Nettie Stevens (1861 - 1912)

Courtesy Bryn Mawr College Special Collections

When a baby is born, what makes them a boy versus a girl? About a century ago, scientists were mystified by this question -- at the time, genetics itself was an uncharted frontier. To navigate the waters of this field, Nettie Stevens stood out as a late-bloomer scientist who accomplished more in a few years than most scientists do in their entire career.

Nettie Stevens was born on July 7th, 1861 in Cavendish, Vermont. She was a stellar student and one of three women to graduate from her high school during a time when education for women was uncommon. Following an untraditional career route at the time, Stevens aimed to become a scientist. However, she initially worked as a high school teacher to fund her education. Teaching was not her greatest passion, but she became a dedicated teacher thanks to her enthusiasm for biology-related subjects.

Stevens would continue working and saving money until graduating from Westfield State University with a B.A. Continuing her journey toward scientific research, Stevens attended Stanford University in 1896 and graduated with a masters in biology. She developed an interest in histology and physiology during this time, building her background in precise, microscopic work. Stevens then pursued her Ph.D. at Bryn Mawr College, focusing her studies on cellular-level topics like cell division, cell regeneration, and the development of sperm and eggs. After obtaining her Ph.D. in 1903, Stevens would continue to conduct her research at Bryn Mawr for most of her remaining career.

Stevens received a fellowship from the Carnegie Institution of Washington in 1904, which eventually generated her most famous work on sex determination. At the time, scientists were unsure of the factors that determined an offspring’s sex. Many believed it was due to external factors like temperature and nutrition acting on the egg, and only a few thought it was due to chromosomal factors. Since the link between chromosomes and Mendel’s genetic rules were still unclear, Stevens sought to investigate the histological side of this mystery. She first began by experimenting on the germ cells of aphids to look for chromosomal differences between the two sexes. After studying more insects, she discovered that the sperm cells would differ by one chromosome -- some carried a large chromosome, while others carried a smaller variant. They were later named X and Y chromosomes, respectively. Unfertilized eggs did not have this difference, and Stevens ultimately concluded that the presence of the Y chromosome carried by the sperm cell was responsible for sex determination. Stevens’ discovery had provided a firm link between Mendelian and chromosomal theories of inheritance, and for the first time proven that chromosomal differences could explain phenotypic differences.

Unfortunately, most scientists did not embrace Stevens's theory immediately. She was often overshadowed by Edmund Wilson, another researcher who independently made a similar discovery. Wilson was often credited with the discovery because of his other contributions and higher reputation, but his conclusions were not as strong or accurate as Stevens’. Stevens’ time in the spotlight would not arrive until after her death, when progress in science would be followed by progress in the way we recognize revolutionary women.

Photo Credit: "Courtesy Bryn Mawr College Special Collections"