Monday, July 8, 2013

Nettie Stevens

Nettie Maria Stevens was born on July 7, 1861 in Cavendish, Vermont. Her family settled in Westford, Vermont, where her father was a carpenter and handyman. He did well enough that he was able to invest in real estate and he was able to send his children away to school. Stevens attended Westfield Academy and Westfield Normal School where she completed a four year program in two years. After working as a teacher for a while, in 1896 Stevens began attending Stanford University, in Palo Alto, California, where she earned a BA in 1899 and a MA in 1900 completing her thesis using a microscope to describe new species of marine life. Stevens next went to Bryn Mawr College where she studied cytology. While at Bryn Mawr Stevens won a fellowship that allowed her to travel to Warzburg, where she studied in the laboratory of Theodore Boveri. Boveri was studying the role that chromosomes have on heredity. Stevens returned to Bryn Mawr finishing her PhD in 1903. After finishing her doctorate she remained at Bryn Mawr as an assistant.

Stevens research while at Bryn Mawr was studying the chromosomes of sex cells of meal worms. Sex cells are the cells produced by males and females that give rise to progeny. These cells have half the number of chromosomes that normal cells do. When male and female sex cells combine, a process called fertilization, it gives rise to a single cell with half of its chromosomes from the father and half from the mother that will eventually develop into progeny. Stevens noted in her research that some male sex cells have a chromosome not found in female sex cells. She proposed that this extra chromosome was responsible for determining the sex of the offspring. Today we call this extra chromosome the Y-chromosome and when a male sex cell, with a Y-chromosome, fertilizes an egg the offspring will be male. Half of male sex cells have an X-chromosome and when it fertilizes a female sex cell the offspring will be female. At the time it was believed that the gender of offspring was determined by the mother and environmental factors and Stevens' research was not widely accepted. Today we know that the sex cells of the father, with either Y or X chromosomes, determine the gender of the offspring through the mechanism discovered by Stevens.

Stevens died of breast cancer on May 4, 1912 at the early age of 39.


DNA Learning Center; "Nettie Maria Stevens (1861-1912)"; Retrieved from:

Scitable; "Nettie Stevens: A Discoverer of Sex Chromosomes"; Retrieved from:

Nettie Stevens Wikipedia Entry

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