Just got me wondering. Sexual differentiating is a complicated thing. Not as straight foward as XX or XY, as the existence of freemartins seems to bear out. It’s all about the hormones. So…
If this happenes in cows why doesn’t it happen in humans? Or does it? Are females with male twins more likely to be transexual? sterile? gay? hirsute? I’ve never heard that they were but if the hormonal interaction exists (in cows apparently) why not us? What about other animals?
I went to graduate school in the sexual differentiation of neuroscience. You are correct that genetic sex only starts the ball rolling. Most other physical aspects of sexual differentiation including brain differentiation are actually caused by the sex hormones. This includes everything from the development of the penis or vagina to facial hair later and much more. If this process is interrupted such when genetic XY males have an androgen (male sex hormones like (testosterone and DHT) insensitivity disorder, they won’t respond to their own hormones and will develop as a female (this isn’t all that rare).
There has been lots of research of the type that you describe. One well described phenomenon of uterine position in rats. Female rats that are close to male rats in the uterus show more male typical sexual behaviors than others. This is believed to be because of the masculinizing effect of being in close proximity to male sex hormones during some of the early critical period
“It is well known that female rats developing close to a male in utero show a higher frequency of heterotypical or male-like behavior in adulthood, and have longer anogenital distances.”
I have seen some studies attempting to replicate something similar in humans but I am not aware of any conclusive ones. There are lots of studies of this type with different animals. It isn’t an effect isolated to cows and there is no reason to rule out something like it in humans.
There is also some evidence that birth order and sibling sex ratio may be related to development of homosexuality, which could potentially be due to hormonal effects. The following article provides a review of some of the data:
The reason why Freemartins exist is because there was a fusion of the placenta in utero. Although this condition is not unknown in other creatures, cattle are known to be particularly prone to it for some reason.
Basically, freemartins are chimeras (sites:here, here) Chimeras are individuals composed of cells of more than one zygote. This is either due to exchange of blood or twins (or more) that fused back together. The only medically documented case of true hermaphroditism in humans was an XY/XX chimera. It is impossible to say how many chimeras there are. Most chimeras were discovered due to medical problems - some unrelated to their chimerism (for example, the woman who discovered that the sons she gave birth to were not her own). Without knowing what part of the individual belonged to the “lost” twin, you’d have to test multiple locations.
There is some evidence that hormone imbalances are a cause of transsexuality. I know more than a few trans individuals whose mothers had hormone problems and were on medication during their pregnancies.
In addition to chimerism, there are other conditions which can result in an individual having more than one set of chromosome. Mosaics occur when cells differ in their chromosomes. For example, about half of all individuals with Turner Syndrome are mosaics. Turner’s syndrome is characterized by a missing or damaged X chromosome (XO). Some individuals have Y chromosomes in some of their cells (XY/XO) and are known as xy-Turners. The majority of xy-Turners are normal males, although some are born female and others with ambiguous genitalia. Many of the fetuses convinced with this condition die in utero.
Although rare, mosaicism can also result in identical twins of opposite sex. Blood tests on a set of twins revealed that 99% of their composition was identical. They were so identical, in fact, that skin grafts between the twins reacted as if they were autographs. The boy was revealed to be XY and the girl XO. Somehow, a single mosaic embryo had developed into two individuals of opposite sex.
A less dramatic case occurred among a pair of fraternal male/female twins. A blood test revealed that the male twin had 70 XX and 30 XY cells compared with the female twin who had 78 XX and 22 XY.
I know a family with a Turner’s daughter; first one I’ve ever seen (to my knowledge). The child has lots of medical problems, and has had to deal with many different kinds of medical interventions to keep her as functional as possible. It seems unlikely she’s a chimera, based on what her mother has to say.
She has a very outgoing personality, but limited mentality. Claire never meets a stranger, and is very charming.
Thanks, kimera, everybody. This is all fascinating.
I guess I’m particualrly interested in the transexual question, as so many people are ready to say it’s just some sort of refelection of our decadent culture :rolleyes: when it seems to me very plausible to think it has as much of a physiological explaination as any other gender disorder even if it’s not completely understood.
There is no question that some of it is physiological. You can google “androgen insensitivity disorder” and learn about well studied conditions that leave the body and brain in various combinations of unusual sexual configurations. That doesn’t explain every case but it is a good starting point if you are interested and there are plenty of facts.