Genetics: Multiplying by Four

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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 Williams. The tough, armor-plated animal offered him a chance to check the theory that there is something in a fertilized egg cell besides genes that influences an animal's inherited characteristics.

Unequal Blood. Because armadillo quads develop from the same fertilized egg, they have identical genes. Thus, according to accepted theory, each inherits the same characteristics from its parents. Any differences between "identical" quads, or triplets or twins—in armadillos or any other animal, including man—have long been explained away as the result of differences in environment. But there is a growing suspicion that there are other influences. And that suspicion was strengthened by the recent discovery that deoxyribonucleic acid (DNA), the molecule that contains the coded message of heredity, exists outside of the genes.

To find out just how identical armadillo quads really are, Williams and Graduate Student Eleanor Storrs killed 16 sets of newborn quadruplets, thus eliminating any environmental differences that might have developed as the animals grew older. Then the two scientists analyzed 20 different features of each animal. Among the "identical" armadillos in each set, they report in the Proceedings of the National Academy of Sciences, there were many variations too great to have been caused by any pre-birth conditions such as different positions in the uterus or unequal supplies of blood from the mother.

Unsymmetrical Arrangement. In individual sets of quadruplets, some animals had hearts, or spleens, or adrenal glands as much as twice as large as those of their siblings. Analysis of organs showed a great disparity in the acids and other chemicals they contained. One baby armadillo, for example, had in its adrenal glands 140 times as much norepinephrine (a hormone that constricts blood vessels and maintains blood pressure) as could be found in the adrenals of an identical brother.

Having ruled out both environmental and genetic influences, Williams can only suggest that there are "unknown factors" that affect heredity. Although the gene-bearing chromosomes duplicate themselves precisely in each new cell nucleus after the division of a fertilized egg cell, the little-understood structures and particles in the cytoplasm of the egg cell are not symmetrically arranged. During division, Williams suggests, one new cell may not receive the same quantity of the unknown factors as another, and the result is marked differences between the four armadillos—or between identical human twins—that eventually form from the same egg.