Chromosome theory of inheritance

The chromosome theory of inheritance emerged because two scientists on different continents—studying completely different organisms—arrived at the same revolutionary conclusion simultaneously. This convergent discovery united the abstract laws of Mendel with the physical reality of cells.

In 1902, Theodor Boveri in Germany was studying sea urchin embryos. He observed that proper embryonic development required all chromosomes to be present—removing even one produced abnormal offspring. Meanwhile, Walter Sutton, a Kansas farm boy turned graduate student at Columbia University, was examining grasshopper testes. He had discovered that the lubber grasshopper Brachystola magna offered an unusually clear view of meiosis: its cells contained 11 distinguishable chromosome pairs plus an accessory singleton he correctly identified as a sex chromosome.

Mendel's laws of heredity, rediscovered in 1900 after decades of obscurity, described how traits passed from parents to offspring in predictable ratios. But Mendel had worked with pea plants and abstract 'factors'—he never observed anything physical carrying the hereditary information. The laws worked, but no one knew what made them work.

Sutton watched chromosomes during meiosis and recognized what he was seeing. Chromosomes came in pairs, just as Mendel's factors did. During gamete formation, paired chromosomes separated, with each gamete receiving one member of each pair—exactly as Mendel's law of segregation predicted. Different chromosome pairs assorted independently of each other, matching Mendel's law of independent assortment.

In his 1902 paper, Sutton articulated what would become the foundation of genetics: 'I may finally call attention to the probability that the association of paternal and maternal chromosomes in pairs and their subsequent separation during the reducing division may constitute the physical basis of the Mendelian law of heredity.'

Boveri published a similar hypothesis in 1904. The theory became known as the Sutton-Boveri theory, named by E.B. Wilson—who had been Sutton's teacher and Boveri's friend. Some historians have questioned whether Boveri deserved equal credit, since Sutton published first and more explicitly connected chromosomes to Mendelian heredity.

The theory faced resistance. Thomas Hunt Morgan, who would later win the Nobel Prize for chromosome research, initially opposed it. Full acceptance came only gradually, as Morgan's own fruit fly experiments provided overwhelming evidence. By the 1910s, the Morgan laboratory had mapped dozens of genes to specific chromosomes, confirming what Sutton had proposed: chromosomes were indeed the physical carriers of hereditary information.

The Sutton-Boveri theory unified two previously separate fields—Mendelian genetics and cell biology. Abstract patterns of inheritance now had a physical substrate. The path from Mendel's peas to Watson and Crick's double helix runs directly through Sutton's grasshopper testes.