THE QUANTUM PHYSICIST Richard Feynman once gave a lecture on color vision in Caltech’s Beckman Auditorium. He explained the molecular events that take place in the human eye and brain to show us red, yellow, green, violet, indigo, and blue. This chain of reactions was one of the early discoveries of molecular biology, and it fascinated Feynman. “Yeah,” someone in the audience said, “but what is really happening in the mind when you see the color red?” And Feynman replied, “We scientists have a way of dealing with such problems. We ignore them, temporarily.”
That line still makes Benzer smile in the middle of the night, which is the middle of his working day. He often repeats it to the postdoctoral students in his laboratory. “We ignore them, pause, temporarily. I thought that was a wonderful statement,” he says. “You know, we do that all the time. The problem you have an instinct for, a feeling for, may be a problem that you’re not going to be able to solve, and you sort of shy away from it, temporarily. The philosophers, of course, don’t ignore them. Maybe they have a penchant for unsolvable problems. If it’s solvable, it’s not really interesting.”
The problems that Benzer and his students are solving are problems that scientists and philosophers from the beginning of recorded history have been unable to solve and also unable to ignore. In the fifth century B.C. Hippocrates, the great-grandfather of modern medicine, dropped in on Democritus, the great-grandfather of atomic physics. He found Democritus sitting in a garden with the bodies of dead beasts cut open all around him. Democritus was sleeping and waking over his notes, trying to find the cause of melancholy, so that he could cure it in himself and teach others to cure it too. It was a first attempt at the dissection of behavior, approximately twenty-five centuries too soon.
In the second century A.D. the Greek physician Galen reported that he and a few of his friends had delivered a young goat by cesarean section “so that it would never see the one who bore it.” They took the kid from its mother’s womb and placed it in a room in which there were bowls of wine, oil, honey, milk, grains, and fruits. “We observed that kid take its first steps as if it were hearing that it had legs,” Galen wrote; “then, it shook off the moisture from its mother; the third thing it did was to scratch its side with its foot; next we saw it sniff each of the bowls in the room, and then from among all of these, it smelled the milk and lapped it up. And with this everyone gave a yell, seeing realized what Hippocrates had said: ‘The natures of animals are untutored.’ ”
Hippocrates and Galen tried to relate human temperaments to the elements: fire, air, earth, and water. The word temperament comes from the Latin temperare, “to mix”; in Galen’s scheme every human being is a mix of those elements. Astrologers tried to relate temperaments to the stars, looking for connections between the two infinites, the skies over our heads and the skies inside our heads. Some of the symbols on fly bottles are astrological. Virgo is one of the twelve signs of the zodiac, part letter, part hieroglyph, part Hebrew, part Phoenician, and on a fly bottle it means what it has always meant, Virgin. The circle with the pendant cross, meaning Female, was once the sign for the planet Venus, with connotations of fertility. The circle with the angry arrow, meaning Male, was once the sign for the planet Mars, with connotations of calamity.
“What a wonderful thing it is that that drop of seed from which we are produced bears in itself the impressions, not only of the bodily shape, but of the thoughts and inclinations of our fathers!” Montaigne wrote in the sixteenth century. “Where can that drop of fluid harbor such an infinite number of forms? And how do they convey those resemblances, so heedless and irregular in their progress, that the great-grandson shall be like his great-grandfather, the nephew like his uncle?” The questions were just as unanswerable in Montaigne’s century as in Galen’s.
Shakespeare seems to have been the first to use the words “nature” and “nurture” in brooding about these mysteries. In his last play, The Tempest, which he completed in 1612, Prospero (the character who comes closest to a self-portrait in any of Shakespeare’s plays; the archetype of all artists, scientists, and philosophers) complains about his adopted son, Caliban:
A devil, a born devil, on whose nature
Nurture can never stick; on whom my pains
Humanely taken, all, all lost, quite lost.
In one way or another, the paradoxes of nature and nurture fascinated every poet, every playwright, and every pair of parents from the first. Abel was a keeper of sheep, but Cain was a tiller of the ground. Yet they both sprang from Adam and Eve. Esau was a cunning hunter, a man of the field; but Jacob was a plain man, dwelling in tents. Esau was also a hairy man, but Jacob was a smooth man. Yet Jacob followed Esau out of the womb with his hand on Esau’s heel.
In the late 1830s, when Charles Darwin first began scribbling a theory of evolution, his handwriting in his secret notebooks reminded him of the handwriting of his old radical grandfather Erasmus, who had published a theory of evolution the century before. Darwin wondered if he was looking at an “instance of hereditary mind.” His cousin Francis Galton later closed his copy of the Origin wondering if he had just devoured the book because of a “bent of mind that both its illustrious author and myself have inherited from our common grandfather, Dr. Erasmus Darwin.” Darwin and Galton each spent decades compiling examples and anecdotes of what they vaguely called the power of inheritance—vaguely, because even though evolution depended on it, they had no idea how the power of inheritance actually works. They never got much closer to that secret than Galen had, although Darwin’s theory framed the question for those who came after him like the framing of a doorway.
Until the twentieth century, the passage of time made less difference with this problem than with almost any other basic problem in the study of life. Every argument about nature and nurture was like the mutant fly pirouette, spiraling inward until it died. Every new system of thought that claimed to solve the problem floated apart from every other, cut off from the rest of human knowledge like the brain in Benzer’s workroom, with its severed cord drifting in formaldehyde. The problem was unsolvable before the discovery of the gene.
THAT DISCOVERY was made by a monk in what is now Brno, in the Czech Republic, in a report on garden peas. Gregor Mendel tended a monastery garden and an experimental greenhouse in the 1850s. While crossing different strains of peas two by two—dusting the flowers of one strain with the pollen of the other—Mendel got a clearer view of the patterns of inheritance than anyone before him. The strains he crossed had smooth peas or wrinkled peas, yellow peas or green; their plant stems were tall or short; and so on: seven pairs of contrasting strains. When Mendel crossed these strains, the traits did not blend together but passed on intact, often skipping several generations. This experiment was so simple that it could have been done by Hippocrates or Democritus, and what made it revolutionary was its demonstration that inheritance comes in something like the Greek idea of atoms. Tallness never blended with shortness. They do blend in humans, but they don’t blend in peas—one reason Mendel was lucky to work with peas. There the patterns stayed crisp and clear. The traits stayed separate generation after generation, and Mendel assumed that they must be governed by separate factors. Much later, biologists who reread Mendel’s paper would think of these factors, with a nod to physics and chemistry, as particles of inheritance. No one knows how the monk pictured them, although he, like Benzer, had been trained as a physicist.
Thirty years ago, Benzer and another founder of molecular biology, Gunther Stent, climbed Mount Fuji, a climb that Benzer still remembers fondly because on it he tasted his first mandarin orange. They stopped for the night in a Buddhist temple halfway up the volcano. The temple’s caretaker was an ancient woman, who asked Benzer and Stent, through their interpreter, what they did. As they began to explain, she said, “Ah, Mendel.”
While Mendel planted peas, Francis Galton, Darwin’s cousin, caught glimpses of the particles of inheritance in human beings. Reading the Origin had inspired Galton to send out hundreds of letters and questionnaires to friends, acquaintances, acquaintances of acquaintances, asking about family resemblances and especially about twins. Galton spent the rest of his life collecting such data and inventing new statistical tools to analyze the patterns. Along the way he sent Darwin an example of behavior that skipped generations:
A gentleman of considerable position was found by his wife to have the curious trick, when he lay fast asleep on his back in bed, of raising his right arm slowly in front of his face, up to his forehead, and then dropping it with a jerk, so that the wrist fell heavily on the bridge of his nose. The trick did not occur every night, but occasionally, and was independent of any ascertained cause. Sometimes it was repeated incessantly for an hour or more. The gentleman’s nose was prominent, and its bridge often became sore from the blows which it received.…
Many years after his death, his son married a lady who had never heard of the family incident. She, however, observed precisely the same peculiarity in her husband; but his nose, from not being particularly prominent, has never as yet suffered from the blows.…
One of his children, a girl, has inherited the same trick.
“We seem to inherit bit by bit,” Galton concluded in 1889 in his book Natural Inheritance. That was the only way Galton could interpret the peculiar persistence of bits of family resemblance and bits of behavior. But the patterns of inheritance in his data were not as clean and clear-cut as they were in Mendel’s experiments; and neither Galton nor Darwin ever read his paper, which was published in the journal of Mendel’s local natural history society in 1866.
No one realized what Mendel’s paper might mean for the inheritance problem until a botanist cited it in a paper in January 1900. By then the time was right, and two other citations followed that same year. All three papers attracted attention, although the existence of atomic particles was still considered speculative, and so was the existence of Mendel’s particles of inheritance. One of the biologists who read the new papers closely but skeptically was Thomas Hunt Morgan, born in Lexington, Kentucky, in 1866, the same year that Mendel had published his paper.
In the fall of 1907, Morgan, then a professor of zoology at Columbia University, told one of his students to put a few bananas on the ledge of his laboratory window to attract some fruit flies. Neither teacher nor student was thinking about Mendel at the time; the student wanted to breed animals in the dark and see if they would lose their instinct to go toward light. Morgan told him to use fruit flies because his laboratory in Schermerhorn Hall was cramped, only about sixteen by twenty-three feet. It was cramped and crowded because Morgan, who had been trained as an old-fashioned naturalist, already took pleasure in working with pigeons, chickens, starfish, rats, and yellow mice.
So Morgan’s student Fernandus Payne trapped some flies, put them in darkness, and let them breed. Within a short time he thought he could detect a change. The tenth generation seemed to move toward the light a little more slowly than the first. This student project, which Payne wrote up in a paper, “Forty-nine Generations in the Dark,” was soon lost in the late prehistory of genetics. Benzer knew nothing about it in 1966 when he built his countercurrent machine.
Next Morgan decided to see if he could force fruit flies to change faster. Morgan had money, but he was a miser in the laboratory, which was another reason he liked working with flies. For microscope lamps, he used ordinary lightbulbs with shades that he and Payne cut out of tin cans. For fly bottles, according to legend, he and his students stole empty half-pints from milk boxes on Manhattan stoops on their early-morning walks to the lab, and they lifted more from the Columbia student cafeteria. This was the milk-bottle tradition that Benzer would inherit.
Morgan subjected his flies to heat, cold, and X rays, trying to create a fly that looked different in some way from all the other flies. He also injected the flies’ private parts with acids, bases, salts, sugars, and alcohol. Beneath the hand lens each fly had the same six legs, the same veined and cross-veined wings, the same brilliant red eyes; but Morgan kept watching and waiting for a mutant. And this was the enterprise that Benzer would inherit, although Benzer would manipulate his flies with more sophistication, and he would watch for changes not in their bodies but in their behavior.
Early in the fall of 1909, Morgan began trying to speed up the evolution of his flies with a new approach. He focused on a dark pattern on the thorax of the fly, a variable pattern in the shape of a trident. Week after week he bred only those flies with the most variable tridents and waited to see if the pressure of this artificial selection would somehow set off an explosion of mutations in one of his fly bottles. Week after week, he and Payne saw nothing but ordinary tridents on ordinary flies.
“There’s two years’ work wasted,” Morgan told a visitor in the first days of January 1910, waving at shelves of stolen milk bottles. But just a few days later, Morgan found a fly with a trident that was slightly darker and more sharply defined than before. Then he found a fly that had a dark blotch where the wing met the thorax. Then, after all those tens of thousands of more or less identical red-eyed flies, he found a single fly with white eyes.
Morgan’s wife, Lilian, who was fascinated by his work and who later (after their children were out of the house and busy in school) made important contributions in the laboratory, was pregnant that year; and long afterward the birth of the new baby became mingled in the family history with the arrival of the mutant. Lilian loved to recall the scene when Morgan walked into her hospital room.
“Well, how is the white-eyed fly?” she asked. According to family lore, he was carrying the fly home at night to sleep in a jar next to their bed.
Morgan told her the fly looked feeble but it was hanging on. “And how is the baby?”
Within a week, one of their two new arrivals was old enough to breed (still another reason to work with flies). Morgan paired the white-eyed fly, which was a male, with normal virgin female flies, and together they produced 1,237 young flies. The flies’ children (as Morgan called them) had red eyes. The next week, Morgan arranged marriages for the children. He was fascinated to see that among the grandchildren, although all of the females had red eyes, about one in two of the males had white eyes. Naturally Morgan thought of Mendel’s peas. When Mendel crossed short peas with tall peas, the first generation was all tall, and in the next generation three quarters of the plants were tall and one quarter was short. Shortness in Mendel’s pea plants is what is now known as a recessive trait, like blue eyes among human beings. Morgan wondered if white eyes among male fruit flies could be a recessive trait too.
As one latter-day drosophilist likes to say now, “In the beginning there was white.” The mutant fly white was the point of entry through which Morgan would establish the modern theory of the gene, the atomic theory of inheritance. In much the same way, decades later, the arrival of the first clock mutant, a fly without a sense of time, would open the atomic theory of behavior. That mutant would give Benzer a point of entry through which he and his students could begin the remarkable series of experiments in which they took basic instincts apart and put them back together, like clock makers who had unscrewed the back of a clock.
The arrival of white intrigued Morgan because ever since 1900, biologists had been looking for Mendel’s elements. Through microscopes they could see tiny threads called chromosomes in the nucleus of every egg. They could also see that when a spermatozoan enters an egg, it contributes a matching set of threads. To many biologists, this looked like a physical explanation for Mendel’s results in his pea garden. A pea plant might inherit a tallness factor on one chromosome, for example, and a shortness factor on the other chromosome in that particular pair. The logic seemed compelling: Mendel saw traits in pairs; microscopists saw chromosomes in pairs. And when a young body begins making new eggs or new sperm, each egg cell and sperm cell receives only one chromosome from each chromosome pair. That way, when sperm finds egg, the single chromosomes can meet inside the fertilized egg and the whole process can start over to make a new life.
All of this fit Mendel’s observations. But Morgan was a contrarian. He used to complain that his best friends at Columbia were “wild over chromosomes.” It did look as if chromosomes were “the thing,” he said, but he wanted hard evidence: “I cannot but fear that we are rapidly developing a sort of Mendelian ritual by which to explain the extraordinary facts.” Morgan’s “show-me” attitude was about to lead him into the simple line of experiments that started the revolution in twentieth-century biology. It was the same attitude that Benzer would later bring to the study of genes and behavior in his own Fly Room. Feynman, the quantum physicist, once knocked on the door of Benzer’s workroom and asked him to show his son a fly’s brain. Benzer sat the boy at a microscope and told him, “There’re a hundred thousand transistors in that brain.” Benzer’s own work as a physicist had helped lead to the invention of the transistor. By comparing the fly’s one hundred thousand neurons to transistors, Benzer was trying to convey an idea of the fly brain’s magnificent miniaturization. At the same time Benzer was also nodding to the father over the son’s head, physicist to physicist.
But Feynman said, “No, no. Tell it straight. They’re not transistors, they’re neurons. Don’t oversimplify.”
Benzer liked that line too. Feynman was right. A neuron is actually a much more complicated object than a transistor, and the path from gene to neuron and from neuron to behavior is longer and more mysterious than the path from an electron to a radio or a computer. Benzer shared with Feynman an aggressively simple and direct style of talking—a trait both men also shared with T. H. Morgan. Benzer and Feynman came from families of New York Jews, Eastern European immigrants; Morgan came from an old family of Kentucky aristocrats. But all three spoke in the kind of down-home, common-as-flies style that is the lingua franca of great scientists, conveying a contempt for pretension, a contempt for cant, a delight in common sense, combined with uncommon curiosity about what is really there.
Sitting in his first Fly Room in Columbia’s Schermerhorn Hall, T. H. Morgan could see through his microscope that a fruit fly has four pairs of chromosomes. In female flies, all four pairs look alike: short, featureless threads. But in male flies, the fourth pair looks different: one is bigger than the other. This is the chromosome pair that is now known, famously, as the X and the Y. Morgan focused on this mismatched fourth pair. He knew that a fly, like a pea plant or a human being, always inherits one chromosome from each parent. In each pair, one chromosome comes from the father and one from the mother. Since a female fly has two Xs and a male has an X and a Y, Morgan could deduce that a son must inherit his X from his mother and his Y from his father. If his father has white eyes and his mother has red eyes, then he will have red eyes. But if his father has red eyes and his mother has white, then he will have white eyes. So Morgan wondered if the fly has a gene for eye color on the X chromosome.
In his Fly Room at Columbia University, in the early years of the twentieth century, Thomas Hunt Morgan transformed the study of life. Because Morgan hated being photographed, his students stole this picture by hiding the camera in an incubator and pulling a string. The camera, along with the books and microscopes that are visible behind Morgan, belonged to his favorite student, Alfred Sturtevant, whose eureka one night in 1911 helped establish the theory of the gene. (Illustrations credit 2.1)
A female fly gets one X chromosome from each of her parents. If her mother gives her an X with a gene for white eyes and her father also gives her an X with a gene for white eyes, then she will have white eyes. But if either of her parents gives her an X with a gene for red eyes, then she will have red eyes, because red is dominant over white in flies, just as purple is dominant over white in the flowers of Mendel’s pea plants.
Human beings have many more pairs of chromosomes (twenty-three pairs, although Morgan’s generation could not sort them out and count them properly). Twenty-two of those twenty-three pairs look like identical twin threads under the microscope. The last pair is mismatched, just as it is in flies: two X chromosomes in women, an X and a Y in men. And color blindness in men, Morgan realized, as he thought about all this, “follows the same scheme as does white eyes in my flies.”
Morgan began to suspect that genes might exist, and that there really might be a gene for eye color hidden somewhere on the fly’s X chromosome. By now he and his first students had examined so many flies through their hand lenses and microscopes that the slightest aberration leaped out, and they began to find more and more mutants in their fly bottles. One was a fly with abnormally short wings. When Morgan bred it he saw that wing length, like eye color, seemed to be on the X.
Now Morgan experimented with these mutants. Suppose a female has red eyes and long wings: she is a normal fly. Suppose a male has white eyes and short wings: he is a double mutant. If they mate, every one of their daughters will inherit one X from the mother and one X from the father. So, since red and long are dominant over white and short, the daughters should have red eyes and long wings, too. Morgan arranged that cross, and so it was.
Morgan thought he knew exactly what was on each of those two Xs, if the gene theory was correct. One X carried genes for red eyes and long wings; the other X carried genes for white eyes and short wings. If so, then when these normal-looking females mated with normal males, each mother should produce just two kinds of sons, depending on which X each son inherited. Some of the sons should be normal like their mother; some of the sons should be double mutants like their grandfather.
Morgan arranged this cross. Just as he expected, some of the sons were normal and some were double mutants. But others had white eyes and normal wings, and still others had red eyes and short wings: they were single mutants. At first sight, that seemed impossible. Each of the sons could inherit just one X from his mother, and his mother did not have an X with a white gene and a long gene, or an X with a red gene and a short gene. It was as if the mother had mixed and matched bits of her two Xs before passing out an X to each of her sons.
After much thought, Morgan could explain that. He considered the microscopic action that takes place when a female fly makes an egg. Her egg has to receive four chromosomes, one strand apiece from each of her four pairs of chromosomes. But the specialized cell that prepares the chromosomes for the new egg has the chromosomes in pairs. So to produce the egg, in the process known as meiosis, each of those pairs of chromosomes has to split up. Just before they part, each pair does something almost gaudily bizarre. The two strands twist and twine around each other as if they themselves are mating. They writhe together like copulating snakes.
So Morgan imagined two of these X chromosomes mating, so to speak—wrapping around each other, aligning themselves so that every point on one X touches the corresponding point on the other. During that intimate moment, Morgan thought, bits of each chromosome might somehow trade places. Genes might cross over from one X to the other. Afterward, the solitary X chromosome that passes into the female fly’s egg would carry some genes from her father’s X and some genes from her mother’s X. And all this shuffling and crossing-over might have caused the oddities that Morgan was trying to explain. The genes on the X chromosomes had shuffled; they had mixed and matched.
Morgan arrived at this vision of crossing-over toward the end of 1911. In the Fly Room, he shared it with his favorite student, Alfred Sturtevant, then a senior at Columbia. What happened next is one of the most important eurekas in twentieth-century science and deserves to be better known outside the field of genetics. The moment would help to define both the style and substance of the study of life for the rest of the century.
This crossing-over idea had interesting implications, Morgan told Sturtevant. Picture that female fly and her two Xs just before the shuffle and scuffle of crossing-over. On one X, she has genes for red eyes and long wings. On the other X, she has genes for white eyes and short wings. Suppose these two genes lie very close together on the X. If they are close, then during crossing-over the two genes will be likely to stay together, like two people who are standing right next to each other in a swirling crowd. But suppose the two genes lie at opposite ends of the X chromosome. Then they will have more chance of being separated, like two people who are standing farther apart.
Morgan thought he could apply this idea to the results of their breeding experiment. If single mutants were rare, that would imply that the gene for eye color and the gene for wing length must lie close together on the X, because the two genes had not been separated very often during crossing-over. But if single mutants were common, that would imply that the two genes must lie far apart on the X, because they had been separated so often. And in fact, the experiment had produced quite a few single mutants: about 30 percent of the sons were single mutants.
Sturtevant not only followed all this: there in the Fly Room, as he listened to Morgan, he had the idea of his life. By this time, he and Morgan and the other students in the Fly Room had found quite a few genes that seemed to lie on the fly’s X chromosome. Sturtevant realized that if Morgan was right about crossing-over, he might actually be able to figure out where each one of these genes lies on the X. He could test Mendel, he could test Morgan, and he could make a map of genes on a chromosome, all in one stroke.
That afternoon Sturtevant collected a stack of laboratory records, the complete records of crosses involving half a dozen genes, and he took the papers home. At home he spread out the papers. He imagined a half-dozen beads on a string, or points on a line:
If genes are real and if they lie in a straight line on a chromosome, they must be in this linear order. A must be closer to B than to C and so on. So in each generation of flies, there should be more ABs than ACs, because A and B are more likely to travel together than A and C. By now Morgan and his students had crossed tens of thousands of flies and they had kept records of each cross. So Sturtevant checked to see which genes had stayed together more often and which genes had parted more often when the flies were crossed.
In the beginning there was white. Which genes are close to white? Sturtevant thought he could guess one of them. In the Fly Room they had found a gene that affects the body color of the fly. They called this gene yellow because they had first inferred its existence when they came across a yellow-bodied mutant. For his purposes now, Sturtevant needed to see the results of a cross in which one parent had white eyes and a normal brown body, and the other parent had red eyes and a yellow body. If the eye-color gene and the body-color gene were very close together, virtually all of the descendants should be of two kinds: either white-eyed and brown-bodied, or else red-eyed and yellow-bodied. But if the genes were farther apart, they would be separated more often during crossing-over. In that case, many of the two flies’ descendants would have white eyes and yellow bodies.
Proof that genes are real. This is T. H. Morgan’s own diagram of the phenomenon known as crossing-over, which one biologist has called “arguably the most intimate event in sexual reproduction.” (a) A schematic drawing of a single pair of chromosomes. Morgan pictures the genes on the chromosomes as pearls on strings. The black pearls come from the mother, the white from the father. (b) Just before the creation of an egg cell, the two chromosomes twine together, and genes cross over. (c) Now each chromosome in the pair carries some genes from the mother and some genes from the father. Morgan and his students used crossing-over to make the first genetic maps, in one of the most extraordinary series of experiments in the twentieth century. (Illustrations credit 2.2)
Sturtevant checked the breeding records he had brought home. In the records, exactly 21,736 flies were the descendants of such parents. Out of those 21,736 fly children, only 214, or about 1 percent, had white eyes and yellow bodies. So those two genes had rarely been separated during crossing-over. That meant the yellow gene must lie very close to white.
Sturtevant decided to call 1 percent one map unit. He would say that white and yellow are one map unit apart.
The lab records he had brought home also included a cross between flies with yellow bodies and flies with vermilion eyes. Those crosses had produced 4,551 fly children. Of those children, 1,464 flies, about 32 percent, had inherited both yellow bodies and vermilion eyes. If 1 percent is one map unit, 32 percent is thirty-two map units. So yellow is one map unit away from white and thirty-two map units away from vermilion.
“Who could have foreseen such a deluge?” Morgan wrote when flies and fly bottles began crowding out everything else in his lab at Columbia. The science was glamorous; the Fly Room was anything but. Morgan and his students arrived at the lab each day bearing more and more empty half-pint milk bottles, which they stole from Manhattan stoops and from the Columbia student cafeteria. Note the bananas hanging in the corner: food not only for the flies but also for the worlds first geneticists. This photograph was taken around 1920. (Illustrations credit 2.3)
Next came a third cross. In the records, there were 1,584 fly children that had one white parent and one vermilion parent. Of those 1,584 children, he found that 471, about 29 percent, had inherited both white and vermilion. So white was twenty-nine map units away from vermilion.
Now came the moment that mathematicians, when they describe a brilliant equation, call the beauty part. Generations of geneticists have since retraced Sturtevant’s big night and shaken their heads over the simplicity of the trick that started their revolution. Much later Benzer would return to this trick, give it a twist, and start a second revolution.
Sturtevant was looking at a simple mathematical puzzle. He knew that white is closer to yellow than to vermilion. He knew that yellow is closer to white than to vermilion. And he also knew that the distance between yellow and vermilion is greater than the distance between white and vermilion. There is only one way to explain those numbers if the genes are on a straight line. They have to be arranged like this:
yellow white vermilion
Sturtevant checked the numbers. He had one map unit between yellow and white; thirty map units between white and vermilion; thirty-two map units between yellow and vermilion. So far, then, everything seemed to be in order, “at least mathematically,” as he wrote later. The arithmetic was close enough, given the slight fuzziness in the data. And when he checked the rest of the breeding data, all of the other numbers and distances fit too. He placed a mutation called miniature wing about three map units away from vermilion. The wings of miniature are normal in shape but very short, like human arms and hands that reach only to the belt. He placed rudimentary wing about twenty-four map units from miniature. The wings of rudimentary are a bit of a mess: Some are wrinkled and blistered; some are truncated; some have irregularly spaced hairs.
Before dawn, Sturtevant was finished. As a senior at Columbia, he had a full load of assignments from other courses, and he had just put in an all-nighter on a long-shot project that no one had assigned. “I had quite a lot of homework to do,” Sturtevant used to say long afterward, “but I didn’t do any of it; but I did come back with a map the next morning.” In the Fly Room, he laid out the first genetic map, with the genes spaced out in a line:
yellow white vermilion rudimentary
Looking at this simple map, Morgan and his students could see that what they had been supposing and assuming month after month in their flyspecked laboratory was almost certainly true. Genes are real, genes are on chromosomes, and genes can be surveyed and explored. It was the biggest lightning (lash and thunderclap in biology since the rediscovery of Mendel in 1900. Morgan, who was not given to hyperbole, once called the view the map opened “one of the most amazing developments in the whole history of biology.” Sturtevant was nineteen years old.
Morgan and his students would spend the next several decades mapping more and more genes on the X and the other three fly chromosomes, and convincing the doubters inside and outside biology that genes are real.
Alfred Sturtevant, T. H. Morgans favorite student, figured out how to draw a map of the genes in 1911, at the age of nineteen. His discovery started biology on a long march inward. Sturtevant made gene mapping his life’s work; he never left Morgan. This photo was taken around 1925. (Illustrations credit 2.4)
Long afterward, when Benzer and his student Ronald J. Konopka found the fly without a sense of time, they would trace its eccentric behavior to that same first chromosome, the X. And when they mapped the mutant gene, they would locate it right next to Morgan’s starting point, less than one map unit away from white.