Stevens, Nettie Maria (1861–1912)

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Stevens, Nettie Maria (1861–1912)

First scientist to demonstrate that gender is determined by a particular chromosome. Born on July 7, 1861, in Cavendish, Vermont; died of breast cancer on May 4, 1912, in Baltimore, Maryland; daughter of Ephraim Stevens (a carpenter) and Julia (Adams) Stevens; graduated from Westford Academy, 1880, and Westford Normal School, 1883; Stanford University, B.A., 1899, M.A., 1900; Bryn Mawr College, Ph.D., 1903.

Nettie Maria Stevens was born in 1861 to Ephraim and Julia Adams Stevens . One of four children, she and her sister Emma Julia Stevens were the only ones to live to adulthood. Little else is known about Stevens' background, except that she was an able and intelligent student. She received her early education at the Westford Academy in Westford, Massachusetts, where she graduated in 1880 with a concentration in the sciences; in 1883, she graduated at the top of her class from the Westford Normal School.

For the next 13 years, Stevens worked as a teacher and a librarian in Westford, Chelmsford, and Billerica, Massachusetts, before traveling across the country in 1892, at age 31, to enroll as a special status undergraduate student at Stanford University. Stevens' father and sister later followed her to California.

At Stanford, Stevens studied physiology with Oliver Peebles Jenkins and histology with Frank Mace MacFarland. She spent her summer months at the Seaside Laboratory in Pacific Grove, working first with MacFarland, and then in 1898 with Jacques Loeb, physician and associate professor of physiology at the University of Chicago. Her resultant study of Boveria, a protozoan parasite of sea cucumbers, was published in 1901 in the Proceedings of the California Academy of Sciences. She received her bachelor's degree in 1899, and her master's degree in 1900. Her master's thesis was published as Studies on Ciliate Infusoria a year later.

In 1900, Stevens return to the East to enroll in Bryn Mawr College as a graduate student in biology, a department dominated by two eminent faculty biologists, Edmund Beecher Wilson and Thomas Hunt Morgan, future Nobel laureate in genetics. Stevens' stellar research earned her an overseas study fellowship at the Naples Zoological Station for the 1901–02 term and later at the Zoological Institute of the University of Würzburg, Germany, where she worked with prominent German biologist Theodor Boveri. Stevens received her Ph.D. in 1903 and her dissertation, Further Studies on the Ciliate Infusoria, Licnophora and Boveria, was published later that year.

Stevens remained affiliated with Bryn Mawr as a Carnegie research fellow in biology from 1903 to 1905, and as an associate in experimental morphology from 1905 to 1912, during which time she returned to work with Boveri from 1908 to 1909. In 1905, Stevens received the Ellen Richards Prize of $1,000—an award created to promote scientific research by women. As well, Bryn Mawr later endowed a research professorship for her.

Stevens published widely in the fields of cytology and experimental physiology. However, her most important contribution to science was the discovery that a particular chromosome was responsible for determining gender. At this time in scientific research, investigators were exploring the relationship between chromosomes and Gregor Mendel's laws of heredity, first described in 1866, but no direct relationship had been confirmed by experiments. Research up to this point had suggested only that a specific chromosome could be linked to a specific trait.

In 1903, Stevens first described, in an application for a Carnegie Institution grant, her research interest in this area. The same idea had been proposed, independent of her interests, by Edmund Beecher Wilson at Columbia University. Although the issue of priority in these two independent investigations has been questioned, it is accepted that the two came to the same conclusions separately from each other. Stevens' paper, Studies in Spermatogenesis with Especial Reference to the Accessory Chromosome (1905), published in the Publications of the Carnegie Institution, described her observation of the common mealworm. The nuclei of the egg always contained ten large chromosomes but the spermatocytes could have either ten large or have one of the large chromosomes replaced by a small chromosome, referred to as X and Y chromosomes. Stevens deduced that since a cell of the female contained 20 large chromosomes and the male had only 19 large chromosomes plus one small chromosome, this represented gender determination by a difference in the size of pairs of chromosomes. This theory was not universally accepted by other biologists. Stevens conducted additional research by observing other species of insects, and found further confirmation of her theory. The discovery had a profound effect upon the study of genetics and the theory of gender determination when it ended the long debate over whether sex was determined by heredity or by environmental influence of the embryo.

An esteemed research scientists, Stevens was also an inspiring and enthusiastic teacher. Before she could occupy the chair created for her by the trustees of Bryn Mawr, however, she died of breast cancer at Johns Hopkins Hospital in Baltimore in 1912. She was buried in Westfield, Massachusetts.