<P> We (are) entering this new era in molecular biology of sex determination where it's a more subtle dosage of genes, some pro-males, some pro-females, some anti-males, some anti-females that all interplay with each other rather than a simple linear pathway of genes going one after the other, which makes it very fascinating but very complicated to study . </P> <P> In mammals, including humans, the SRY gene is responsible with triggering the development of non-differentiated gonads into testes, rather than ovaries . However, there are cases in which testes can develop in the absence of an SRY gene (see sex reversal). In these cases, the SOX9 gene, involved in the development of testes, can induce their development without the aid of SRY . In the absence of SRY and SOX9, no testes can develop and the path is clear for the development of ovaries . Even so, the absence of the SRY gene or the silencing of the SOX9 gene are not enough to trigger sexual differentiation of a fetus in the female direction . A recent finding suggests that ovary development and maintenance is an active process, regulated by the expression of a "pro-female" gene, FOXL2 . In an interview for the TimesOnline edition, study co-author Robin Lovell - Badge explained the significance of the discovery: </P> <P> We take it for granted that we maintain the sex we are born with, including whether we have testes or ovaries . But this work shows that the activity of a single gene, FOXL2, is all that prevents adult ovary cells turning into cells found in testes . </P> <P> Looking into the genetic determinants of human sex can have wide - ranging consequences . Scientists have been studying different sex determination systems in fruit flies and animal models to attempt an understanding of how the genetics of sexual differentiation can influence biological processes like reproduction, ageing and disease . </P>

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