Dr. Nettie Stevens

Dr. Nettie Stevens, circa 1905

Introduction

What determines the sex of an offspring? Many debated that question for centuries. Aristotle posited that heat played a crucial role, suggesting that warmer conditions favored the conception of males. During the Middle Ages and Renaissance, it was common to believe that factors like the moon's phase or the wind's direction at conception could influence whether a child would be male or female. In the 1800s, people believed that environmental conditions during pregnancy, such as a mother's health, diet, and emotional state, amongst other factors, played a role. 

Then came Dr. Nettie Stevens. Her research determined the answer.

In a groundbreaking paper published in 1905, Nettie asserted that genetics determines sex. She wrote that an egg fertilized by a sperm carrying an X chromosome results in a female (XX), while an egg fertilized by a sperm carrying a Y chromosome results in a male (XY)​.

The public reaction to Nettie's groundbreaking discovery was mixed and largely underwhelming. At the time, many scientists, including her supervisor, continued to believe in the theories of environmental factors or maternal influence, which made them skeptical of Nettie's findings.

History, however, would prove Nettie right, and her discovery became one of the foundations for modern genetics​.

Biography

Early Life and Education

Information about Nettie's early years and family is sparse. She was born on July 7, 1861. We know she descended from ancestors who made their way to Boston from England centuries prior, arriving in 1635. And that her parents married in 1854. She was the third of their four children. In 1863, Nettie's mother died. Shortly after, Nettie's father remarried.