An excerpt from The Gene by Siddhartha Mukherjee:
Morgan began to breed flies sometime around 1905. A year later, he was breeding maggots by the thousands, in milk bottles filled with rotting fruit in a third-floor laboratory at Columbia University. Bunches of overripe bananas hung from sticks. The smell of fermented fruit was overpowering, and a haze of escaped flies lifted off the tables like a buzzing veil every time Morgan moved. The students called his laboratory the Fly Room.
Like Mendel, Morgan began by identifying heritable traits. Did fruit flies have mutations as well? By scoring thousands of flies under the microscope, he began to catalog dozens of mutant flies. A rare white-eyed fly appeared spontaneously among typically red-eyed flies. The mutants would prove crucial to experiments: only the outliers could illuminate the nature of normal heredity.
Between 1910 and 1912, Morgan and his students crossed thousands of fruit fly mutants with each other. The result of each cross was meticulously recorded: white-eyed, sable-coloured, bristled, short-winged. When Morgan examined these crosses, he found a surprising pattern: some genes acted as if they were “linked” to each other. The gene responsible for creating white eyes, for instance, was inescapably linked to the Y chromosome: no matter how Morgan crossed his flies, the white-eyed trait tracked with that chromosome. Similarly the gene for sable colour was linked with the gene that specified the shape of a wing.
Morgan’s experiments had established that genes that were physically linked to each other on the same chromosome were inherited together. If the gene that produces blue eyes is linked to a gene that produces blond hair, then children with blond hair will inevitably tend to inherit blue eyes. However, occasionally, a gene could unlink itself from its partner genes and swap places from the paternal chromosome to the maternal chromosome, resulting in a rare blue-eyed, dark-haired child.
It is fascinating to learn that gene theory was found amidst the buzz of fruit flies!