Nettie Stevens

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Nettie Stevens
Nettie Stevens.jpg
Born Nettie Maria Stevens
(1861-07-07)July 7, 1861
Cavendish, Vermont, United States
Died May 4, 1912(1912-05-04) (aged 50)
Baltimore, Maryland, United States
Education Westford Academy
Alma mater Westfield Normal School
Stanford University
Bryn Mawr College
Known for XY sex-determination system
Scientific career
Fields Genetics
Doctoral students Alice Middleton Boring
Influences Edmund Beecher Wilson
Thomas Hunt Morgan

Nettie Maria Stevens (July 7, 1861 – May 4, 1912) was an early American geneticist. In 1906, she discovered that male beetles produce two kinds of sperm, one with a large chromosome and one with a small chromosome. When the sperm with the large chromosome fertilized eggs, they produced female offspring, and when the sperm with the small chromosome fertilized eggs, they produced male offspring. This pattern was observed in other animals, including humans, and became known as the XY sex-determination system. .[1][2][3]

Early life[edit]

Nettie Maria Stevens was born on July 7, 1861, in Cavendish, Vermont, to Julia (née Adams) and Ephraim Stevens. In 1863, after the death of her mother, her father remarried and the family moved to Westford, Massachusetts. Her father earned enough money to provide Nettie and her sister with a strong education. In public and high school, Stevens was amongst the top of her class. She and her sister were amongst the 3 women to graduate from Westford Academy between 1872-1883. After graduating in 1880, Stevens moved to Lebanon, New Hampshire to teach high school Zoology, physiology, math, English, and Latin. After three years, she returned to Vermont to continue her studies.


After returning home,Stevens continued her education at Westfield Normal School (now Westfield State University) She completed the four-year course in two years and graduated with the highest scores in her class.[5] In 1896, Stevens enrolled in Stanford University, where she received her B.A. in 1899 and her M.A. in 1900. She became increasingly focused on histology after completing one year of graduate work in physiology under Professor Jenkins and his former student, and assistant professor, Frank Mace Macfarland. .[4]

Stevens enrolled in Bryn Mawr College, where she focused her graduate studies on topics such as the regeneration in primitive multicellular organisms, the structure of single cell organisms, the development of sperm and eggs, germ cells of insects,[1] and cell division in sea urchins and worms. During her time at Bryn Mawr College, Stevens studied for a year (1901–02) at the Zoological Station in Naples, Italy, where she studied marine organisms, and at the Zoological Institute of the University of Würzburg, Germany, where she was named a President’s European Fellow. Stevens returned to the US where she continued her studies in cytology at Bryn Mawr, which was heavily influenced by the work of the previous head of the biology department, Edmund Beecher Wilson, and by that of his successor, Thomas Hunt Morgan.[5] Stevens later received her Ph.D. from Bryn Mawr in 1903 and remained at the college as a research fellow in biology for a year, as reader in experimental morphology for another year. She worked at Bryn Maw as associate in experimental morphology from 1905 until her death. [5]

Stevens' work at the Carnegie Institution required fellowship support, and both Wilson and Morgan wrote recommendations to the Carnegie Institution of Washington on her behalf. She applied for funding for research on heredity related to Mendel’s laws, specifically sex determination. She received an assistantship at the Carnegie Institute in 1904. After receiving the grant from the Carnegie Institution, she used germ cells of aphids to examine possible variation in chromosome structure between the two sexes. One paper, ( written in 1905,) won Stevens an award of $1,000 for the best scientific paper written by a woman. Another work, "Studies in Spermatogenesis," highlighted her entry into the increasingly promising focus of sex-determination studies and chromosomal inheritance.[1] At the Carnegie Institute, Stevens published her major sex determination work as a report in 1905. In 1908, Stevens received the Alice Freeman Palmer Fellowship from the Association of Collegiate Alumnae, now the American Association of University Women. [6] After receiving her Ph.D. from Bryn Mawr, Stevens was given an assistantship at the Carnegie Institute of Washington in the year 1904–1905. In 1908, Stevens received the Alice Freeman Palmer Fellowship from the Association of Collegiate Alumnae, now the American Association of University Women. During her fellowship year, Stevens studied at the Naples Zoological Station and the University of Wurzburg, in addition to visiting laboratories throughout Europe.[7]


Nettie Stevens's microscope, Bryn Mawr College

Stevens was one of the first American women to be recognized for her contribution to science. Her research was completed at Bryn Mawr College. The highest rank she attained was associate in experimental morphology (1905–1912). At Bryn Mawr, she expanded the fields of genetics, cytology, and embryology.[1]

Although Stevens did not have a university position, she made a career for herself by conducting research at leading marine stations and laboratories. Her record of 38 publications includes several major contributions which further the emergence of ideas of chromosomal heredity. By experimenting on germ cells, Stevens interpreted her data to conclude that chromosomes have a role in sex determination during development. As a result of her research, Stevens provided critical evidence for Mendelian and chromosomal theories of inheritance.[1]

Following her death, Thomas Hunt Morgan wrote an extensive obituary for the journal Science.[8] In an earlier letter of recommendation he wrote, "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."[9]

Using observations of insect chromosomes she discovered that, in some species, chromosomes are different between the sexes. The discovery was the first time that observable differences of chromosomes could be linked to an observable difference in phenotype or physical attributes (i.e., whether an individual is male or female). This work was published in 1905. Her experiments used a range of insects. She identified the small chromosome currently known as the Y chromosome in the mealworm, Tenebrio. She deduced that the chromosomal basis of sex depended on the smaller Y chromosome carried by the male. An egg fertilized by a sperm that carries the small chromosome becomes a male while an egg fertilized by a sperm with the larger chromosome becomes female. Studying egg tissue and the fertilization process in beetles and flies, Stevens saw that there were chromosomes that existed in small-large (now known as XY) pairs and she also saw chromosomes that were unpaired, XO. Hermann Henking. had studied firebug chromosomes earlier but didn't find the small chromosome now called Y. Stevens realized that the previous idea of Clarence Erwin McClung, that the X chromosome determines sex, was wrong and that sex determination is, in fact, due to the presence or absence of the small (Y) chromosome. Stevens did not name the chromosomes X or Y, Their current names came later.[10][11] Edmund Wilson worked on spermatogenesis preparations simultaneously with Stevens' studies. He performed tests only on the testes, claiming that eggs were too fatty for his staining procedures. After her discoveries, Wilson reissued his original paper and acknowledged Stevens for the finding of sex chromosomes.

At Bryn Mawr, Stevens bred Drosophila melanogaster in the laboratory as subjects of her research some years before Morgan adopted it as his model organism.[12]

Sex Discrimination[edit]

Although Stevens and Wilson both worked on chromosomal sex determination, many textbooks have credited Wilson for the results. Wilson did not realize how significant the small (Y) chromosomes are for sex determination until Stevens had completed her research. He had believed that environmental factors played a role in sex determination. Additionally, Thomas Hunt Morgan has been credited with the discovery of sex chromosomes although at the time of their discoveries, he argued against Wilson's and Stevens' interpretations. Morgan's recognition came in part from his work on the white mutant of fruit flies and was especially heightened by his Nobel Prize award. Morgan and Wilson were invited to speak at a conference to present their theories on sex determination in 1906. Stevens was not invited to speak.


At 50 years old, and only 9 years after completing her Ph.D., Stevens died of breast cancer on May 4, 1912, in Baltimore, Maryland. Her career span was short, but she published approximately 40 papers.[14] She was buried in the Westford, Massachusetts, cemetery alongside the graves of her father, Ephraim, and her sister, Emma.[6] [13] [5]


Her single-mindedness and devotion, combined with keen powers of observation; her thoughtfulness and patience, united to a well-balanced judgment, account, in part, for her remarkable accomplishment.

— Thomas Hunt Morgan, in an obituary note following Stevens's death in 1912[4]

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's death in 1912 (The Scientific Work of Miss N. M. Stevens. Science, Vol. 36 (No. 928), October, 1912)[4]


To celebrate her 155th birthday on July 7th, Google created a doodle showing Stevens peering through a microscope at XY chromosomes.

On May 5, 2017, Westfield State University honored Stevens through the naming ceremony of the Dr. Nettie Maria Stevens Science and Innovation Center. The center is where the university’s STEM-related degree programs in Nursing and Allied Health, Chemical and Physical Sciences, Biology, Environmental Science and the soon-to-be launched master’s in Physician Assistant Studies are all based.


  1. ^ a b c Brush, Stephen G. (June 1978). "Nettie M. Stevens and the Discovery of Sex Determination by Chromosomes". Isis. 69 (2): 162–172. doi:10.1086/352001. JSTOR 230427. 
  2. ^ "Nettie Maria Stevens – DNA from the Beginning". Retrieved 2016-07-07. 
  3. ^ John L. Heilbron (ed.), The Oxford Companion to the History of Modern Science, Oxford University Press, 2003, "genetics".
  4. ^ a b c "Nettie Maria Stevens (1861–1912)". The Marine Biological Laboratory. Archived from the original on March 31, 2013. Retrieved August 18, 2013. 
  5. ^ a b "ST17no1.pdf". Google Docs. 
  6. ^ Gilgenkrantz, Simone (October 15, 2008). "Nettie Maria Stevens (1861–1912)" (in French). Clérey-sur-Brénon, France: Médecine/Sciences. Archived from the original on August 17, 2013. Retrieved August 18, 2013. 
  7. ^ Maltby, Margaret (1929). History of the Fellowships Awarded by the American Association of University Women. American Association of University Women. pp. 41–42. 
  8. ^ Morgan, T.H. (October 12, 1912). "The Scientific Work of Miss N. M. Stevens". Science. 36 (298): 468–70. Bibcode:1912Sci....36..468M. doi:10.1126/science.36.928.468. JSTOR 1636618. PMID 17770612. 
  9. ^ Wessel, Gary M. (September 13, 2011). "Y does it work this way?" (PDF). Molecular Reproduction and Development. Wiley. 78 (9). doi:10.1002/mrd.21390. Retrieved August 20, 2013. 
  10. ^ David Bainbridge, 'The X in Sex: How the X Chromosome Controls Our Lives, pages 3-5, 13, Harvard University Press, 2003 ISBN 0674016211.
  11. ^ James Schwartz, In Pursuit of the Gene: From Darwin to DNA, pages 170-172, Harvard University Press, 2009 ISBN 0674034910
  12. ^ Lois N. Magner, A History of the Life Sciences, 3rd ed., Marcel Dekker, 2002, p. 346.
  13. ^ "Nettie Stevens: A Discoverer of Sex Chromosomes". Nature. Archived from the original on March 14, 2016. Retrieved July 6, 2016. 

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