In the comments here, rosko points me to a study on the effects on MC4R, a gene implicated in natural variation in human weight, on pathways involved in sexual function. It's well known, of course, that genetic pathways can be involved in multiple physiological processes--in particular, signaling pathway can generate many different phenotypes depending on what the downstream target of the signal is.

The effects of MC4R simulation in humans are, as rosko comments, kind of interesting:

Methods. Ten subjects were enrolled in a double-blind, placebo-controlled, crossover study. Melanotan II (0.025 mg/kg) and vehicle were each administered twice by subcutaneous injection; real-time RigiScan monitoring and a visual analog were used to quantify the erections during a 6-hour period. The level of sexual desire and side effects were recorded with a questionnaire.

Results. Melanotan II initiated subjectively reported erections in 12 of 19 injections versus only 1 of 21 doses of placebo. The mean rigidity score of the responders was 6.9 on a scale of 0 to 10. The mean duration of tip rigidity greater than 80% was 45.3 minutes with Melanotan II versus 1.9 for placebo (P = 0.047). The level of sexual desire after injection was significantly higher after Melanotan II administration than after placebo. Nausea and stretching/yawning occurred more frequently with Melanotan II, and 4 of 19 injections were associated with severe nausea.

I wondered what a "Rigiscan" is--find out here. Hypothetically, one could test whether natural variation in sexual behavior in humans is also affected by MC4R polymorphism, though I can't imagine that being a particularly fun study to carry out (one for agnostic's new series? 23andme + free time = association studies about erections).

This reminds of the MC1R story about increased pain sensitivity in redheads in the vague sense that both involve melanocortin receptors and pleiotropy.

Labels: Genetics, Pigmentation