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The '''X:A ratio''' is the ratio between the number of [[X chromosome]]s and the number of sets of [[autosome]]s in an [[organism]]. This ratio is used primarily for determining the sex of some species, such as [[drosophila]] flies and the [[Caenorhabditis elegans|C. elegans]] nematode.{{sfn|Meurant|1990|p=122}} The first use of this ratio for sex determination is ascribed to Victor M. Nigon.{{sfn|Meurant|1990|p=122}}

Generally, a 1:1 ratio results in a female and a 1:2 ratio results in a male. When calculating the ratio, [[Y chromosome]]s are ignored. For example, for a [[diploid]] drosophila that has XX, the ratio is 1:1 (2 Xs to 2 sets of autosomes, since it is a diploid). For a diploid drosophila that has XY, the ratio is 1:2 (1 X to 2 sets of autosomes, since it is diploid).{{sfn|Gilbert|2000|loc=Chromosomal Sex Determination in Drosophila}}
Drosophilla sex chromosome ratio determines the factors it encodes which enhances the synthesis of sxl protein which in turn activates the female specific pathway.

==See also==
* {{section link|Drosophila melanogaster|Sex determination}}

==Notes==
{{reflist}}

==References==
{{refbegin}}
*{{cite book|last=Gilbert|first=Scott F.|title=Developmental Biology|publisher=Sinauer Associates|edition=6th|date=2000|isbn=9780878932436|url-access=registration|url=https://archive.org/details/developmentalbio00gilb}}
*{{cite book|title=Advances in Genetics|last=Meurant|first=Gerard|editor-last=Wright|editor-first=Theodore R.F.|volume=27|publisher=Academic Press|date=1990|isbn=0120176270|lccn=47-30313}}
{{refend}}

[[Category:Genetics]]
[[Category:Drosophila| X]]


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