Susumu Ohno was born of Japanese parents in Seoul, Korea, on February 1, 1928. The second of five children, he was the son of the minister of education of the Japanese Viceroyship of Korea. The family returned to Japan after the war in 1945. He later became a citizen of the United States of America. Susumu Ohno married Midori Aoyama in 1951. They had two sons and one daughter.
His passion for science derived from his lifelong love of horses. He earned a Ph.D. in veterinary science at Tokyo University of Agriculture and Technology in 1949, and later a Ph.D. and D.Sc. from Hokkaido University. He went to the United States in 1951, as a visiting scholar to UCLA, and in 1952 joined the new research department at City of Hope Medical Center, where he remained in active research until 1996.
Ohno postulated that gene duplication plays a major role in evolution in his classic book Evolution by Gene Duplication (1970). While subsequent research has overwhelmingly confirmed the key role of gene duplication in molecular evolution, research to evaluate Ohno's model for the preservation of duplicate genes (now termed neofunctionalization) is ongoing and very active. He also discovered in 1956 that the Barr body of mammalian female nuclei was in fact a condensed X chromosome. In Evolution by Gene Duplication, he also suggested that vertebrate genome is the result of one or more entire genome duplications; variations of this idea have come to be known as the 2R hypothesis (also called "Ohno's hypothesis"). He indicated that mammalian X chromosomes are conserved among species; it has been referred to as Ohno's law. He also invented the term Junk DNA for segments of the DNA that have no known function.
^Ohno S, Kaplan WD, Kinosita R (1959). "Formation of the sex chromatin by a single X-chromosome in liver cells of Rattus norvegicus.". Experimental cell research18 (2): 415–418. doi:10.1016/0014-4827(59)90031-X. PMID14428474.
^Ohno S (1967). Sex Chromosomes and Sex-Linked Genes. Berlin, New York: Springer-Verlag