Sex-determination system

A sex-determination system is a biological system that determines the development of the organism's sex. Most organisms that create their offspring using sexual reproduction have two common sexes, males and females, and in other species there are hermaphrodites, organisms that can function reproductively as either female or male, or both.
There are also some species in which only one sex is present, temporarily or permanently. This can be due to parthenogenesis, the act of a female reproducing without fertilization, mostly seen in plant species. In some plants or algae the gametophyte stage may reproduce itself, thus producing more individuals of the same sex as the parent.
In some species, 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, generally through combinations of XY, ZW, XO, ZO chromosomes, or haplodiploidy. The sexual differentiation is generally triggered by a main gene, with a multitude of other genes following in a domino effect.
In other cases, the sex of a fetus is determined by environmental variables. The details of some sex-determination systems are not yet fully understood.
Some species such as various plants and fish do not have a fixed sex and instead go through life cycles and change sex based on genetic cues during corresponding life stages of their type. This could be due to environmental factors such as seasons and temperature. In some gonochoric species, a few individuals may have conditions that cause a mix of different sex characteristics.

Discovery

The scientific understanding of sex determination has evolved significantly over the centuries through a series of landmark discoveries across plants, animals, and insects.
In 1694, German botanist Rudolf Jakob Camerarius conducted pioneering experiments on pollination, during which he identified the existence of distinct male and female reproductive structures in plants, including maize. His findings laid the groundwork for understanding plant reproduction and sexual differentiation.
In 1866, Gregor Mendel, an Austrian monk and scientist, published his seminal work on the inheritance of traits in pea plants. These findings—now known as Mendelian inheritance—introduced the concept of heritable units passed from two gametes. Mendel's principles eventually became the foundation of modern genetics.
In 1902, American zoologist C.E. McClung proposed that sex chromosomes played a central role in determining sex, based on cytological studies in insects. This was one of the earliest steps toward a chromosomal theory of sex determination.
In 1903, American geneticist Nettie Stevens made a groundbreaking discovery while studying the mealworm, demonstrating that sex is determined by specific chromosomes, now known as X and Y chromosomes. Her findings provided the first concrete evidence for chromosomal sex determination.
In 1917, botanist Charles Elmer Allen extended this understanding to the plant kingdom by discovering sex chromosomes in plants, confirming that the chromosomal mechanism is not exclusive to animals.
In 1922, geneticist Calvin B. Bridges introduced the genie balance theory, based on experiments with Drosophila melanogaster. He proposed that the ratio of X chromosomes to sets of autosomes determines sexual development, adding complexity to the previously accepted XX/XY system.
In 1928, Swiss biologist Fritz Baltzer was the first to describe environmental sex determination, showing that external environmental factors, such as temperature, could influence the development of sex in certain organisms. This discovery expanded the understanding of sex determination beyond purely genetic mechanisms.

Chromosomal systems

Among animals, the most common chromosomal sex determination systems are XY, XO, ZW, ZO, but with numerous exceptions.
According to the Tree of Sex database, the known sex determination systems are:

Taxonomic group	XY	XO	ZW	ZO	Other¹	XO/XY ratio	ZO/ZW ratio
Vertebrates	722	15	480	3	254	0.02	0.01
Insects	4415	1857	37	25	156	0.42	0.68
Angiosperms	23	0	1	0	19	0.00	0.00
Total	5160	1872	518	28	429	0.36	0.05

^{1. complex sex chromosomes, homomorphic sex chromosomes, or others}

XX/XY sex chromosomes

The XX/XY sex-determination system is perhaps the most familiar as it is found in humans and most other mammals, as well as in some insect species. In the XX/XY system, karyotypic females usually have two X chromosomes, while karyotypic males usually have a single X and a single Y chromosome. The X and Y sex chromosomes are different in shape and size from each other, unlike other chromosome pairs, and are sometimes called allosomes. In some species, including humans, individuals remain phenotypically undifferentiated for some time during development ; in others, however, such as fruit flies, sexual differentiation occurs as soon fertilization occurs.

Y-centered sex determination

Some species have a gene SRY on the Y chromosome that triggers development of the male phenotype. Members of SRY-reliant species can also have other chromosomal combinations such as XXY. In humans, karyotypic sex is generally determined by the presence or absence of a Y chromosome with a functional SRY gene. If the SRY gene is present and activated during fetal development, cells create testosterone and anti-müllerian hormone which typically leads to male phenotypic development. In embryos that lack a functioning SRY gene, such as most XX individuals, the individual develops phenotypically female.
In Y-centered sex determination, the SRY gene is the gene that triggers male phenotype development, however multiple genes are required for this process to complete. In XY mice, lack of the gene DAX1 on the X chromosome results in sterility, but in humans it causes adrenal hypoplasia congenita. However, when an extra DAX1 gene is placed on the X chromosome, the result is phenotypically female, despite the existence of SRY, since it overrides the effects of SRY. Even when there are functional X chromosomes in XX females, duplication or expression of SOX9 causes testes to develop. Gradual sex reversal in developed mice can also occur when the gene FOXL2 is removed from females. Even though the gene DMRT1 is used by birds as their sex locus, species who have XY chromosomes also rely upon DMRT1, contained on chromosome 9, for sexual differentiation at some point in their formation.

X-centered sex determination

Some species, such as fruit flies, use the presence of two X chromosomes to determine femaleness. Species that use the number of Xs to determine sex are nonviable with an extra X chromosome.

Other variants of XX/XY sex determination

Some fish have variants of the XY sex-determination system, as well as the regular system. For example, while having an XY format, Xiphophorus nezahualcoyotl and X. milleri also have a second Y chromosome, known as Y', that creates XY' females and YY' males.
At least one monotreme, the platypus, presents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds and lacks the SRY gene. The platypus has sex chromosomes. The males have, while females have. During meiosis, 5 of X form one chain, and 5 of Y form another chain. Thus, they behave effectively as a typical XY chromosomal system, except each of X and Y is broken into 5 parts, with the effect that recombinations occur very frequently at 4 particular points. One of the X chromosomes is homologous to the human X chromosome, and another is homologous to the bird Z chromosome.
Although it is an XY system, the platypus' sex chromosomes share no homologues with eutherian sex chromosomes. Instead, homologues with eutherian sex chromosomes lie on the platypus chromosome 6, which means that the eutherian sex chromosomes were autosomes at the time that the monotremes diverged from the therian mammals. However, homologues to the avian DMRT1 gene on platypus sex chromosomes X3 and X5 suggest that it is possible the sex-determining gene for the platypus is the same one that is involved in bird sex-determination. More research must be conducted in order to determine the exact sex determining gene of the platypus.

XX/X0 sex chromosomes

In this variant of the XY system, females have two copies of the sex chromosome but males have only one. The 0 denotes the absence of a second sex chromosome. Generally in this method, the sex is determined by amount of genes expressed across the two chromosomes. This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches. A small number of mammals also lack a Y chromosome. These include the Amami spiny rat and the Tokunoshima spiny rat and Sorex araneus, a shrew species. Transcaucasian mole voles also have a form of XO determination, in which both sexes lack a second sex chromosome. The mechanism of sex determination is not yet understood.
The nematode C. elegans is male with one sex chromosome ; with a pair of chromosomes it is a hermaphrodite. Its main sex gene is XOL, which encodes XOL-1 and also controls the expression of the genes TRA-2 and HER-1. These genes reduce male gene activation and increase it, respectively.

ZW/ZZ sex chromosomes

The ZW sex-determination system is found in birds, some reptiles, 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, and males have two of the same kind of chromosomes. 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 deactivate 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. However, reportedly, the silkworm Bombyx mori uses a single female-specific piRNA as the primary determiner of sex. Despite the similarities between the ZW and XY systems, these sex chromosomes evolved separately. In the case of the chicken, their Z chromosome is more similar to humans' autosome 9. The chicken's Z chromosome also seems to be related to the X chromosome of the platypus. When a ZW species, such as the Komodo dragon, reproduces parthenogenetically, usually only males are produced. This is due to the fact that the haploid eggs double their chromosomes, resulting in ZZ or WW. The ZZ become males, but the WW are not viable and are not brought to term.
In both XY and ZW sex determination systems, the sex chromosome carrying the critical factors is often significantly smaller, carrying little more than the genes necessary for triggering the development of a given sex.