Appropriately enough, considering some recent conversations here, AlDaily links to an interesting article in New York Magazine on the biology of human sexuality. The concept of natural variation was apparently quite the shock to the field:

"The brain was considered pretty hardwired," says Roger Gorski, a neurobiologist at UCLA who researches sexual differentiation. "It was male or female, period. Then Simon's study shows that there could be intermediates. That wasn't just a watershed-it pushed the water over the waterfall."

The study being alluded to in the quote apparently showed that the size of a particular part of the brain in gay men tends to be smaller-- closer to the female mode than that of the heterosexual male.

One interesting tidbit:

A large-scale study within the next year is expected to determine more conclusively if a gene (or genes) is linked to sexual orientation. Alan R. Sanders, a psychiatrist from Northwestern University, is enrolling 1,000 pairs of gay brothers in one of the largest sexual-orientation studies ever undertaken. With the experiment, funded by an NIH grant of over $1 million, Sanders will attempt to map genes that influence sexual orientation.

Sexual orientation is obviously "genetic", in the sense that without brains (constructed by genetically controlled developmental pathways) it wouldn't exist. But if there is genetic variation that predisposes one to homosexuality, it would likely be eliminated fairly quickly from the population (assuming this variation has no effects on any other phenotype, a likely false assumption). However, if the mutational target size is large enough (ie. if slight alterations in any of 100 different pathways could lead to slights shifts in preference), mutation-selection balance might lead to a much higher rate than what one might expect.

Then again, the MZ twin concordance rate is about 50% (according to this article), which mean both environmental (probably intra-uterine) and stochastic forces are also rather important. I find the association between hair whorls/handedness and homosexuality interesting; both those traits are hypothesized to be influenced by developmental stochasticity.

FYI, here's one current model (likely wrong or a major simplification, but as they say: all models are wrong, but some are useful) for the genetics of handedness:

On the basis of these findings, combined with other results (KLAR 1996), I hypothesized that a single dominant gene, RGHT1 (for right-handedness), causes the development of both RH preference and clockwise hair whorls and that both traits are randomly distributed to the left and the right side in individuals homozygous for the nonfunctional, recessive r (for random-handedness) allele. Handedness discordance of monozygotic twins was also attributed to randomness resulting from the deduced r/r genotype. Thus, genetic etiology for handedness was supported for the unselected human population by results of both hair-whorl correlation and familial inheritance of the handedness trait.

Labels: Genetics