During World War I, he was on an eighteen-month leave of absence when he served in the U.S. Army as a supply officer in a depot brigade.
During World War II, he was on another leave of absence when he served in 1944 and 1945 as a personnel officer at the Los Alamos Laboratory.
[1] As a graduate assistant of William Albert Setchell, Clausen began investigating the genus Nicotiana and continued this research until he died in 1956.
[3][4] Clausen's pioneering studies on unbalanced chromosomal types—haploids, trisomics, monosomic—culminated in the recognition of a complete set of twenty-four monosomics in N. tabacum.
However, he was not content merely to isolate and describe the monosomics, but through their use to develop a powerful new tool of genetic analysis.