Nature is a DJ
Jack Molay takes a look at the current state of biological research and what it can mean for the transgendered.
In my post on What brain science says about M2F transsexuals I presented some recent research on various nuclei (brain sectors) that may or may not govern sexual orientation and gender identification.
I believe it could be useful to have a popular summary of what contemporary science says about genes, hormones, sex and gender.
These sex difference genes can be dichotomous (which is a fancy word for an on/off switch) or they can be continuous, which large variation between two extremes. The combination of all the sex limited genes forms the biological basis of the sex identifying aspects of a personality.
The traditional narrative these days is that the male Y chromosome has a gene complex called TDF (the Testes Determining Factor). The TDF has a gene called SRY (Sex Determining Region of the Y-chromosome. Duh!).
All embryos -- male or female -- have the genes that encode the blueprint for both male testes and female ovaries. If the TDF is not there, the genes that encode the blueprint for the ovaries will turn on instead. You can say that the female bodyplan is the default. We are all girls before the TDF swings into action, according to this story.
Joan Roughgarden (who I presented in my Sex, Gender, Nature series) argues that this narrative is too simple, and that the SRY can only partly influence gendered presentation.
"The idea is in instead of having a simplistic mechanism by which you have pro-male genes going all the way to make a male, in fact there is a solid balance between pro-male genes and anti-male genes and if there is a little too much of anti-male genes, there may be a female born and if there is a little too much of pro-male genes then there will be a male born. 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."
The hormonal balance produced by the testes or the ovaries is not dichotomous (an on/off variable). The amount of hormones will vary from fetus to fetus, and that variation will influence the development of the fetal brain and the "sex mix" of that individual. It seems the hypothalamus is organized differently in males and females.
The hypothalamus controls the pituitary gland, which again controls hormone production in other glands. Because of this fetal brain development may also influence the future production of sex hormones in the body, and through this also future sex differences.
But that is not all: The hypothalamus also govern various natural drives, including hunger, thirst and sex drives. Different aspects of the sex drive (like sexual orientation and sex drive intensity) are regulated by different nuclei (sectors) of the hypothalamus. Most researcher believes that the basis for gender identity is also found here.
Other parts of the brain may also respond to sex hormones, and some researchers believe these are relevant to the development of brain lateralety, visuospatial learning, spatial memory, aggression, motor activity, exploratory activity etc, which all are associated with typically male and female abilities according to this line of research.
Studies of birds and rats show that their gender behavior may be changed by giving them hormones or by removing certain parts of their brain.