Nettie Maria Stevens (July 7, 1861 – May 4, 1912) was an early American geneticist. She was either the scientist who deserves the most credit for the discovery of sex chromosomes or she discovered them at the same time as Edmund Beecher Wilson. In 1905, soon after the rediscovery of Mendel's paper on genetics in 1900, she discovered that male mealworms produced 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. She and Wilson both credited her with the discovery of sex chromosomes, but later scientists disagreed about who made the discovery. The pair of sex chromosomes that she studied later became known as the X and Y chromosomes.  .
Nettie Maria Stevens|
July 7, 1861
Cavendish, Vermont, United States
May 4, 1912 (aged 50)|
Baltimore, Maryland, United States
Westfield Normal School|
Bryn Mawr College
|Known for||XY sex-determination system|
|Institutions||Bryn Mawr College, Carnegie Institution of Washington|
|Doctoral advisor||Thomas Hunt Morgan|
|Doctoral students||Alice Middleton Boring|
Edmund Beecher Wilson|
Thomas Hunt Morgan
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 Emma with a strong education through high school. During her education, Stevens was near the top of her class. She and her sister were 2 of the 3 women to graduate from Westford Academy between 1872-1883. After being graduated in 1880, Stevens moved to Lebanon, New Hampshire to teach high school zoology, physiology, mathematics, English, and Latin. After three years, she returned to Vermont to continue her studies.
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 Seeking additional training in sciences, in 1896, Stevens enrolled in newly established 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.
After learning physiology and histology at Stanford, Stevens enrolled in Bryn Mawr College to pursue her PhD, where she focused her graduate studies on topics such as the regeneration in primitive multicellular organisms, the structure of single celled organisms, the development of sperm and eggs, germ cells of insects, and cell division in sea urchins and worms. During her graduate studies at Bryn Mawr College, Stevens spent a year (1901–02) at the Zoological Station in Naples, Italy, where she worked with 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 to continue her studies in cytology at Bryn Mawr. Her PhD advisor was the celebrated geneticist Thomas Hunt Morgan, who later moved to Columbia University. In addition, Stevens' experiments were influenced by the work of the previous head of the biology department, Edmund Beecher Wilson, who had moved to Columbia University earlier. Stevens received her Ph.D. from Bryn Mawr in 1903 and remained at the college as a research fellow in biology for a year. She continued there as reader in experimental morphology for another year and worked at Bryn Mawr as Associate in Experimental Morphology from 1905 until her death. She was offered the position she had long sought, as Research Professor at Bryn Mawr Colloge, just before cancer took her life, but she was unable to accept it due to her ill health.
After receiving her Ph.D. from Bryn Mawr, Stevens was awarded a research assistantship at the Carnegie Institute of Washington in 1904–1905. Stevens' postdoctoral year of work at the Carnegie Institution required fellowship support, and both Wilson and Morgan wrote recommendations on her behalf. She applied for funding for research on heredity related to Mendel’s laws, specifically sex determination. After receiving the grant, she used germ cells of aphids to examine possible variation in chromosome sets 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. Her major sex determination work was published by the Carnegie Institution of Washington in the two part monograph, "Studies in Spermatogenesis,"  which highlighted her increasingly promising focus of sex-determination studies and chromosomal inheritance. In 1908, Stevens received the Alice Freeman Palmer Fellowship from the Association of Collegiate Alumnae, now the American Association of University Women. During that fellowship year, Stevens again conducted research at the Naples Zoological Station and the University of Würzburg, in addition to visiting laboratories throughout Europe.
Stevens was one of the first American women to be recognized for her contribution to science. Most of 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.
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 emerging concepts 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.
Following Nettie Stevens' death, Thomas Hunt Morgan wrote an extensive obituary for the journal Science. In that article, Morgan said she had "a share in a discovery of importance.”  He continued in the obituary to describe in some detail the implications of this work. Writing such a tribute in the major journal Science was a tremendous compliment to Nettie Stevens for her scientific accomplishments. But there were a few issues in that same obituary. Morgan said she confirmed McClung's hypothesis about sex chromosomes  when she actually refuted his main claim that the larger (X) chromosome determined sex. Morgan recognized Wilson's parallel but less complete and convincing studies  as producing a "joint discovery" with Stevens. But Wilson said in a later footnote that she made the discovery. Also, Morgan claimed Stevens seemed to "appear at times wanting in that sort of inspiration that utilizes the plain fact of discovery for wider vision.”  Apparently, Morgan was forgetting the fact that she was often excluded from scientific dialogue, for example she was not invited to speak at the meetings where he and Wilson expounded the theory of sex chromosomes. In spite of these issues in Morgan's tribute, it seems clear that he valued her scientific work. However, in his textbook, The Mechanism of Genetics, published in 1915, he didn't credit either Stevens or Wilson with the discovery of sex chromosomes. He described sex linkage of the white gene in the chapter immediately before the one in which he described Stevens' results without mentioning her name, implying that his own laboratory's sex linkage analysis was the basis on which one should understand sex determination. At first glance this omission seems unfair, but he left out credit for many researchers as well, including Alfred Sturtevant who discovered linkage in Morgan's own laboratory as an undergraduate student. No one was credited with that discovery or a number of others, so her omission in his textbook need not be due to sexism. 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."
Using observations of insect chromosomes, Stevens discovered that, in some species, chromosomes are different between the sexes and when chromosome segregation occurs in sperm formation, this difference leads to outcomes of female versus male progeny. Her 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 continuing 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 aphids, mealworms, beetles, and flies, Stevens saw that there were chromosomes that existed in small-large pairs (now known as XY chromosome pairs) and she also saw chromosomes that were unpaired, XO. Hermann Henking had studied firebug chromosomes earlier and noticed the chromosome now called X, 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. Edmund Wilson worked on spermatogenesis preparations simultaneously with Stevens' studies. He performed cytological examination only on the testes, that is he didn't examine the female germ cells (eggs) but only the male germ cells (sperm) in his studies. His paper claimed that eggs were too fatty for his staining procedures. After reading the papers describing Stevens' discoveries, Wilson reissued his original paper and in a footnote acknowledged Stevens for the finding of sex chromosomes.
At Bryn Mawr, following her 1905-6 publications,Stevens bred and studied Drosophila melanogaster in the laboratory. She worked with these fruit flies as subjects of her research for some years before Morgan adopted them as his model organism.
Although Stevens and Wilson both worked on chromosomal sex determination, many authors have credited Wilson alone for the discovery. Wilson did not realize how significant the small (Y) chromosomes are for sex determination until Stevens had completed and published her research. Before he read her papers, 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 these cytological discoveries, he argued against Wilson's and Stevens' interpretations. Morgan's recognition came in part from his work on sex linkage of the white mutant gene of fruit flies and was especially heightened by his Nobel Prize award in 1933. Stevens was not even recognized immediately after her discovery. For example, Morgan and Wilson were invited to speak at a conference to present their theories on sex determination in 1906 but 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. She was buried in the Westford, Massachusetts cemetery alongside the graves of her father, Ephraim, and her sister, Emma. 
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.
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.
To celebrate her 155th birthday, on July 7, 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 then soon-to-be launched masters degree program in Physician Assistant Studies are all based.
- 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.
- "Nettie Maria Stevens – DNA from the Beginning". www.dnaftb.org. Retrieved 2016-07-07.
- John L. Heilbron (ed.), The Oxford Companion to the History of Modern Science, Oxford University Press, 2003, "genetics".
- MB Ogilvie, CJ Choquette (1981) "Nettie Maria Stevens (1861-1912): her life and contributions to cytogenetics. Proc Amer Phil Soc US 125(4):292-311.
- NM Stevens. (1905) “A Study of the Germ Cells of Aphis rosae and Aphis oenotherae.” Journal of Experimental Zoology 2 (3):313-334.
- NM Stevens, (1905) “Studies in Spermatogenesis, with Especial Reference to the ‘Accessory Chromosome,’” Washington, DC, Carnegie Institution of Washington, Publication 36, NM Stevens, (1906) “Studies in Spermatogenesis Part II, A Comparative Study of Heterochromosomes in certain Species of Coleoptera, Hemiptera, and Lepidoptera with Especial Reference to Sex Determination,” Washington, DC, Carnegie Institution of Washington, Publication 36, Part II, (1906).
- 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.
- Maltby, Margaret (1929). History of the Fellowships Awarded by the American Association of University Women. American Association of University Women. pp. 41–42.
- 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.
- E Wilson. (1905) “A study of the ideochromosomes in Hemiptera." Journal of Experimental Zoology 2(3):371-405.
- David Bainbridge, 'The X in Sex: How the X Chromosome Controls Our Lives, pages 3-5, 13, Harvard University Press, 2003 ISBN 0674016211.
- Morgan, TH, Sturtevant, AH Mu(UMLAUT)ller, HJ, and Bridges, C. (1915). The Mechanism of Mendelian Genetics. New York, Henry Holt and Company.
- 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.
- "Nettie Stevens: A Discoverer of Sex Chromosomes". Nature. Archived from the original on March 14, 2016. Retrieved July 6, 2016.
- James Schwartz, In Pursuit of the Gene: From Darwin to DNA, pages 170-172, Harvard University Press, 2009 ISBN 0674034910
- Lois N. Magner, A History of the Life Sciences, 3rd ed., Marcel Dekker, 2002, p. 346.
- "ST17no1.pdf". Google Docs.
- "Nettie Maria Stevens (1861–1912)". The Marine Biological Laboratory. Archived from the original on March 31, 2013. Retrieved August 18, 2013.
- "Westfield State University News". May 5, 2017. Retrieved August 14, 2018.
- Brush, S G (Jun 1978). "Nettie M. Stevens and the discovery of sex determination by chromosomes". Isis; an international review devoted to the history of science and its cultural influences. United States. 69 (247): 163–72. doi:10.1086/352001. ISSN 0021-1753. PMID 389882.
- Ogilvie, M B; Choquette C J (Aug 1981). "Nettie Maria Stevens (1861–1912): her life and contributions to cytogenetics". Proceedings of the American Philosophical Society. United States. 125 (4): 292–311. ISSN 0003-049X. PMID 11620765.
- Gilgenkrantz, Simone (Oct 2008). "Nettie Maria Stevens (1861–1912)". Médecine/sciences (Paris). France. 24 (10): 874–78. doi:10.1051/medsci/20082410874. ISSN 0767-0974. PMID 18950586.
- Biography of Nettie Stevens
- Google Doodle for Nettie Stevens
- Episode 8: Nettie Stevens from Babes of Science podcasts
- Cleveland, Hillary. "Women in Science". 3/6/09. Bryn Mawr College. Retrieved June 20, 2013.
- Works by Nettie Maria Stevens at Project Gutenberg
- Works by or about Nettie Stevens at Internet Archive