Every time I read or hear the "...the hormonal imbalance in the mothers womb" BS I feel like screaming, ARGH!!
I am an expert in genetics, I actually earned a living doing the research stuff, so do listen to me if you want a summary of what science knows now. I stopped posting on the subject because as incredible as it may sound some of the people in this forum refuse to believe what is so, and rather forge ago with some mickey mouse explanations on the cause of gender identity disorder.
First disclaimer is that most of what we currently know has come in the last 10 yeas,r and thus the story is far from complete. Second disclaimer s that in order to make it more understandable, I am lumping animal studies with human studies.
First we need to separate sexual orientation from gender orientation. I am going to talk only about gender identity.
The BST size (male bigger, smallest female) is determined before week 9 in uterus by the presence or absence and amounts of Testosterone (T) and Estrogen (E), by the presence or absence of steroid (T&E) receptors in the surface of the embryonic BST cells, by the presence or absence of the steroid bind protein alpha feto protein 1 (aF1), and by the presence or absence of a protein (aromatase) inside the embryonic BST cells. With this information on hand, then this is what happens to generate either a female or a male BST:
Let's start by the simple one. If you are an XX individual and you have normal levels of aF1, then the local levels of both T and E are nearly zero. In the absence of T&E the embryonic BST cell do not divide and grown and after week 8 your BST is female. Nothing after that can change the BST. But if there is not enough aF1 then you will be a FTM, a male in a female body
For the complicated male pathway several thing have to happens simultaneously: there has to be high levels of T&E and little or none of aF1, there have to be receptors for T&E on the surface of th embryonic BST cells so that T&E can be brought inside the embryonic BST cells, once inside the BST cell the T has to be converted into E (yes, into E!) by the action of Aromatase enzyme; then E goes into the nucleus of the embryonic BST cell and induces them to divide and grow to become a male BST. Failure to express a fully functional aromatase protein, or to have receptors for T&E, or low level of T&E, and the BST will be female, you get a MTF, or female person in a a male body.
So, finally, for the genetics. Mutations on the T&E receptor genes, or the AF1 gene, or the aromatase gene, they all would result in individuals who are transgender. The aromatase gens has indeed been found to be mutated in MTF. It seems that gender idenity is not a simply trait governed by a single gene but rather a poly-genetic trait, thus the genetics of it is going to be complicated to elucidate. There anecdotal evidence in the medical literature that indicates that indeed transgender like homosexuality does runs in some families, but much work remains.
Love,
Kate D, MD PhD
