Nettie Stevens discovered that an organism’s sex is determined by its chromosomes – now known as the XY sex-determination system: females have two of the same kind of sex chromosome (XX); males have two different sex chromosomes (XY).
The discovery was also the first time that a link was demonstrated between a physical characteristic (sex) and differences in chromosomes.
Beginnings
Nettie Maria Stevens was born on July 7, 1861 in Cavendish, Vermont, USA.
Her father was Ephraim Stevens, a carpenter. Her mother was Julia Adams, who died when Nettie was just two years old.
In 1865 her father married Ellen Thompson. The Stevens family, consisting of father, step-mother, Nettie and her younger sister Emma, then relocated to Westford, Massachusetts.
Nettie attended public elementary schools in Westford, where her remarkable academic ability was noticed by her teachers.
Her father’s business prospered in Westford and Nettie attended Westford Academy, a private school. She enrolled there in 1872, aged 11, and graduated in 1880, aged 18. Nettie’s younger sister Emma also attended Westford Academy, and she too proved to be an exceptionally talented student. Emma went on to become a high school teacher.
Westford Academy taught its students a rather traditional classical curriculum emphasizing Latin, Greek, English, and Mathematics.
Teachers’ College
After graduating, Nettie Stevens got work in New Hampshire, about 100 miles from her family, teaching at Lebanon High School for a year. She enjoyed the work enough to enroll at the teachers’ college in Westfield, Massachusetts, again about 100 miles from her family.
It took her only two years to complete the college’s four year course. She studied all of the sciences, scoring exceptionally high grades; her grades in Algebra, Chemistry, and Geometry were perfect.
School Teaching and Saving
In 1883, aged 22, Stevens returned to the family home and began teaching at Minot’s Corner School in Westford. She was quickly recognized as a highly talented teacher, working as both a teacher and a supervisor.
In 1884 she returned to Westford Academy, the private school she had attended between the ages of 11 and 18, and taught there for six years, until 1892.
She saved hard while working because, at some point during her teaching career, she decided she wanted to become a scientist. She needed to save enough money to get through several years of studying for a university degree.
Stanford University
In September 1896, aged 35, Stevens moved to California. She had been accepted by Stanford University to study for a bachelor’s degree, majoring in Physiology. She got her degree in 1899.
She spent her summers working at Stanford’s Hopkins Seaside Laboratory, where she specialized in studying the microscopic anatomy of organisms (histology) and cells (cytology). The work fascinated her and in 1900 she earned a master’s degree at Stanford with her thesis Studies on Ciliate Infusoria.
Bryn Mawr College, Europe, and a Ph.D.
At age 39, early in 1901, Stevens moved to Bryn Mawr College, close to the city of Philadelphia in Pennsylvania. There she began working for a doctorate in cytology. The head of biology was Thomas Hunt Morgan, who would later win a Nobel Prize for his work clarifying the chromosome’s role in heredity.
In 1901 Stevens was awarded a President’s European Fellowship, and spent the 1901-1902 academic year carrying out research at the Naples Zoological Station in Italy and the University of Würzburg in Germany. In Würzburg she worked with Theodor Boveri, who, like Thomas Hunt Morgan, was working on the role of the chromosome in heredity.
In 1903, aged 42, Stevens submitted her thesis, which expanded upon the theme of her master’s degree. Its title was: Further Studies on the Ciliate Infusoria Lichnophora and Boveria. She got her Ph.D. and accepted an offer to do postdoctoral research work at Bryn Mawr.
Nettie Stevens’ Contributions to Science
Discovery of New Species
While she was working for her master’s degree, Stevens discovered two new species of single-celled organisms: Licnophora macfarlandi and Boveria subcylindrica; she also documented their life cycles.
Chromosomes Determine the Sex of Offspring
In 1866 Gregor Mendel had established the rules of heredity – the rules governing how parental traits pass to offspring. Nobody noticed the huge significance of his work until it was rediscovered in about 1900.
By 1900 it was already well-known to cytologists that an offspring inherits equal numbers of chromosomes from each of its parents – they had seen this under their microscopes.
We now know that cells carry their genetic code – heredity instructions – in structures called chromosomes, which contain DNA. (The role of DNA in chromosomes was unknown in Stevens’ time.)
Nobody, however, had been able to prove a link between Mendel’s rules and the role of chromosomes. Establishing what part, if any, chromosomes played in heredity had become a very hot topic.
Furthermore, nobody knew how the sex of an offspring was determined. There were different theories, including a theory that chromosomes determined sex. Most scientists, however, believed sex was decided by external factors, such as temperature and nutrition, acting on a fertilized egg. They did not believe that sex was decided by chromosomes at the instant of fertilization.
In 1903 Stevens applied for funding from the Carnegie Institute of Washington saying:
“I am especially interested in the histological side of the problems in heredity connected with Mendel’s Law, and I know that there is need of a great deal of painstaking work along that line.”
She received a $1,000 grant for the 1904-1905 year to investigate problems relating to sex determination. She won a further $1,000 in 1905 – the Ellen Richards Prize for the best scientific paper written by a woman – for A Study of the Germ Cells of Aphis rosae and Aphis oenotherae. Stevens actually wrote this paper in December 1904, at which time she was still uncertain about the source of sex determination.
“…we find that the evidence is overwhelmingly on the side of the view that sex is determined in the egg; but to the question of how sex is determined in the egg, no thoroughly convincing answer has yet been given.”
In 1905 Stevens published a series of papers in which she demonstrated that the sex of an offspring is determined by the chromosomes it inherits from its parents.
“Since the male somatic cells have 19 large and 1 small chromosome, while the female somatic cells have 20 large ones, it seems certain that an egg fertilized by a spermatozoon which contains the small chromosome must produce a male, while one fertilized by a spermatozoon containing 10 chromosomes of equal size must produce a female.”
Nettie Stevens identified a large chromosome and a small chromosome – we now call these X and Y. An individual that inherits XX will be female and XY will be male.
“In 1906 she found that the male of a beetle (Tenebrio molitor) produced two kinds of sperm, differing in that one half of the sperms have a large chromosome and the other half a smaller chromosome… the small chromosome is confined to the male line while in the female its place is taken by the larger one. [Stevens] drew the correct inference that since all unfertilized eggs are alike in their chromosomal content, therefore a female results from the fertilization of an egg by the sperm containing the larger chromosome, and the male by the sperm containing the smaller chromosome. A similar relation was discovered at the same time by Professor E. B. Wilson. Their joint discovery marks the turning point in the history of the theory of sex-determination.”
In her papers Stevens showed that an organism’s sex is determined by specific chromosomes — today we would say that males have XY and females XX sex chromosomes.
In doing so she also provided the first evidence that a physical characteristic – in this case the sex of an individual – is linked to differences in chromosomes.
Edmund Beecher Wilson of Columbia University, America’s first cell biologist, independently made the same discovery as Stevens, also in 1905.
Some Personal Details and the End
In 1905 Stevens became an associate in experimental morphology at Bryn Mawr College. In 1912 a research professorship was created for her, but she died before she could begin working in her new role.
Nettie Maria Stevens died aged 50 of breast cancer on May 4, 1912 in Baltimore, Maryland. She never married and had no children.
She was remembered by her students as a rather shy, unassuming teacher. She was remembered by other members of the faculty at Bryn Mawr as a talented researcher working in a difficult field who had achieved a worldwide reputation among biologists. She was always on hand to help willingly with teaching or any other college related work.
Nettie Stevens was laid to rest in the cemetery at Westfield, Massachusetts beside the graves of her father and step-mother. Her sister Emma’s ashes were also placed there in 1945.
Author of this page: The Doc
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Further Reading
N. M. Stevens
A Study of the Germ Cells of Aphis rosae and Aphis oenotherae
The Journal of Experimental Zoology, Vol. 2, pp 313-334, 1905
Nettie Maria Stevens
Studies in Spermatogenesis Part I
Carnegie Institution of Washington Publication 36, 1905
The Faculty of Bryn Mawr College
The Death of Nettie Maria Stevens
Science, Vol. 35, May 17, 1912
Thomas Hunt Morgan
The Scientific Work of Miss N. M. Stevens
Science, Vol. 36, Oct. 11, 1912
Marilyn Bailey Ogilvie and Clifford J. Choquette
Nettie Maria Stevens (1861-1912): Her Life and Contributions to Cytogenetics
Proceedings of the American Philosophical Society, Vol. 125, No. 4, pp. 292-311, Aug. 21, 1981