TsBiology

To many lay people, the idea that something as seemingly basic as sexual and gender identity can be so complicated is often disturbing. And scientists are far from being able to explain why there can be such variation. But they do have some clues.
In a basic sense, sexual fate is set at the moment of fertilization by the incoming sperm. The sperm carries either a Y chromosome, which determines maleness, or an X chromosome, said David Page, a geneticist at the Whitehead Institute in Cambridge. (The egg always contains one X chromosome, resulting in XY chromosomes for boys and XX for girls.)

At this stage, the embryo has both male and female internal structures. One of these - Mullerian ducts - has the potential to become fallopian tubes and a uterus; the other - Wolffian ducts - has the potential to become the sperm-making machinery and tubes to carry sperm. Early embryos also contain primitive gonads that are capable of turning into either ovaries, which make eggs, or testes, which make sperm.

"But, at about seven weeks, the embryo sort of takes stock of whether it got that Y chromosome seven weeks back at the moment of fertilization,"
Page said. On the Y chromosome lies a key gene called SRY, which stimulates the primitive gonads to become testes. If the SRY gene is not present, the gonads become ovaries.

Once testes form, they begin pumping out the male hormone, testosterone, which causes the Wolffian ducts to become the sperm production and transport system. The testes also pump out a chemical called MIS that causes the Mullerian ducts to shrivel up, so they cannot form fallopian tubes and the uterus.

As fetal development continues, male and female hormones then imprint the brain, nudging it toward masculinization or feminization.

One of the things we believe is that it is more common for men to become female in transgender change than for females to become men." said Dr. Marshall Forstein, medical director of Fenway Community Health in Boston.

"If something goes wrong in (the fetal development) process, or something is variable in that process, some of the brains of those men don't become masculinized at the appropriate development time: Their brains are female, even though their bodies are male."

So far however, the idea that incomplete brain imprinting causes transgenderism is merely a hypotheses, and it makes some transgendered people like Nancy Nangerini and Joan Roughgarden, a Stanford University biologist who how lives as a woman but was born male, cringe because it suggest that there might be something wrong with them. They also strongly dispute that men are more likely to be transgendered than women.

Medical people are the worst," Roughgarden said. "They don't know any biology or zoology. They just superimpose their preconceptions on the data. They try to construct a norm and pathologize all states that differ from the norm."