Genetics: Multiplying by Four

As its name suggests, the nine-banded armadillo is encased in nine bands of armorlike plate. When alarmed, it curls into a nearly impenetrable ball or jumps straight up into the air. It spends much of its time burrowing holes in the earth; after swallowing air to increase its buoyancy, it is at home in the water too. But the strange beast has an even more remarkable characteristic: it normally gives birth only to quadruplets.

Its aptitude for multiple delivery was what brought the armadillo to the attention of University of Texas Biochemist Roger...