The Violinist's Thumb: And Other Lost Tales of Love, War, and Genius, as Written by Our Genetic Code by Sam Kean Read Free Book Online Page A

chromosomes on to their children. All the genetic traits on each chromosome should therefore always be inherited together—they should always be linked. To take a hypothetical example, if one chromosome’s set of genes call for green bristles and sawtooth wings and fat antennae, any fly with one trait should exhibit all three. Such clusters of traits do exist in flies, but to their dismay, Morgan’s team discovered that certain linked traits could sometimes become unlinked—green bristles and sawtooth wings, which should always appear together, would somehow show up separately, in different flies. Unlinkings weren’t common—linked traits might separate 2 percent of the time, or 4 percent—but they were so persistent they might have undone the entire theory, if Morgan hadn’t indulged in a rare flight of fancy.
    He remembered reading a paper by a Belgian biologist-priest who had used a microscope to study how sperm and eggs form. One key fact of biology—it comes up over and over—is that all chromosomes come in pairs, pairs of nearly identical twins. (Humans have forty-six chromosomes, arranged in twenty-three pairs.) When sperm and eggs form, these near-twin chromosomes all line up in the middle of the parent cell. During division one twin gets pulled one way, the other the other way, and two separate cells are born.
    However, the priest-biologist noticed that, just before thedivvying up, twin chromosomes sometimes interacted, coiling their tips around each other. He didn’t know why. Morgan suggested that perhaps the tips broke off during this crossing over and swapped places. This explained why linked traits sometimes separated: the chromosome had broken somewhere between the two genes, dislocating them. What’s more, Morgan speculated—he was on a roll—that traits separating 4 percent of the time probably sat farther apart on chromosomes than those separating 2 percent of the time, since the extra distance between the first pair would make breaking along that stretch more likely.
    Morgan’s shrewd guess turned out correct, and with Sturtevant and Bridges adding their own insights over the next few years, the fly boys began to sketch out a new model of heredity—the model that made Morgan’s team so historically important. It said that all traits were controlled by genes, and that these genes resided on chromosomes in fixed spots, strung along like pearls on a necklace. Because creatures inherit one copy of each chromosome from each parent, chromosomes therefore pass genetic traits from parent to child. Crossing over (and mutation) changes chromosomes a little, which helps make each creature unique. Nevertheless chromosomes (and genes) stay mostly intact, which explains why traits run in families. Voilà: the first overarching sense of how heredity works.
    In truth, little of this theory originated in Morgan’s lab, as biologists worldwide had discovered various pieces. But Morgan’s team finally linked these vaguely connected ideas, and fruit flies provided overwhelming experimental proof. No one could deny that sex chromosome linkage occurred, for instance, when Morgan had ten thousand mutants buzzing on a shelf, nary a female among them.
    Of course, while Morgan won acclaim for uniting these theories, he’d done nothing to reconcile them with Darwinian natural selection. That reconciliation also arose from work insidethe fly room, but once again Morgan ended up “borrowing” the idea from assistants, including one who didn’t accept this as docilely as Bridges and Sturtevant did.
    Hermann Muller began poking around the fly room in 1910, though only occasionally. Because he supported his elderly mother, Muller lived a haphazard life, working as a factotum in hotels and banks, tutoring immigrants in English at night, bolting down sandwiches on the subway between jobs. Somehow Muller found time to befriend writer Theodore Dreiser in Greenwich Village, immerse himself in socialist politics, and