In the frigid New York winter of 1910, experimental biologist Thomas Hunt Morgan etherized a family of fruit flies and began sorting through their sleeping forms. After a while he saw something unusual—a fly with white eyes.

In all his time studying Drosophila melanogaster, Morgan had seen only red eyes, or more specifically, red compounds with 760 individual lenses. What confronted him now was a rare genetic mutant. He bred the fly, eventually yielding a cohort with those same ghostly features. His experiment established the material basis of heredity in the chromosome, providing an essential “How?” that had been missing from Darwin’s theory of evolution.

It also earned Morgan the Nobel Prize, and made the fruit fly the sine qua non of 20th-century biological research. Subse...