Nettie Stevens

The GENETICS PIONEER who discovered what makes animals male or female

Late starter

Born in Vermont, USA, in 1861, Nettie Stevens worked as a teacher before becoming a scientist in her thirties. At the time, nobody understood what made an animal male or female. Stevens began RESEARCHING CHROMOSOMES, the thread-like structures made of DNA inside a cell’s nucleus. Chromosome

Cell

A cell’s nucleus contains chromosomes.

Stevens solved a mystery that had long puzzled scientists by proving that an animal’s sex is determined by genetics and not external factors. How she changed…

the world

X and Y

By studying mealworms, Stevens found that males produce sperm containing either X OR

Y CHROMOSOMES, while females produce eggs with only X chromosomes. If an egg is fertilized by a sperm with the Y chromosome, a male offspring is created. If the sperm has an X chromosome, then the offspring is female.