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The Intersexion of Boy or Girl September 3, 2009

Posted by Geekgirl in anatomy, Developmental, genetics.
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Many people think that gender is simple. If you have an X and a Y chromosome, you are a boy. If you have an X and an X chromosome, you are a girl. But, it doesn’t always work this way.

Why not? Because the development of genitalia has a fork in the road. One direction – girl. Another direction – boy. And sometimes there is a new road, where something happens just a little bit differently in development and a child is born with ambiguous genitalia. Some individuals are XY but anatomically are girls. Here is a portion from a very recent scientific publication and the link. It looks hard to read but it really isn’t that bad. It contains a nice explanation of the developmental pathway.

A balance between antagonistic signals in the choice between the male or female pathway

Mammalian embryos, like those of most animals, are initially sexually undifferentiated and can develop into either male or female individuals following one of two alternative processes. The paternal transmission of a Y chromosome triggers testicular differentiation, whereas the presence of a paternal X chromosome pushes gonads towards ovarian differentiation.

Characterization of the pathways that promote testicular or ovarian differentiation is essential for a better understanding of sex determination pathologies, including gonadal dysgenesis, gonadal agenesis and sex reversal [1]. In humans, XY sex reversal is relatively frequent (occurring in about 1 in 3,000 newborns) and is genetically heterogeneous, with loss of function of the SRY male-sex-determining gene accounting for only 15% of the cases. By contrast, XX sex reversal is rare (about 1 in 20,000 newborns) and is usually caused by the translocation of SRY onto another chromosome. The underlying cause of sex reversal still cannot be identified in approximately 75% of patients, indicating that a significant number of genes (or regulatory regions of known genes) involved in sex determination are yet to be revealed. Several recent reports [2,3], including work by Garcia-Ortiz et al. in BMC Developmental Biology [4], are shedding new light on the genes involved in female sex determination.

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