Editing Sex-determination system (section)

=== XX/X0 sex chromosomes ===
{{Main |X0 sex-determination system}}

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. 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 (order [[cockroach|Blattodea]]). A small number of mammals also lack a Y chromosome. These include the Amami spiny rat (''[[Tokudaia osimensis]]'') and the Tokunoshima spiny rat (''[[Tokudaia tokunoshimensis]]'') and ''Sorex araneus'', a [[shrew]] species. Transcaucasian mole voles (''[[Ellobius lutescens]]'') also have a form of XO determination, in which both sexes lack a second sex chromosome.<ref name="Chandra-1999"/> The mechanism of sex determination is not yet understood.<ref name="Kuroiwa-2011">{{cite journal | vauthors = Kuroiwa A, Handa S, Nishiyama C, Chiba E, Yamada F, Abe S, Matsuda Y | title = Additional copies of CBX2 in the genomes of males of mammals lacking SRY, the Amami spiny rat (Tokudaia osimensis) and the Tokunoshima spiny rat (Tokudaia tokunoshimensis) | journal = Chromosome Research | volume = 19 | issue = 5 | pages = 635–644 | date = July 2011 | pmid = 21656076 | doi = 10.1007/s10577-011-9223-6 | s2cid = 23311263 }}</ref>

The [[nematode]] ''[[Caenorhabditis elegans|C. elegans]]'' is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite.<ref name="Majerus-2003">{{Cite book | vauthors = Majerus ME | title = Sex wars: genes, bacteria, and biased sex ratios | publisher = Princeton University Press | pages = 250 | url = https://books.google.com/books?id=vDHOYPQ2mmYC&q=zo,+zww,+zzww+lepidoptera | isbn = 978-0-691-00981-0 | access-date = 4 November 2011 | year = 2003 }}
</ref> Its main sex gene is XOL, which encodes [[XOL-1 Switch protein N-terminal domain|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.<ref name="Kuwabara-1992">{{cite journal | vauthors = Kuwabara PE, Okkema PG, Kimble J | title = tra-2 encodes a membrane protein and may mediate cell communication in the Caenorhabditis elegans sex determination pathway | journal = Molecular Biology of the Cell | volume = 3 | issue = 4 | pages = 461–473 | date = April 1992 | pmid = 1498366 | pmc = 275596 | doi = 10.1091/mbc.3.4.461 }}</ref>