Nettie Stevens discovered the chromosomal XY sex-determination system.

Nettie Stevens Discovered the XY Chromosomal Basis of Sex

Nettie Stevens and Edmund Beecher Wilson independently discovered the chromosomal XY sex-determination system (that males have XY and females have XX sex chromosomes). Wilson made some of the discoveries in his partially correct 1900 paper, while Steven’s 1905 paper filled in the blanks and corrected some of Wilson’s errors.[1][2]

Stevens and Wilson worked independently of each other in their discovery of the chromosomal basis of sexual differentiation, both doing research at the same Bryn Mawr College (Wilson from 1885 – 1891, and Stevens later under the new head of the biology department Thomas Hunt Morgan, who also contributed to the field).[3][4][5][6][7]

All three of the aforementioned (Stevens, Wilson, and Morgan) made discoveries in genetics and cytology, and all studied insects and reproduction. While Wilson and Morgan’s work predate Steven’s work, Stevens can be said to have properly discovered the chromosomal basis of sex, as Wilson’s focus was on male insects only and Morgan’s research was in other areas like epigenetics.

Wilson, who also studied sexual differentiation and chromosomes, published first, but, after Stevens made her discoveries Wilson revised his paper and thanked her. Due to this fact, it is fair to credit Nettie Stevens with the discovery of the XY Chromosome, but better to credit Stevens and Wilson with “multiple discovery” (with a nod to Morgan).

READ: Studies in Spermatogenesis by N.M. Stevens 1905 and The Cell in Development and Inheritance Edmund B. Wilson 1900.

Nettie Stevens (1861 – 1912) Birthday Doodle to celebrate Nettie Stevens’ 155 birthday. Google: Google honors Nettie Maria Stevens. She was an early American geneticist. She and Edmund Beecher Wilson were the first researchers to describe the chromosomal basis of sex.

FACT: Due to her discovery, Nettie Stevens became one of the first American women to be recognized for her contribution to science.

TIP: Here in 2016 we know sexual differentiation is more complex than just an A or B choice. Genetic sex is not determined by the presence or absence of a Y chromosome alone, although it is a primary factor. See Is Sex and Gender Binary?

The Quick History of Sex Differentiation Theories

Thinkers had long tried to understand what determined the sex of a child. Past idea’s included Aristotle’s belief that sex was determined by the body temperature of the father. There was a theory prevalent in Europe during the 1800’s that nutrition was key; poor nutrition led to males, good nutrition to females. There were also other theories that wouldn’t be corrected until Steven’s work, which was done long after Darwin’s famous Origin of Species).

Sex Differentiation. We all start as females, and then our genes express. The study of how genes express is called “epigenetics,” a field that Stevens, Wilson, and Morgan founded.

Stevens and the XY Discovery

Stevens was studying insect reproduction at Bryn Mawr College where she achieved an associates degree in experimental morphology (1905–1912). Her main focus was on “how or if sex was passed on through genetic inheritance.”[8]

By observing insect chromosomes such as the mealworm beetle, Stevens discovered that, in some species, chromosomes are different among the sexes.

The discovery was the first time that observable differences of chromosomes could be linked to an noticeable difference in physical attributes (i.e., whether an individual is male or female).

Stevens observed that the female mealworm’s cells had 20 large chromosomes. The male had 20 chromosomes as well, but the 20th was notably smaller than the other 19.

She correctly deduced that this could be traced back to differences in the male’s sperm. She theorized that the sperm that contained the smaller chromosome determined male sex and when all are equal size, the female sex resulted. She deduced that the chromosomal basis of sex depended on the presence or absence of the Y chromosome.

Even though she didn’t call them XY, she had correctly for the first time discovered how sex determination worked, successfully expanding the fields of embryology and cytogenetics.

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). – Nettie Stevens: A Discoverer of Sex Chromosomes

How is that Different From Wilson’s Discovery? Who Deserves Credit?

Wilson published first, but Wilson’s theories were off-base. He believed environmental factors played a role, and he notably only studied the male insects and not the females.

Stevens was the first to recognize that females have two large sex chromosomes, but importantly Wilson published first and had been working on the problem longer. The effect of this was that Wilson was celebrated as the discoverer of the XY, while Stevens was marginalized by history.

The other aspect is, of course, the Matilda effect, where women’s roles are downplayed. Regardless of what happened in the past, today we are seldom so fickle as to judge contributions depending on whether a scientist has XX or XY chromosomes. Also, we know a lot more about sexual differential today than either did at the time (see all mammals start as “female” and people are born gay).

As noted above, Wilson ultimately reissued his original paper, correcting his mistakes, and thanking Stevens for her finding confirming the chronology and Steven’s contribution.

Today we can say that Steven’s made the discovery, but we should be quick to note multiple discoveries and tell the full story which includes all three researchers at Bryn Mawr.

TIP: See Vox’s Nettie Stevens discovered XY sex chromosomes. She didn’t get credit because she had two Xs for a different take on the gender issue over the discovery of gender expression.

FACT: Another similar story, where an early woman of note gets “just a little more credit than the story merits” (likely as a response to previously going without as much credit as she deserved), is Ada Lovelace “the second programmer” and “prophet of the computer age.” Ada is often called “the first programmer,” but history has shown her mentor Charles Babbage created the first computer program 5 – 6 years earlier.


Nettie Stevens can be said to have discovered the chromosomal XY sex-determination system, but just barely. She found the missing piece of the puzzle, but the story can’t be properly told without including the other contributors (who came first and mentored Stevens) Wilson and Morgan.

Today we know sex determination is a bit more complex than XX/XY, as is the story of its discovery, but we can still accurately credit Stevens with discovering the sex determination system.


  1. XY sex-determination system
  2. Nettie M. Stevens and the Discovery of Sex Determination by Chromosomes
  3. Nettie Stevens
  4. Edmund Beecher Wilson
  5. Thomas Hunt Morgan
  6. Bryn Mawr College
  7. Sexual differentiation
  8. Nettie Stevens discovered XY sex chromosomes. She didn’t get credit because she had two X’s.

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