Nettie Stevens: The Genetics Pioneer Who Discovered Sex Chromosomes

At a time when women mostly married and stayed home, or were teachers or nurses if they wanted to work, Nettie Stevens became a research scientist and her discoveries changed genetics forever.

NettieStevensGeneticsPioneerOnce she graduated with her PhD in 1903, she and a colleague (Thomas Morgan) began a collaboration on the controversial and unresolved question of how sex is determined in the developing egg. Did external factors, like food and temperature, set the sex of an egg? Or was it something inherent to the egg itself? Or was sex inherited as a Mendelian trait?

She examined the yellow mealworm, Tenebrio melitor, and made a striking observation. She had observed that this species produced two classes of sperm: a type that carried ten large chromosomes, and a type that carried nine large and one small chromosome. Body cells in the females contained 20 large chromosomes while males carried 19 large and one small chromosome.

Stevens reasoned that when an egg is fertilized by a sperm that carries the small chromosome, the result is a male offspring. The presence of the small chromosome might be what decided the individual’s “maleness.”

She published her research in 1905 and it eventually evolved into the XY sex-determination system we know today: The father’s sperm, which can carry either X or Y chromosomes, determines the sex of the offspring. Before Stevens’ work, scientists thought that the mother or the environment determined if a child was born male or female.

Biography

Nettie StevensNettie Maria Stevens was one of the first American women to be recognized for her contribution to science. Yet she didn’t begin her career in genetics until later in life.

Stevens was born on July 7, 1861, in Cavendish, Vermont, to Ephraim and Julia Stevens. After the death of her mother, her father remarried and the family moved to Westford, Massachusetts.

Initially, Stevens taught high school and was a librarian for more than a decade. Her teaching duties included courses in physiology and zoology, as well as mathematics, Latin, and English. Her first career allowed her to save up for college; at the age of 35, she resigned from a high school teaching job in Massachusetts and traveled across the country to enroll at Stanford University in California.

At Stanford, she received her B.A. in 1899 and her M.A. in 1900. She also completed one year of graduate work in physiology under Professor Jenkins and histology and cytology under Professor McFarland.

Stevens continued her studies in cytology at Bryn Mawr College, where she obtained her Ph.D. Here, she was influenced by the work of Edmund Beecher Wilson and by that of his successor, Thomas Hunt Morgan. Her work documented processes that were not researched by Wilson and she used subjects that he later would adopt along with the results of her work.

At age 50 years, only 9 years after completing her Ph.D., Nettie Stevens died of breast cancer on May 4, 1912 in Baltimore, Maryland.

Bring it Home

▶︎ Dive into Genetics with a fun unit study

▶︎ Enjoy a slide show presentation on genetics

▶︎ Try out this jigsaw format activity to explore the sex determination mechanisms of seven organisms, Xs and Os

▶︎ Learn about the Father of Genetics: Gregor Mendel

▶︎ Try this Gummy Bear Genetics lab from The Science Teacher (a NSTA publication)

▶︎ Use pipecleaners and beads to show how genes and chromosomes are inherited in this Pipecleaner Babies lab.

▶︎ Use pennies to do this How Well Does a Punnet Square Predict the Actual Ratios? lab.

Science Milestones

Visit my Science Milestones page to learn more about scientists whose discoveries and advancements have made a significant difference in our lives or who have advanced our understanding of the world around us.

The bloggers of the iHomeschool Network have teamed up to create fun and original unit studies on fascinating people who were born in July.