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A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. Occasionally there are hermaphrodites in place of one or both sexes. There are also some species that are only one sex due to parthenogenesis, the act of a female reproducing without fertilization. In many 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 (a “sex locus“), with a multitude of other genes following in a domino effect. In other cases, sex is determined by environmental variables (such as temperature) or social variables (e.g. the size of an organism relative to other members of its population). Environmental sex determination preceded the genetically determined systems; it is thought that a temperature-dependent amniote was the common ancestor of animals with sex chromosomes. Some species do not have a fixed sex, and instead change sex based on certain cues. The details of some sex-determination systems are not yet fully understood. 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 (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. 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 chromosomes of the platypus. When a ZW species, such as the Komodo Dragon, reproduce parthenogenetically, males are usually only 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. #geneticInformation #chromatin #hardyWeinbergEquilibrium #cytosine #TumorCells #FrancisCrick #AminoAcids #hardyWeinbergPrinciple #population #centromere #DaughterCells #Inbreeding #PCR #NaturalSelection #Morphogenesis #ChromosomeArm #SisterChromatid #TurnerSyndrome #Diploid #CellCulture #Nucleotide
