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way—that is, independently of the rest of the plant’s characteristics. Rather than a blending of flower color, the progeny of a purple-flower pea plant and a white-flower pea plant were not pinkish, but resulted in a predictable number of white ones and purple ones. By repeating these crossbreeds until the patterns were clear, Mendel’s pea experiments also resulted in his determining that characteristics were inherited equally, one from each parent, but that some of those characteristics were more equal than others. He bred tall plants with short ones, and their offspring were always tall, rather than an average of the two heights. When he crossed those offspring together, three-quarters of their offspring were tall and one was short. In those proportions he had uncovered not only that characteristics were passed down individually, but also that some characteristics were dominant over others.
    The story of Mendel and his peas is high-school biology. What he had discovered (though the name came much later) was the existence of genes—discrete units of inheritance. 2
    Having been largely ignored, at the beginning of the twentieth century Mendel’s papers were rediscovered. What followed was observation beyond that which is visible to the naked eye. New technologies of the twentieth century meant that the scale of biology was reducing from the organism to the cell, to the molecular and atomic level, and with this zooming in came the birth of modern genetics.
    â€œIt has not escaped our notice . . .”
    Between Mendel’s death in 1884 and the 1950s, there were major successive advances in the study of genes. Mendel had established that inheritance occurred in discrete units. Italian marine biologists looked at the cells of sea urchins and observed chromosomes—neat structures inside the nucleus of all cells, which became visible, resembling tiny sausages, when the cells divided. They came in specific numbers depending on the host, and these marine biologists discovered that altering the number of chromosomes resulted in abominations in their offspring, or prevented reproduction altogether. In the 1920s, Thomas Hunt Morgan inbred fruit flies to show that Mendel’s units of inheritance were positioned very precisely on these chromosomes. German researchers, meanwhile, had shown that chromosomes were made of a molecule called DNA, whose chemical composition was clearly different from the proteins that made up much of the cells ingredients, as it contained phosphate.
    In the 1940s, Oswald Avery, Colin MacLeod, and Maclyn McCarty demonstrated it was DNA that conferred characteristics and passed them on by performing a neat resurrection trick in New York, perversely a fatal one. They were following the work of Fred Griffiths, a British medical officer who more than a decade earlier had noticed that, during the course of pneumonia, virulent and benign bacteria were both present, but that the latter could acquire the malign characteristics of the former, even if these malign bacteria were themselves dead. He demonstrated this by boiling the lethal bacteria, thereby killing them, and then adding the resultant broth to the benign bugs, which then acquired the ability to kill. Avery and his team repeated Griffiths’ experiment, but to figure out how it worked, they systematically eliminated each of the components of the bacteria that were candidates for passing on this characteristic. It was only when they destroyed the deadly bacteria’s DNA that the transformation failed to occur. It seemed that DNA, not proteins or anything else in the cell’s milieu, was the essential genetic material, the stuff that bestowed characteristics and passed them on.
    DNA was clearly a—possibly
the
—key component of inheritance. But it was unclear how this could work. The answer lay in its construction. At King’s College London in 1952, a group of researchers including Maurice Wilkins,