Sex-determination is a biological process that determines the development of sexual gender. Most individual organisms that procreate by sexual reproduction have one of two possible sexes, male or female. In many cases, determination of the gender of a new infant of a species that reproduces sexually is genetic: males and females have different genes that specify their sexual morphology and their ability to produce gametes. In many animals,like humans and birds, this is often accompanied by chromosomal differences. In other cases, sex is determined by environmental variables (such as temperature) during development or social variables (the size of an organism relative to other members of its population) during adult life.
XX/XY sex chromosomes
The XX/XY sex-determination system is found in human beings and most other mammals. In the XY sex-determination system, most pairs of chromosomes are autosomes, but one pair are sex chromosomes. 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).
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).
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 (ZWW) or a single Z (Z0) have been discovered. Probably that either condition causes embryonic death, and both chromosomes are responsible for gender selection.
In Lepidoptera, examples of Z0, ZZW and ZZWW females can be found. This suggests that the W chromosome is essential in female determination in some species (ZZW), but not in others (Z0). In Bombyx mori (the commercial silkworm), the W chromosome carries the female-determining genes.
Chromosomes in the ZW region in birds are autosomal in mammals, and vice-versa; therefore, it is theorized that the ZW 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 ZW-equivalent chromosomes at the other end.
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 ¾ of their genes with each other, not ½ 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.
Non-genetic sex-determination systems
In some reptiles like sea turtles and alligators the genital ridges in their early stages of development form just like the vertebrates whose sex is determined by chromosomes. But in these reptiles, sex chromosomes do not contribute to the way gender proceeds. The primary factor is not a set of genes, but the temperature at which the embryo is incubated during this period.
In the American alligator there is a temperature sensitive period for their eggs. Incubate a clutch at 33 degrees Centigrade and every one will hatch a male. Drp the temperature, and incubate at 30 degrees Centigrade and instead, every hatchling in the clutch is a female. Raise the temperature above the 100% male temperature of 33, to 34.5 - and the clutch hatches out 95% females and 5% males.
In tropical clown fish, the dominant individual in a group becomes female while the other ones are male.
Other species, such as some snails, practice sex change: adults start out male, then become female.
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.