Biologist Nettie Maria Stevens (1861-1912) discovered that chromosomes determine sex.
Nettie Maria Stevens was a biologist and cytogeneticist and one of the first American women to be recognized for her contributions to scientific research. "She … produced new data and new theories," wrote Marilyn Bailey Ogilvie in Women in Science, "yet beyond these accomplishments passed along her expertise to a new generation…. illustrat[ing] the importance of the women's colleges in the education of women scientists." Although Stevens started her research career when she was in her thirties, she successfully expanded the fields of embryology and cytogenetics (the branch of biology which focuses on the study of heredity), particularly in the study of histology (a branch of anatomy dealing with plant and animal tissues) and of regenerative processes in invertebrates such as hydras and flatworms. She is best known for her role in genetics—her research contributed greatly to the understanding of chromosomes and heredity. She theorized that the sex of an organism was determined by the inheritance of a specific chromosome—X or Y—and performed experiments to confirm this hypothesis.
Stevens, the third of four children and the first daughter, was born in Cavendish, Vermont, on July 7, 1861, to Ephraim Stevens, a carpenter of English descent, and Julia Adams Stevens. Historians know little about her family or her early life, except that she was educated in the public schools in Westford, Massachusetts, and displayed exceptional scholastic abilities. Upon graduation, Stevens taught Latin, English, mathematics, physiology and zoology at the high school in Lebanon, New Hampshire. As a teacher she had a great zeal for learning that she tried to impart both to her students and her colleagues. Between 1881 and 1883, Stevens attended the Normal School at Westfield, Massachusetts, consistently achieving the highest scores in her class from the time she started until she graduated. She worked as a school teacher, and then as a librarian for a number of years after she graduated; however, there are gaps in her history that are unaccounted for between this time and when she enrolled at Stanford University in 1896.
In 1896, Stevens was attracted by the reputation of Stanford University for providing innovative opportunities for individuals aspiring to pursue their own scholastic interests. At the age of thirty-five she enrolled, studying physiology under professor Oliver Peebles Jenkins. She spent summers studying at the Hopkins Seaside Laboratory, Pacific Grove, California, and pursuing her love of learning and of biology. During this time, Stevens decided to switch careers to focus on research, instead of teaching. While at Hopkins she performed research on the life cycle of Boveria, a protozoan parasite of sea cucumbers. Her findings were published in 1901 in the Proceedings of the California Academy of Sciences. After obtaining her master's degree—a highly unusual accomplishment for a woman in that era—Stevens returned to the East to study at Bryn Mawr College, Pennsylvania, as a graduate biology student in 1900. She was such an exceptional student that she was awarded a fellowship enabling her to study at the Zoological Station in Naples, Italy, and then at the Zoological Institute of the University of Würzburg, Germany. Back at Bryn Mawr, she obtained her doctorate in 1903. At this time, she was made a research fellow in biology at Bryn Mawr and then was promoted to a reader in experimental morphology in 1904. From 1903 until 1905, her research was funded by a grant from the Carnegie Institution. In 1905, she was promoted again to associate in experimental morphology, a position she held until her death in 1912.
While Stevens' early research focused on morphology and taxonomy and then later expanded to cytology, her most important research was with chromosomes and their relation to heredity. Because of the pioneering studies performed by the renowned monk Gregor Mendel (showing how pea plant genetic traits are inherited), scientists of the time knew a lot about how chromosomes acted during cell division and maturation of germ cells. However, no inherited trait had been traced from the parents' chromosomes to those of the offspring. In addition, no scientific studies had yet linked one chromosome with a specific characteristic. Stevens, and the well-known biologist Edmund Beecher Wilson, who worked independently on this type of research, were the first to demonstrate that the sex of an organism was determined by a particular chromosome; moreover, they proved that gender is inherited in accordance with Mendel's laws of genetics. Together, their research confirmed, and therefore established, a chromosomal basis for heredity. Working with the meal worm, Tenebrio molitor, Stevens determined that the male produced two kinds of sperm—one with a large X chromosome, and the other with a small Y chromosome. Unfertilized eggs, however, were all alike and had only X chromosomes. Stevens theorized that sex, in some organisms, may result from chromosomal inheritance. She suggested that eggs fertilized by sperm carrying X chromosomes produced females, and those by sperm carrying the Y chromosome resulted in males. She performed further research to prove this phenomenon, expanding her studies to other species. Although this theory was not accepted by all scientists at the time, it was profoundly important in the evolution of the field of genetics and to an understanding of determination of gender.
Stevens was a prolific author, publishing some thirty-eight papers in eleven years. For her paper, "A Study of the Germ Cells of Aphis rosae and Aphis oenotherae, " Stevens was awarded the Ellen Richards Research Prize in 1905, given to promote scientific research by women. Stevens died of breast cancer on May 4, 1912, before she could occupy the research professorship created for her by the Bryn Mawr trustees. Much later, Thomas Hunt Morgan, a 1933 Nobel Prize recipient for his work in genetics, recognized the importance of Stevens' ground-breaking experiments, as quoted by Ogilvie in the Proceedings of the American Philosophical Society, "Stevens had a share in a discovery of importance and her name 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."
Ogilvie, Marilyn Bailey, Women in Science: Antiquity through the Nineteenth Century, Massachusetts Institute of Technology, 1986.
Isis, June, 1978, pp. 163-72.
Proceedings of the American Philosophical Society, Held at Philadelphia for Promoting Useful Knowledge, Volume 125, American Philosophical Society, 1981, pp. 292-311.