Johannes Henoch ('Oom Jan') Neethling, expert in plant breeding, was the son of Reverend Hendrik Johannes Neethling and his wife Cicilia Louisa, born Steyn. His education was interrupted by the Anglo-Boer War (1899-1902), but after the war he received his secondary schooling at the Boys' High School, Stellenbosch, passing the matriculation examination of the University of the Cape of Good Hope in 1906. Continuing his studies at Victoria College, Stellenbosch, he passed the intermediate examination for the Bachelor of Arts (BA) degree (equivalent to the first year of study) in 1907. The next year he was one of eight candidates selected to study agriculture overseas in order to stimulate the local development of agriculture. He attended Cornell University in the United States, where he obtained the degree Master of Science (MSc) in Agriculture in 1911 with plant breeding as a major subject. His MSc thesis was entitled A study of the third year's potato crop developed asexually from seedlings (1911). During his holidays he attended additional agricultural courses at other universities. After graduating he studied genetics in Munich and Amsterdam for a while.
Neethling returned to South Africa in 1912 and in November that year was appointed as lecturer in botany and plant breeding at the agricultural school at Elsenburg, near Stellenbosch. He also continued the wheat-breeding experiments of Dr Eric A Nobbs* and Russell W Thornton* and in 1913 started the selection of pasture plants. In 1917 a Faculty of Agriculture was created in Victoria College (from 1918 the University of Stellenbosch) and in August 1917 Neethling was appointed professor of agricultural botany. In 1921 his post was changed to professor of genetics, the first such post in South Africa. He retained this position until his retirement at the end of 1959. His influence on genetics locally is indicated by the fact that seven later professors of genetics at South African universities studied under him.
Neethling had a profound influence on wheat-breeding in the Western Cape. In 1914 he obtained his famous Union selections of wheat, from varieties produced by Thornton, and these were followed by many others. It has been estimated that between 1915 and 1956 he cultivated 95% of the wheat varieties in the winter rainfall area. His work contributed much to the effective control of wheat diseases in South Africa, while early on he also bred barley and oats for rust resistance. He published several papers in the South African Journal of Science, including 'A preliminary note on dwarfs appearing in Gluyas Early (wheat) hybrids' (1917), 'Will the inhibitor (dwarfness) throw new light on the Multiple Factor Hypothesis of quantitative characters?' (1929), and 'Some genetic aspects of a wheat x rye hybrid' (1935). Other publications by him were Wheat varieties in South Africa, their history and development until 1912 (1932) and Die ontstaan en ontwikkeling van landbou-onderwys op Stellenbosch (with J.C. Swart, 1968).
Neethling was a life member of the American Genetic Society and a member of the Royal Society of South Africa. He was elected a member of the Suid-Afrikaanse Akademie vir Wetenskap en Kuns in 1941 and was awarded its medal of honour for achievement in the natural sciences in 1963. He had a lively interest in farming and was for many years a member of the Western Cape Agricultural Union. He became a member of the South African Association for the Advancement of Science in 1917. In 1927 he was elected chairman of the Stellenbosch Horticultural Society. In his spare time he bred homing pigeons and for some time was the president of the Boland Pigeon Club. He served also as vice-president of the Western Province Baseball Union. One of the buildings of the agricultural faculty at the University of Stellenbosch was named after him in 1982. He was married to Marguerite (Rita) Joubert, but they had no children.