How did John Dalton's preserved eyeball settle a 200-year-old question?

By having its genes read. In 1794, the English chemist John Dalton gave the first scientific account of color blindness, his own, and suggested a cause: a discoloration of the liquid inside the eyeball. In 1995, genetic analysis of his preserved eyeball, some 150 years after his death, gave the real answer. Dalton had deuteranopia, a type of congenital red-green color blindness in which the gene for the medium-wavelength, or green, light-sensitive pigment is missing. His theory about the fluid was wrong.

Dalton presented his paper to the Manchester Literary and Philosophical Society, and it was published in 1798 as "Extraordinary Facts Relating to the Vision of Colours." In it he described how the world looked to him: red appeared "little more than a shade, or defect of light," while orange, yellow and green seemed one color. Because both he and his brother were color blind, he recognized that the condition must be hereditary. In some languages, red-green color blindness is still called daltonism after him.

On heredity, he was right. Color blindness is usually a sex-linked inherited variation in one or more of the three classes of cone cells in the retina. The most common form, congenital red-green color blindness, affects up to 1 in 12 males but only about 1 in 200 females, because the opsin genes responsible sit on the X chromosome. Males have only one X chromosome, so if their one copy of the gene is affected, they will be color blind; females have two.