Thomas Hunt Morgan

This long string of seminal discoveries began with the discovery of a single male white-eyed fly among the many thousands of normal red-eyed ones. Morgan bred this mutant male with a red-eyed female. All the offspring were red-eyed, indicating the white form of the gene (called the white allele) was "recessive" to the dominant red allele: Flies carried the mutant allele, but its effects did not show up. When these offspring were crossed, the ratio of red to white was 3:1, just what would be expected for a classical recessive trait.

However, Morgan noted an unusual fact about the white-eyed flies—all of them were male. Morgan knew that the female Drosophila had two so-called X chromosomes, while the male had only one. Combining this fact with his discovery that only males showed the white-eye trait, he reasoned that the white-eye mutant allele must be on the X chromosome. Males show the white-eye trait because the mutant white allele is the only one they have—they don't have a second X chromosome with a normal red allele. Females rarely show the white-eye trait, because they have a normal redeye allele on the other X chromosome.

A cross between two double heterozygotes indicates linkage. Very few recombinants (long wing-purple eye or short wing-red eye) are formed if the two genes are close together.

Morgan's results showed that the white-eye allele is inherited on the X chromosome, and confirmed the discovery that the X chromosome helps determine sex, first shown in 1905 by Sutton and Nettie Stevens. In one step, his discovery proved that genes, the factors governing inheritance, are carried on chromosomes, and that specific genes are carried on specific chromosomes. This provided the crucial evidence that genes are indeed discrete physical objects.