Males are XY and Females are XX. You're born as one of the two, and this can not and does not change.
That conventional wisdom is constantly being rewritten, revised and updated as new scientific evidence points to a sexuality spectrum due to easily disrupted gender physiology in utero, and researchers asking whether we all have female and male abilities (ie: the ability to produce estrogen and testosterone in our gonads depending on conditions in utero).
Using a mouse model, researchers in Germany recently deleted a single gene (abbreviated FOXL2) in sexually mature mouse ovaries and found that the mice essentially became males, in that they started producing testosterone from their ovaries. This finding suggests that female mammals may be 'actively suppressing their male pathways', according to the study's author Mathias Treier (quoted in Nature this month).
However, in most mammals, sexual development in the fetus occurs when genes on the Y chromosome activate male-specific pathways, including those that induce testes development. (Meaning, we will default to female pathways without the Y chromosome.) This new study (published in Cell this month) shows that the product of the FOXL2 gene acts with estrogen receptors (which bind estrogen to transmit its molecular signals and activities) to suppress the gene that induces testes development, so without FOXL2, ovaries will produce testosterone.
Apparant sex changes in utero have also been found previously in mice lacking estrogen receptors, and a wealth of data show that mammals are capable of exhibiting a range of sexual physiology, estimated at 50-100 distinct scenarios including males with XXY or XXXY chromosomes (a list of some of these conditions and their frequencies can be found here: http://www.isna.org/faq/frequency).
Although the cause of transsexual (or intersex) births is unclear, it is thought to arise from genetic or environmental (ie: hormones bathing the fetus in utero) causes, and it is now widely accepted as a distinct phenomena from being homosexual/heterosexual. (Read more here in the ABCs of sexual health: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC516664/)
Scientific research into our sexual spectrum is not new. As early as 1981, the prestigious journal Science published:
Effects of prenatal sex hormones on gender-related behavior
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Michael Steinberg
doctorwascher
Paul Hinrichs
Mary Elizabeth Williams
Cary Tennis
Dave Cullen
Salon.com
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