Nettie Stevens: A Discoverer of Sex Chromosomes

Nettie Maria Stevens (July 7, 1861 – May 4, 1912) was an early American geneticist. In 1906, she discovered that male beetles produce 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. This pattern was observed in other animals, including humans, and became known as the XY sex-determination system. source: wikipedia

Nettie Stevens Discovered XY Sex Chromosomes | She Didn’t Get Credit | Nettie Stevens 155th Birthday source: youtube

Following her own path.

Nettie Maria Stevens grew up in America just after the Civil War. Beyond teaching, nursing, or secretarial work, little opportunity was available to women looking for a profession; most simply hoped to marry well. Stevens, however, would not go that route. She was a teacher, but it was only a means to an end. Stevens wanted to be a scientist and worked her way through school, eventually reaching her goal and earning her place in the history of genetics.

Her education begins.

Stevens was born July 7, 1861, in Cavendish, Vermont, to a middle-class family that had lived in New England for five generations. At that time, education for women was not common, but Stevens was lucky enough to attend the Westford Academy, which was open to men and women of all nationalities. When she graduated at age 19, she became a teacher even though she still longed to further her education. She taught for only three terms, while saving her money to attend the Westfield Normal School, a teachers college.

Student and teacher.

Stevens’s pattern of working, saving, and returning to school would repeat itself after graduation from Westfield. At age 35, she enrolled at Leland Stanford University. She began graduate school at Bryn Mawr College four years later. Stevens was an exceptionally bright student, and, although teaching was not her first love, her enthusiasm for science made her a caring and dedicated teacher. She once told a hesitant student, “How could you think your questions would bother me? They never will, so long as I keep my enthusiasm for biology; and that, I hope will be as long as I live.”

A late start.

Finally, at age 39, Stevens began working as a research scientist, and the next 11 years would be the most productive of her life. Stevens was interested in the process of sex determination. While studying the mealworm, she found that the males made reproductive cells with both X and Y chromosomes whereas the females made only those with X. She concluded that sex is inherited as a chromosomal factor and that males determine the gender of the offspring.

Not appreciated, yet.

At the time, the chromosomal theory of inheritance was not yet accepted, and it was commonly believed that gender was determined by the mother and/or environmental factors. Most scientists did not embrace Stevens’s theory immediately. Another researcher, Edmund Wilson, made a similar discovery at about the same time, but Stevens is generally considered to have made the larger theoretical leap (one which was ultimately proven correct).

Gone too soon.

Stevens continued to do research and teach at Bryn Mawr and Cold Spring Harbor Laboratories for the rest of her life. Her scientific career started late, and ended much too soon when she died of breast cancer on May 4, 1912. However, in the intervening decade prior to her death, she had managed to contribute more to her field than many scientists have with much longer careers.

This page appears in the eBook Essentials of Genetics, Unit 3.5 / via

Humans, like other mammals, have what’s called “chromosomal sex”. Women have two copies of a medium-sized chromosome called X (which stands for “unknown” because it was originally a mystery). Males have a single X and a tiny Y.

Jenny Graves, Author provided

The X bears about 1,600 genes with varied functions. But the Y has hardly any genes; maybe 50, and only 27 of these are in the male-specific part of the Y. Many are present in multiple copies, most of them inactive, lying in giant loops of DNA. Most of the Y is made of repetitive “junk DNA”. Thus the human Y shows all the signs of a degraded chromosome near the end of its life.

But the Y must contain a gene that determines maleness, because XXY people are male, and XO people with a single X but no Y are female. This is part of the artivle “Sex, genes, the Y chromosome and the future of men” originally publishe on : The