Nettie Stevens - dicoverer of the X and Y chromosomes


courtesy of Carnegie Institution of Washington

(higher level of understanding required)


Nettie Maria Stevens (July 7, 1861 – May 4, 1912), was the most crucial figure involved in the discovery of the sex genes, now known as x and y chromosomes.


Nettie Maria Stevens was born on July 7, 1861, in Cavendish, Vermont.Stevens was at the top of her class during her college. Stevens traveled to Lebanon, New Hampshire, after graduation in 1880, to teach zoology, physiology, mathematics, English, and Latin in high school. She returned to Vermont and pursued her studies at Westfield State University. In two years, she finished the four-year course, graduating the top of her class.




Stevens joined Stanford University in 1896, continuing her path towards scientific study, and graduated with a master's degree in biology. During this time, she gained an interest in histology and physiology, developing her experience in correct, microscopic work. Stevens then completed her Ph.D. at Bryn Mawr College, concentrating her thesis on cellular-level topics such as cell differentiation, cell regeneration, and sperm and egg growth.


In 1904, Stevens received a fellowship from Washington's Carnegie Foundation, which subsequently produced her most famous research on sex determination. After researching more species, she observed that the sperm cells would differ by one chromosome—some carried a large chromosome, and others carried a smaller version. At the time scientists were unaware of the causes that determined the sex of an offspring. They were later called the chromosomes of X and Y, respectively.


Unfertilized eggs did not have this difference, and Stevens proposed primarily that sex determination was responsible for the inclusion of the Y chromosome borne by the sperm cell. The discovery of Stevens established a clear correlation between Mendelian and chromosomal inheritance hypotheses and showed for the first time that phenotypic variations could be clarified by chromosomal differences.


While Stevens and Edmund Wilson also collaborated on the determination of chromosomal sex, several scholars have attributed the finding to Wilson alone. The appreciation of Morgan came in part from his work on the sex linkage of the white mutant gene of fruit flies and was especially heightened by his 1933 Nobel Prize.


Immediately after her finding, Stevens was not acknowledged. Thomas Hunt Morgan and Edmund Beecher Wilson, for instance, were invited to appear at a conference in 1906 to discuss their ideas on sex determination, but Stevens was not asked to speak.


Her [Nettie Stevens] 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 obituary, 'The Scientific Work of Miss N.M. Steves', Science (11 Oct 1912)

Stevens died of breast cancer on May 4, 1912, in Baltimore, Maryland, aged 50 years old, and just 9 years after finishing her Ph.D. Her career period was short, but about 40 papers were written by her.



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