The X:A ratio is the ratio between the X chromosome and the number of sets of autosomes in an organism.
This ratio is used primarily for determining sex of drosophila flies. Generally, a 1:1 ratio results in a female and a 1:2 ratio results in a male. When calculating the ratio, Y chromosomes are ignored. For example, for a triploid drosophila that has XXX, the ratio is 1:1 (3 Xs to 3 autosomes, since it is a triploid). For a diploid drosophila that has XY, the ratio is 1:2 (1 X to 2 autosomes, since it is diploid).
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“Official dignity tends to increase in inverse ratio to the importance of the country in which the office is held.”
—Aldous Huxley (1894–1963)