He joined the Royal Air Force as a photographic intelligence officer in April 1941. He served in the Middle East, Algiers and Italy and reached the rank of Squadron Leader. Demobilised in 1945, he joined the cancer research staff, Mount Vernon Hospital Middlesex. He discovered, with John Read, the role of oxygen in Radiobiology and showed that the radiotherapeutic effect of X-rays on tissue growth was largely due to chromosome breakage.
He was appointed Assistant Lecturer at the University of Sheffield in 1947, and taught genetics in the Botany and Zoology Departments. In 1954 he was made Senior Lecturer and Head of the newly-created Department of Genetics. The Professorship was created 1959.
In 1959 he accepted the offer of appointment to the Arthur Balfour Professorship of Genetics at the University of Cambridge. In 1961 he moved the department from a house in Storey's Way to the old Veterinary School site in Milton Road. He established a formal place for the department's teaching in part I of the Natural Sciences Tripos. Such a place had hitherto been deemed inappropriate though the department was established in 1912!
In 1976, he moved the Genetics Department into the Downing site. From 1977 to 1981 he served on the General Board of the Faculties, and was Chairman of its Needs Committee.
His research from 1947 has been largely concerned with the causes and functions of intraspecific genetic variation, on the nature of continuous genetic variation and on the effects of selection on such variation. He has published an important thesis on the meaning of biological progress in evolution and the role of genetic variation in determining long term fitness. He has pioneered a method for the location on chromosomes of genes mediating continuous variation, and showed (contrary to accepted theory) that the genes at different loci affected the quantitative character in qualitatively different ways. He has pioneered experiments into disruptive selection (selection in the same population for both extremes and against intermediates), and (again contrary to theoretical expectation), showed such selection could be extremely effective, increasing variance, establishing and maintaining polymorphisms, and, if the selected individuals were allowed to choose their mates, dividing the population into two partially isolated parts, something which is a step towards speciation.
The University has a short history of the Genetics department in Cambridge