to the paper, platypuses have a ten-chromosome–based system, where the chromosomes form a multivalent chain in male meiosis, segregating into XXXXX-sperm and YYYYY-sperm, with XY-equivalent chromosomes at one end of this chain and the WZ-equivalent chromosomes at the other end.

Haplodiploidy
Haplodiploidy is found in insects belonging to Hymenoptera, such as ants and bees. Haploid individuals are male. Diploid individuals are generally female but may be sterile males. Thus, if a queen bee mates with one drone, her daughters share 3/4 of their genes with each other, not 1/2 as in the XY and WZ systems. This is believed to be significant for the development of eusociality, as it increases the significance of kin selection.

Non-genetic sex-determination systems
Many other exotic sex-determination systems exist. In some species of reptiles, including alligators, 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. 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 [this section still being researched]:
 * Swordtail fish?
 * The Chironomus midge species
 * etc

Reference

 * (2004) Evolution of Sex Chromosomes: The Case of the White Campion.

Kønskromosom Système de détermination sexuelle Determinazione del sesso Y染色体 Kjønnskromosom Детерминација пола Könskromosom