Chapter I
Counting Peas
Before genetics, heredity was thought to blend. A child was a mixture of its parents, as paint mixes. That explained why offspring resemble both parents, but not why traits skip generations, and, as Darwin's critics pointed out, it would dilute any new variation to nothing.
Gregor Mendel, a friar and teacher at the Augustinian monastery in Brno, took a different approach: choose simple traits, breed carefully, and count. Crossing purple-flowered with white-flowered peas gave all purple offspring. Crossing those with each other gave purple and white again, in a ratio of about three to one. The white trait had not blended away. It had been hidden. Mendel explained it with paired factors, one from each parent, that separate cleanly when sex cells form. He presented the work in 1865 and published it in 1866. For thirty-four years almost no one took notice.
Chapter II
Rediscovery and the Fly Room
In 1900 three botanists, Hugo de Vries, Carl Correns and Erich von Tschermak, reported similar results and found Mendel's paper. William Bateson made himself its champion and gave the new science its name. Wilhelm Johannsen coined gene in 1909.
Where were the genes? Chromosomes, the thread-like bodies that pair up and separate when cells divide, behaved exactly as Mendel's factors should. In 1910 Thomas Hunt Morgan found a single white-eyed male fruit fly in his crowded Columbia lab and traced the trait to the X chromosome. His student Alfred Sturtevant, then an undergraduate, realised overnight that how often linked genes are separated measures their distance, and drew the first genetic map.
Chapter III
A Closer Look: Mendel's Counts
Mendel's key experiment crossed pure-breeding round-seeded peas with pure-breeding wrinkled ones. All the offspring were round. He then let those hybrids self-fertilise and counted their seeds:
| Trait | Dominant | Recessive | Ratio |
|---|---|---|---|
| Seed shape | 5,474 round | 1,850 wrinkled | 2.96 : 1 |
| Seed colour | 6,022 yellow | 2,001 green | 3.01 : 1 |
Why three to one? Suppose each plant carries two copies of a factor, now called a gene, one from each parent. Write for round, which is dominant, and for wrinkled. The hybrids are all . Each passes on or with equal chance, so their offspring are
Anything with at least one looks round, so three-quarters are round and a quarter wrinkled. The wrinkled factor was hidden in the hybrids, not lost.
Mendel then tracked two traits at once. If the genes for shape and colour are passed on independently, the combinations should appear in the proportions , , and , a ratio of 9 : 3 : 3 : 1. Of 556 seeds, the expected numbers are 312.75, 104.25, 104.25 and 34.75. He counted 315 round yellow, 101 wrinkled yellow, 108 round green and 32 wrinkled green.
The fit is close, perhaps too close. In 1936 Ronald Fisher calculated that Mendel's data agree with the theory better than chance would usually allow, and suggested that someone, perhaps an assistant, had tidied them. Others have argued that Mendel's methods, such as when he stopped counting, explain the fit. Either way, the ratios have been confirmed countless times since. Independence also has limits: genes close together on the same chromosome tend to travel together, which is how Sturtevant built the first genetic map.
Chapter IV
From Heredity to Chemistry
Genetics then learned to intervene. In 1927 Hermann Muller found that X-rays multiply the mutation rate, giving geneticists an endless supply of mutants and giving the world an early warning about radiation. In 1941 George Beadle and Edward Tatum used that trick on bread mould and found that each mutation disabled one enzyme. Genes work by making proteins.
That turned heredity into a question about molecules. What is a gene made of, and how does it make a protein? Answering it created molecular biology. Meanwhile Mendel's discrete genes rescued Darwin. Because they do not blend, variation is preserved, and the mathematical fusion of the two founded population genetics.