Support for the Chromosomal Theory of Heredity

Even after Sutton and Boveri's proposal, the chromosomal theory remained a live controversy among biologists for several years, because no one had yet tied a specific visible trait to the behaviour of one particular chromosome. That decisive proof came from Thomas Hunt Morgan's work after 1910 on the fruit fly Drosophila melanogaster, a superb genetic model organism because it has only 2n=8 chromosomes and completes its entire life cycle in about two weeks. Morgan discovered a white-eyed mutant fly among an otherwise red-eyed stock and found that the eye-colour gene behaved unlike any autosomal gene Mendel had described: its inheritance pattern tracked exactly with the X chromosome. Because the X and Y chromosomes are not equivalent, males (who carry only one X) express whatever single allele that X carries, whereas females (who carry two X chromosomes) can be heterozygous carriers. Reciprocal crosses between red-eyed and white-eyed flies therefore gave different ratios of red- and white-eyed sons and daughters depending on which parent was which, a result that only makes sense if the eye-colour gene physically resides on the X chromosome. Morgan later found the genes for yellow body colour and miniature wing shape were carr …