What significant discovery did Thomas Hunt Morgan make regarding the inheritance of traits in fruit flies?

Thomas Hunt Morgan's significant discovery regarding the inheritance of traits in fruit flies was that genes are located on chromosomes. This finding was pivotal in advancing the understanding of genetics because it established a clear link between the physical structure of chromosomes and the inheritance of genetic traits.

Morgan conducted experiments using Drosophila melanogaster, commonly known as the fruit fly, and observed specific traits, like eye color, that appeared to be inherited in a patterned manner. By mapping these traits to the chromosomes, he demonstrated that genes are not merely abstract units of heredity but are physically situated on the chromosomes themselves. This was a fundamental shift in the field of genetics and laid the groundwork for the modern understanding of gene linkage and chromosome behavior during cell division.

This discovery also provided evidence against the idea that all traits are inherited independently, as it revealed that genes located close to each other on the same chromosome could be inherited together, influencing the expression of traits in a linked manner. Hence, the assertion that genes are located on chromosomes is a central tenet of genetics, validating the chromosomal theory of inheritance and reinforcing the importance of studying chromosomes in understanding heredity.