 With genetics carved out as a unique field of study, two threads of genetic research ensued. One thread focused on identifying the physical unit of heredity that was passed from generation to generation—to figure out what genes were, where they were located, how they operated, and how that operation produced the hereditary patterns consistent with Mendel’s principles (entry:  theories of biological development).  Thomas Hunt Morgan’s research at Columbia University most contributed to this work. Morgan studied fruit flies, in part because they bred quickly and were easy to maintain. With a team of young scientists, he crossed thousands upon thousands of fruit flies, tracking how traits like eye color and wing shape transmitted across generations. They also induced mutations using a variety of chemical and radiation interventions (see the entry on  experiment in biology).  This research proved that genes were located on chromosomes, and it showed how the physics of chromosomal action during meiosis impacted the hereditary process; for example, genes closer together on chromosomes were more often inherited together, while genes that were further apart more often separated from one another during chromosomal recombination. This realization allowed Morgan and his students to produce the first gene maps—charts of genes’ relative locations to one another—and also determine that certain traits were sex-linked due to their location on sex chromosomes (Darden 1991; Kohler 1994). 