Distinguished for contributions to the genetics of the bacterium Escherichia coli, especially for his discovery of the transmissible agent, the F factor, which determines sexual differentiation and for proof of his hypothesis that only part of the donor chromosome is transferred to the recipient cell to form an incomplete zygote. Studies of the kinetics of the mating process have contributed to knowledge of the timing of transfer, the integration and expression of genes in the recombinants. These discoveries have been vital steps in the exploitation of Escherichia coli as a prime organism of genetical and biochemical research.