Nettie Stevens

Nettie Maria Stevens (July 7, 1861 – May 4, 1912) was an American geneticist who discovered sex chromosomes. In 1905, soon after the rediscovery of Mendel's paper on genetics in 1900, she observed 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. The pair of sex chromosomes that she studied later became known as the X and Y chromosomes.

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 worked as a carpenter and 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 Emma were two of the three women to graduate from Westford Academy between 1872 and 1883. After graduating 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 biology in 1900. She became increasingly focused on histology after completing one year of graduate work in physiology under Oliver Peebles Jenkins and his former student, and assistant professor, Frank Mace MacFarland.

After studying physiology and histology at Stanford, Stevens enrolled in Bryn Mawr College to pursue her Ph.D. in cytology. She focused her doctoral studies on topics such as 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, Stevens was named a President's European Fellow and 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. Returning to the United States, her Ph.D. advisor was the geneticist Thomas Hunt Morgan. In addition, Stevens's experiments were influenced by the work of the previous head of the biology department, Edmund Beecher Wilson, who had moved to Columbia University in 1891. 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 an associate in experimental morphology from 1905 until her death. She was offered the position she had long sought, as research professor at Bryn Mawr College, just before cancer took her life. She was unable to accept the offer 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's post-doctoral 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 differences 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 that 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.

At first, her research and findings about sex determination were not appreciated or believed because of the gender bias in science. Another researcher, Edmund Wilson made similar discoveries to her around the same time but hers were much larger leaps in the world of science and were the ideas which ended up being correct.