American Scientist

Nursery rhymes can be unspeakably violent. Recall, for example, the three unfortunate mice who, as well as being blind, had their tails docked by a sadistic, knife–wielding peasant. The real–life, adults–only version of this tale features a newborn mouse, seemingly perfect in every way except that the front end of its body is oddly truncated, ending in a small mound crowned by two tiny ears. This mouse is certainly blind, since not only does it have no eyes, it also has no head. It looks as cleanly decapitated as if by the farmer's wife of folklore. The mouse was one of four born without a head, as a result of a mutation in a regulatory gene called Lim1. With no mouth or nose to breathe through, [it] died very soon after its birth. More than a hundred other headless mice foetuses did not get as far as being born.

The experiments in which these mice were created were part of an effort to understand the activities of Lim1, one of an increasingly well–documented cadre of regulatory genes whose role it is to ensure that every part of the body develops in the place it should. When such genes are mutated, the result can be as monstrous as anything from nursery folklore.

Jacob’s Ladder: The History of the Human Genome
Henry Gee
W. W. Norton, $25.95