There's a great piece

in Edge, an interview with Armand-Marie Leroi about "the nature of normal human variety." His book Mutants is a must-read. Here's a taste of the interview (he also touches on the question of why certain kinds of human difference--especially racial difference--have become taboo for geneticists--it's a really thoughtful and interesting discussion):

If you go to teratology museums—literally "monstrosity museums"—in places such as Amsterdam and Philadelphia, you can see rows of babies in bottles. These infants, usually stillborn, are deformed in ways that are truly hideous, that really represent the kinds of monstrosities that you might expect from Greek myth. I mean this quite literally. They include children born with a single eye in the middle of their forehead, and who look exactly like the monsters of Greek myth—Polyphemus in The Odyssey, for example. Indeed, it's sometimes suggested that the monsters of Greek myth were inspired by deformed children, and this seems to be a fairly remarkable correspondence, at least with some of them.

These infants, when you see them, are truly horrific. But very quickly, after you look at them, a sort of intellectual fascination takes over because it's clear that these children tell us something very deep about how the human body is built. Take, for instance, these children with a single eye in the middle of their foreheads. The syndrome is called, appropriately, Cyclopia. Cyclopia is caused by a deficiency in a gene called Sonic hedgehog. Sonic hedgehog is named after a fruit fly gene which when mutated causes bristles to sprout all over the fruit fly larva, hence "hedgehog". When the gene was found in mammals, some wit called it Sonic hedgehog after the video game character. If you get rid of this gene, bad things happen. You lose your arms beneath the elbow and legs beneath the knee. The face collapses in on itself, such that you get a single eye in the middle of the forehead and the rest of the face collapses into a long, trunk-like proboscis. The forebrain, which is normally divided such that we have a left and a right brain—the left and right cerebral hemispheres—is fused into a single unitary structure. Indeed the technical name for this syndrome is called Holoprosencephaly.

Now all this is very horrible, and actually that's just an initial list of things that can go wrong in infants that have no Sonic hedgehog. But what's really interesting about it is that by looking at infants of this sort you can reverse-engineer and ask what Sonic hedgehog does in the embryo. Instantly it tells you that one of the things that Sonic hedgehog does is to keep our eyes apart because if you don't have the gene the face collapses. It also separates the left and and right sides of our brains. And it's needed for the formation of our arms and legs. In fact, it is one of the most ubiquitous and powerful molecules in the making of our bodies.

(Link via Arts & Letters Daily.)