Joshua Lederberg
In 1946 and 1947, Lederberg took a leave of absence to study under Edward Tatum at Yale University. Lederberg and Tatum showed that the bacterium Escherichia coli entered a sexual phase during which it could share genetic information through bacterial conjugation.  With this discovery and some mapping of the E. coli chromosome, Lederberg got his PhD. from Yale University in 1947.