If manners maketh a man, they are assisted by a cascade of genetic activity that actively promotes maleness. In a classic series of experiments on rabbits in the 1950s, Albert Jost provided evidence that all embryos develop as females unless specific, male-determining factors intervene. Removal of the testes from male rabbit embryos caused the rabbits to develop as females, but the excision of ovaries from female rabbits had no effect on sexual development. In later years, studies on rare cases of humans who are genetically male but who develop as females have supported this view.
More recently, researchers have challenged the idea that femaleness is a passive, default option, and evidence has come to light of genes that might actively promote the formation and function of the ovaries. One such candidate is AHC (also known as DAX1). But in a report in Nature Genetics, J. Larry Jameson of Northewestern University Medical School, Chicago, Illinois provide evidence to show that the role of AHC is not to promote ovarian function, but instead to act as an ?anti-testis? factor, which is something rather different. This result reopens the question of whether femaleness might not be the default route of sexual development after all.
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