On the instability of polygenic sex determination: the effect of sex-specific selection. So XX is a girl and XY is a boy. Kuwabara; Peter G. EMBO J.
Ueshima N. Complex sex chromosome systems with multiple X chromosomes are present in species. How robust are the data? Gametes possess half the number of chromosomes found in zygote and the cells derived from it. Open in new tab.
So, moot point. Everyone seems to be focused on the possible political side of this posting. Novartis Found Symp. The nematode C. A very interesting article! Forces driving the divergence of sex determination systems Random genetic drift and directional positive selection are the evolutionary forces that can initially drive rare sex-determining variants throughout populations of any given species.
Genotypic sex determination: an individual's sex is established by its genotype e. Changes in the sex determination pathways across taxa are assumed to involve adding a new master-switch gene to this molecular pathway as in some fly taxa;  , with little change to downstream elements of the sex determination pathway .
When this developmental process goes awry, the effects can be catastrophic, causing everything from ambiguous external genitalia which occurs in up to one in 4, infants to sterility which is more cryptic and difficult to diagnose but may be far more common.
In some species, such as humans, organisms remain sex indifferent for a time after they're created; in others, however, such as fruit flies, sexual differentiation occurs as soon as the egg is fertilized. Box 2.