Final answer:
Thomas Hunt Morgan determined the eye color gene in Drosophila was located on the X chromosome through breeding experiments, observing distinct inheritance patterns in F2 generations that were consistent with X-linked inheritance.
Step-by-step explanation:
The discoveries made by Thomas Hunt Morgan on eye color in Drosophila, the common fruit fly, led him to conclude that the gene affecting eye color was located on the X chromosome. When Morgan crossed a red-eyed female with a white-eyed male, the results indicated that the locus affecting eye color was indeed X-linked. Notably, in the F2 generation from a cross involving a homozygous white-eyed female and a red-eyed male, half of the female offspring showed red eyes and the other half white eyes, while half of the male offspring had red eyes and the other half had white eyes. This inheritance pattern could only be explained by the presence of the eye color gene on the X chromosome because males only inherit one X chromosome, making their phenotype directly correspond to their single X-linked allele.
Through his experiments, Morgan demonstrated that reciprocal crosses do not produce the same offspring ratios, which supports the idea of an X-linked trait. In humans too, there are several traits linked exclusively to the X-chromosome, three of which include color blindness, hemophilia, and Duchenne muscular dystrophy.