Going beyond X and Y

In his genetics laboratory at the University of California, Los Angeles, Vilain's findings have pushed the field toward not only improved technical understanding but more thoughtful treatment as well. "What really matters is what people feel they are in terms of gender, not what their family or doctors think they should be," Vilain says. Genital ambiguity occurs in an estimated one in 4,500 births, and problems such as undescended testes happen in one in 100.
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Students have long learned in developmental biology that the male path of sex development is "active," driven by the presence of a Y chromosome. In contrast, the female pathway is passive, a default route. French physiologist Alfred Jost seemed to prove this idea in experiments done in the 1940s, in which castrated rabbit embryos developed into females.
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But studies by Vilain and others have shaped a more complex picture. Instead of turning on male development directly, SRY works by blocking an "antitestis" gene, he proposes. For one, males who have SRY but two female chromosomes range in characteristics from normal male to an ambiguous mix. In addition, test-tube studies have found that SRY can repress gene transcription, indicating that it operates through interference. Finally, in 1994, Vilain's group showed that a male could develop without the gene. Vilain offers a model in which sex emerges out of a delicate dance between a variety of promale, antimale, and possibly profemale genes.
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Despite classic dogma, he is certain that sex hormones do not drive neural development and behavioral differences on their own. SRY is expressed in the brain, he points out, suggesting that genes influence brain sexual differentiation directly. His lab has identified in mice 50 new gene candidates on multiple chromosomes for differential sex expression. Seven of them begin operating differently in the brain before gonads form. 


Source : Sciam - Sally Lehrman

Also :
Making Sperm, No Men necessary
Y chromosome shrinks, End of Men pondered