Sunday, March 07, 2004

More than Meets the Eye

Eric Rescorla of Educated Guesswork makes a post about an issue that perturbed me greatly when I first learnt about it while studying biology:

For example, one type of pseudohermaphrodite looks like a normal woman. Indeed, "she" conforms to the male ideal of female pulchritude even more closely than does the average real woman because "her" breasts are well developed and "her " legs are long and graceful. Hence cases have turned up repeatedly of beautiful women fashion models not realizing that they are actually men with a single mutant gene until genetically tested as adults.

Since this type of pseudohermaphrodite looks like a normal girl baby at birth and undergoes externally normal development and puberty, the problem isn't even likely to be recognized until the adolescent "girl" consults a doctor over failure to begin menstruating. At that point, the doctor discovers a simple reason for that failure: the patient has no uterus, fallopian tubes, or upper vagina. Instead, the vagina ends blindly after two inches. Further examination reveals testes that secrete normal testosterone, are programmed by a normal Y chromosome, and are abnormal only for being buried in the groin or labia. In other words, the beautiful model is an otherwise normal male who happens to have a genetically determined biochemical block in his ability to respond to testosterone.

That block turns out to be in the cell receptor that would normally bind testosterone and dihydrotestosterone, thereby enabling those androgens to trigger the further development steps of the normal male. Since the Y chromosome is normal, the testes themselves form normally and produce normal Mullerian inhibiting hormone, which acts as in any man to forestall development of the uterus and fallopian tubes. However, development of the usual male machinery to respond to testosterone is interruped. Hence development of the remaining bipotential embryonic sex organs follows the female channel by default: female rather than male external genitalia, and atrophy of the Wolffian ducts and hence of potential male internal genitalia. In fact, since the testes and adrenal glands secrete small amounts of estrogen that would normally be overridden by androgen receptors, the complete lack of these receptors in functional form (they are present in small numbers in normal women) makes the male pseudohermaphrodite appear externally superfeminine.

It's rather surprising to think that people who are genetically men might actually make for the most beautiful women, isn't it? I have to admit it's something I'm always wondering about whenever I see an exceptionally tall and beautiful woman - could she be a pseudohermaphrodite too? I suspect that the occurrence of pseudohermaphroditism is far too low for me to ever have actually seen anyone who has it though. Still, I think this goes to show that the usual assumption that great physical beauty and high reproductive fitness are equivalent doesn't always hold up under scrutiny.