The Gene: An Intimate History - Siddhartha Mukherjee Page 0,20

world, at first glance, suggests the existence of a gene; indeed, you have to perform rather bizarre experimental contortions to uncover the idea of discrete particles of inheritance. Unable to arrive at a theory of heredity via experimental means, Darwin was forced to conjure one up from purely theoretical grounds. He struggled with the concept for nearly two years, driving himself to the brink of a mental breakdown, before he thought he had stumbled on an adequate theory. Darwin imagined that the cells of all organisms produce minute particles containing hereditary information—gemmules, he called them. These gemmules circulate in the parent’s body. When an animal or plant reaches its reproductive age, the information in the gemmules is transmitted to germ cells (sperm and egg). Thus, the information about a body’s “state” is transmitted from parents to offspring during conception. As with Pythagoras, in Darwin’s model, every organism carried information to build organs and structures in miniaturized form—except in Darwin’s case, the information was decentralized. An organism was built by parliamentary ballot. Gemmules secreted by the hand carried the instructions to manufacture a new hand; gemmules dispersed by the ear transmitted the code to build a new ear.

How were these gemmular instructions from a father and a mother applied to a developing fetus? Here, Darwin reverted to an old idea: the instructions from the male and female simply met in the embryo and blended together like paints or colors. This notion—blending inheritance—was already familiar to most biologists: it was a restatement of Aristotle’s theory of mixing between male and female characters. Darwin had, it seemed, achieved yet another marvelous synthesis between opposing poles of biology. He had melded the Pythagorean homunculus (gemmules) with the Aristotelian notion of message and mixture (blending) into a new theory of heredity.

Darwin dubbed his theory pangenesis—“genesis from everything” (since all organs contributed gemmules). In 1867, nearly a decade after the publication of Origin, he began to complete a new manuscript, The Variation of Animals and Plants Under Domestication, in which he would fully explicate this view of inheritance. “It is a rash and crude hypothesis,” Darwin confessed, “but it has been a considerable relief to my mind.” He wrote to his friend Asa Gray, “Pangenesis will be called a mad dream, but at the bottom of my own mind, I think it contains a great truth.”

Darwin’s “considerable relief” could not have been long-lived; he would soon be awoken from his “mad dream.” That summer, while Variation was being compiled into its book form, a review of his earlier book, Origin, appeared in the North British Review. Buried in the text of that review was the most powerful argument against pangenesis that Darwin would encounter in his lifetime.

The author of the review was an unlikely critic of Darwin’s work: a mathematician-engineer and inventor from Edinburgh named Fleeming Jenkin, who had rarely written about biology. Brilliant and abrasive, Jenkin had diverse interests in linguistics, electronics, mechanics, arithmetic, physics, chemistry, and economics. He read widely and profusely—Dickens, Dumas, Austen, Eliot, Newton, Malthus, Lamarck. Having chanced upon Darwin’s book, Jenkin read it thoroughly, worked swiftly through the implications, and immediately found a fatal flaw in the argument.

Jenkin’s central problem with Darwin was this: if hereditary traits kept “blending” with each other in every generation, then what would keep any variation from being diluted out immediately by interbreeding? “The [variant] will be swamped by the numbers,” Jenkin wrote, “and after a few generations its peculiarity will be obliterated.” As an example—colored deeply by the casual racism of his era—Jenkin concocted a story: “Suppose a white man to have been wrecked on an island inhabited by negroes. . . . Our shipwrecked hero would probably become king; he would kill a great many blacks in the struggle for existence; he would have a great many wives and children.”

But if genes blended with each other, then Jenkin’s “white man” was fundamentally doomed—at least in a genetic sense. His children—from black wives—would presumably inherit half his genetic essence. His grandchildren would inherit a quarter; his great-grandchildren, an eighth; his great-great-grandchildren, one-sixteenth, and so forth—until his genetic essence had been diluted, in just a few generations, into complete oblivion. Even if “white genes” were the most superior—the “fittest,” to use Darwin’s terminology—nothing would protect them from the inevitable decay caused by blending. In the end, the lone white king of the island would vanish from its genetic history—even though he had fathered more children than any other man of his generation, and even though his