He was born in Tonica, Illinois, on April 8, 1868, the son of George Nelson Jennings and his wife Olive Taft Jenks.

[3][4][5] Tracy Sonneborn would later write:Jennings was so struck by the continued production of hereditarily diverse clones at conjugation, even after many successive inbreedings, that he undertook to examine the matter mathematically.

[6]In 1924, Jennings published an article in Scientific Monthly on "Heredity and Environment" which was prescient for anticipating the double helix, and provocatively liberal for its comments on racial differences and American immigration policy.

[9] In 1930, he published The Biological Basis of Human Nature, discussing eugenics, genetic influence on human traits, and criticizing simplistic interpretations of heritability based on Mendelism, emphasizing gene-environment interaction & polygenicity; Sonneborn would describe the impact of The Biological Basis of Human Nature as widespread: "Probably no book by a geneticist has been so widely quoted by American workers in the fields of education, sociology, anthropology, and psychology.

In spite of inevitable misquotation and misinterpretation, it has exercised a distinctly salutary effect on those fields and is to a large extent responsible for whatever they have assimilated in this country from modern genetics."