Nettie Stevens

Born on July 7, 1861. She was an American  geneticist whose research transformed the understanding of biological sex determination by her discovery of sex chromosome 

Through careful microscopic studies, she discovered that sex is determined by specific chromosomes, now known as X and Y.

In 1905, she observed that male mealworms produced two kinds of sperm, one with a large chromosome and one with a small chromosome. When the sperm with the large chromosome fertilized eggs, they produced female offspring, and when the sperm with the small chromosome fertilized eggs, they produced male offspring

 At the time, this was a groundbreaking insight that challenged previous assumptions about heredity.

Although her work paralleled that of Thomas Hunt Morgan, she received less recognition during her lifetime.

Her discovery laid the foundation for modern genetics and chromosomal research.

 Today, she is recognized as a key figure in the field of biology.