Photograph · Carnegie Institution of Washington · c. 1904
Nettie Stevens
1861 — 1912
She looked down a microscope at a mealworm and found the answer to a question as old as humanity: what makes a boy, and what makes a girl.
For most of history, people guessed. The mother's diet, the warmth of the womb, the season, the side of the body — theory after theory tried to explain why a child is born male or female. In 1905 a 44-year-old researcher at Bryn Mawr College, counting chromosomes in the cells of the common mealworm beetle, found the physical answer. Every female cell carried twenty large chromosomes. Every male cell carried nineteen large ones — and one small one. Sex was decided at fertilisation, by which sperm reached the egg.
Nettie Maria Stevens had spent her twenties and early thirties teaching school to save the money for a university education. She did not enter Stanford until she was 35. She published her discovery with just seven years of life left — and within a generation, the textbooks were crediting it to the men around her.
A late start, a short run
The lifeline
1861
Born 7 July in Cavendish, Vermont, to a carpenter and his wife
1880
Begins teaching high-school zoology, maths, English and Latin in Lebanon, New Hampshire
1896
Enters Stanford University at 35, her savings paying the way
1903
PhD from Bryn Mawr; wins a Carnegie grant to study how sex is determined
1905
Publishes the X–Y answer from the chromosomes of the mealworm
1912
Dies of breast cancer on 4 May, aged 50, before taking up a research professorship
By the numbers
The measure of her
35
Her age when she finally enrolled at Stanford, in 1896
38+
Scientific papers she published before her death in 1912
59
Species surveyed to test the idea: 50 beetles and 9 flies
18
Women among the 1,000 leading American scientists ranked in 1910 — she was one
Female20 large
Male19 large + 1 small
What she saw in Tenebrio molitor, the yellow mealworm: one tiny chromosome — today's Y — marks every male.
What she found under the lens
The work
1899–00
Stanford BA (1899) and MA in biology (1900), with summers of marine research at Stanford's Hopkins laboratory on Monterey Bay.
1901–02
Naples and Würzburg. On Bryn Mawr's President's European Fellowship, a year at the Naples Zoological Station and the University of Würzburg.
1903
Doctorate at Bryn Mawr, under Thomas Hunt Morgan — who would win the 1933 Nobel Prize for showing chromosomes carry heredity.
1905
Studies in Spermatogenesis, Part I. In the mealworm, two kinds of sperm: one carrying the large chromosome makes a daughter; one carrying the small makes a son.
1905
The Ellen Richards Prize — $1,000 for the best scientific paper by a woman, for her study of aphid germ cells.
1906
Part II extends the pattern: X–Y pairs or a lone unpaired chromosome across dozens more insect species.
Credited late
Recognition
In her lifetime & after
The discovery was long credited to E. B. Wilson, who reached a similar result independently in 1905, and to Morgan
The Mechanism of Mendelian Heredity (1915), by Morgan and his students, credited neither her nor Wilson with the discovery
Never held a full professorship; Bryn Mawr's research chair came in 1912, the year she died
A 1916 genetics paper called the men "Dr." — and her "Miss Stevens"
Today
Recognised as a discoverer of chromosomal sex determination — the basis of the X–Y system taught in every biology class
Inducted into the U.S. National Women's Hall of Fame, 1994
Celebrated with a worldwide Google Doodle on her 155th birthday, 7 July 2016
Her 1905 monograph is studied as a founding text of modern genetics
The place that made her
Westford, Massachusetts
After her mother died in 1863, her father remarried and moved the family to Westford, a farming town northwest of Boston. There a carpenter's daughters got a real education: Nettie and her sister Emma were two of only three women to graduate from Westford Academy between 1872 and 1880. And it was to Westford that she came home at the last — she is buried there beside her father and sister.
1872–80
Her years at Westford Academy, one of the oldest co-educational schools in the U.S.
2 yrs
To finish Westfield Normal School's 4-year course, graduating 1883
~10 yrs
Teaching and library work before she could afford university
"Of the graduate students that I have had during the last twelve years, I have had no one that was as capable and independent in research as Miss Stevens."
— Thomas Hunt Morgan, in a letter of recommendation
