Nettie Maria Stevens

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Nettie Maria Stevens

Birthplace: Cavendish, Windsor County, Vermont, United States
Death: May 04, 1912 (50)
Baltimore, Maryland, United States (Breast cancer)
Place of Burial: Westford, Middlesex County, Massachusetts, United States
Immediate Family:

Daughter of Ephriam Asa Stevens and Julia Maria Stevens
Sister of Charles Merrill Stevens; Willie Porter Stevens and Emma Julia Stevens
Half sister of Edward P Stevens

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About Nettie Maria Stevens

Nettie Maria Stevens (July 7, 1861 - May 4, 1912) was an early American geneticist. She and Edmund Beecher Wilson were the first researchers to describe the chromosomal basis of sex.

An outstanding student, Nettie Stevens completed in two years the four-year course at Westfield Normal School (now Westfield State University) in Massachusetts. She graduated at the top of her class. At Stanford, she received her B.A. in 1899 and her M.A. in 1900, having returned to college after an initial career as a school teacher. Stevens continued her studies in cytology at Bryn Mawr, where she was influenced by the work of the previous head of the Biology Department, Edmund Beecher Wilson, and by that of his successor, T. H. Morgan. She also studied marine organisms in Europe.

Stevens was one of the first American women to be recognized for her contribution to science. Her research was done in Bryn Mawr College. She discovered that in some species chromosomes are different among the sexes, by observations of insect chromosomes. The discovery was the first time that observable differences of chromosomes could be linked to an observable difference in physical attributes i.e. if an individual is a male or a female. This work was done in 1905. The experiments done to determine this used a range of insects, she identified the Y chromosome in the mealworm Tenebrio. She deduced the chromosomal basis of sex depended on the presence or absence of the Y chromosome. She did not start her research until her thirties and completed her PhD in 1903. She successfully expanded the fields of embryology and cytogenetics. She died on May 4, 1912 of breast cancer, before she was able to fully take up a new faculty position.

Nettie Stevens is a somewhat controversial character. Following her death, Thomas Hunt Morgan wrote an extensive, if somewhat dismissive, obituary for the journal Science, implying that she was more of a technician than a scientist. This later assessment belies his earlier statement in a letter of recommendation: "Of the graduate students that I have had during the last twelve years I have had no one that was as capable and independent in research as Miss Stevens...". Also Nettie Stevens was the first to recognize that females have two large sex chromosomes. Wilson did not see this because he only performed tests on the testis because eggs are too fatty for the old staining procedures. Wilson even reissues his original paper and thanks Nettie Stevens for this finding. This finding is what then allowed Wilson to combine his idea of idiochromosomes with her heterosomes. This shows that Nettie Stevens was very influential in this process. Most biology textbooks credit Morgan for mapping the first gene locations onto chromosomes of fruit flies Drosophila melanogaster, but what is often missed is that it was Stevens who brought the fruit fly into Morgan's lab in the first place.


Modern cytological work involves an intricacy of detail, the significance of which can be appreciated by the specialist alone; but Miss Stevens had a share in a discovery of importance, and her work will be remembered for this, when the minutiae of detailed investigations that she carried out have become incorporated in the general body of the subject.

— Thomas Hunt Morgan, following Stevens' death in 1912


A 1905 research paper with a long-winded title - "Studies in Spermatogenesis with Especial Reference to the 'Accessory Chromosome'" - written by Bryn Mawr biologist Nettie Stevens, was one of the 20th century's major scientific breakthroughs, showing that the chromosomes known as "X" and "Y" were responsible for determining the sex of individuals. This ended a longstanding scientific debate as to whether sex was determined by heredity or other factors. Now, once and for all, a relatively obscure research biologist had shown that chromosomes influenced human traits, opening the doors for research in science and medicine that continues to this day.

Nettie Stevens, educated at Stanford University and Bryn Mawr College (Ph.D., 1903), taught throughout her relatively short life, inspiring many students to careers in science. She published more than 38 papers from 1901 to her death, in cytology and experimental physiology.

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Nettie Maria Stevens's Timeline

July 7, 1861
Cavendish, Windsor County, Vermont, United States
May 4, 1912
Age 50
Baltimore, Maryland, United States
Westford, Middlesex County, Massachusetts, United States