Demerec , Milislav

(1895–1966) Croatian–American geneticist

Demerec was born in Kostajnica in Croatia and graduated from the College of Agriculture in Krizevci in 1916. After a few years' work at the Krizevci Experimental Station, he moved to America. He gained his PhD in genetics from Cornell University in 1923 and then worked at the Carnegie Institution, Cold Spring Harbor, where he remained for most of his career, becoming director in 1943.

Demerec was concerned with gene structure and function, especially the effect of mutations. He found that certain unstable genes are more likely to mutate than others and that the rate of mutation is affected by various biological factors, such as the stage in the life cycle. He also demonstrated that chromosome segments that break away and rejoin in the wrong place may cause suppression of genes near the new region of attachment. This lent additional support to the idea of the ‘position effect’, first demonstrated by Alfred Sturtevant.

Demerec's work with the bacterium Salmonella revealed that genes controlling related functions are grouped together on the chromosome rather than being randomly distributed through the chromosome complement. Such units were later termed operons. His radiation treatment of the fungus Penicillium yielded a mutant strain producing much larger quantities of penicillin – a discovery of great use in World War II. He showed that antibiotics should be administered initially in large doses, so that resistant mutations do not develop, and should be given in combinations, because any bacterium resistant to one is most unlikely to have resistance to both.

Demerec greatly increased the reputation of Cold Spring Harbor while he was director there and also served on many important committees. He founded the journal Advances in Genetics and wrote some 200 scientific articles.