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Sax was born in Spokane, Washington, his parents were pioneer farmers in the eastern part of the state, and active in civic affairs. His father was the mayor of Colville, Washington. Sax's early education was in the Colville schools, and in 1912 he continued his studies at Washington State College. He majored in agriculture, and his subsequent decision to undertake graduate masters study was influenced by the botanist and plant breeder Edward Gaines there. At college he met and married Dr. Hally Jolivette, his cytology teacher, and they later had three sons. Following his graduation, his wife Dr. Jolivette accepted a position at Wellesley College in Wellesley, Massachusetts and they moved to the East Coast in 1916. Sax enrolled in the doctoral program at the Bussey Institution Graduate School of Applied Biology at Harvard University in Cambridge, Massachusetts and completed his MA in 1917. He served as a Private in the US Army from 1917 to 1918 in World War I.
In 1918 after his war service he was employed as an instructor in the Department of Genetics at the University of California, Berkeley, where he worked with E. B. Babcock on the genetics of the genus Crepis. In 1920 he took an appointment at the Riverbank Laboratories in Geneva, Illinois working on wheat genetics, but he moved on from that job soon after when he took a position at the Maine Agricultural Experiment Station in Orono, Maine. During this period he was also undertaking his doctoral dissertation through Harvard University and received his D.Sc. in 1922. Later, in 1928, he left the Agricultural Station in Orono to take a teaching position in Harvard's genetics department at the Bussey Institution. Before his arrival the department had been dissolved, and he transferred to the cytology department at the University's Biological Laboratories in Cambridge, Massachusetts.
In 1938 Karl Sax published a paper entitled "Chromosome Aberrations Induced by X-rays," which demonstrated that radiation could induce major genetic changes by affecting Chromosomal translocations, a chromosome abnormality. The paper is thought to mark the beginning of the field of radiation cytology, and led him to be called the "father of radiation cytology".
Sax also bred new varieties of ornamental trees and shrubs including Malus domestica-apples, Malus species-crabapples, magnolias, and forsythias, and Prunus-cherries. He hybridized a cross between the Japanese cherry Prunus subhirtella and Prunus apetela which he named Prunus x 'Hally Jolivette' in honor of his wife. and a cultivar of Forsythia bred by Sax was named "Karl Sax" by a nurserymen. In 1946 he was appointed acting director of Harvard's Arnold Arboretum, in 1947 becoming the director – a post he held until 1954.
Sax was also interested in human demography, in 1955 he wrote Standing Room Only: The Challenge to Overpopulation, on the consequences of uncontrolled human population growth. Sax became associated with Planned Parenthood and was a member of the Population Association of America. Sax was an elected member of the National Academy of Sciences and the American Academy of Arts and Sciences.
In 1959 he retired and moved to Media, Pennsylvania where he continued his work on plant breeding. Karl Sax died on October 8, 1973.
- Smocovitis, V. B. Sax, Karl. 'American National Biography Online. Oxford University Press
- Swanson, C. P. 1988. Cytogenetics and Karl Sax. Genetics 119:5–7
- Biographical Memoir of Karl Sax written by Carl P. Swanson and Norman H. Giles for the U.S. National Academy of Sciences, a superb source of information about Sax and his work