In the 20th century, a young American biologist and geneticist was motivated to solve a scientific mystery that had bewildered humanity for ages. The mystery — How do males and females come to be? It is all thanks to Nettie Stevens that we know today that sex is hereditary and the father’s sperm determines the sex of the offspring. Born on July 7, 1861, Nettie grew up during the civil war, a time when women and their scientific innovations were ruled out. When she graduated at 19, she went on to become a teacher but she longed to pursue education further.
She taught for three terms, while simultaneously saving up to attend the Westfield Norma School, a teachers’ college. By 1900, at the age of 35, she had earned a bachelor’s and master’s degree from Stanford. Thereafter, she pursued PhD at Bryn Mawr in Pennsylvania. It was here that she diverted her attention to solving the mystery of sex determination. She made observations with a microscope of the chromosomes in Tenebrio Molitor - the mealworm beetle.
It is here that she discovered that both the male and female mealworm’s cells had 20 large chromosomes. But the 20th chromosome of the male was notably smaller than the other 19. In a report summarising findings, Nettie wrote that it seemed to be a clear case of sex determination.
Thus, she concluded that the sperm had either the small version of the 20th chromosome or the large one. The spermatozoa which contain the small chromosome determine the male sex and those that contain chromosomes of equal sizes determine the female sex.
However, she did not name these chromosomes X or Y. Moreover, despite her discoveries, Nettie’s colleague and mentor EB Wilson is commonly known as the discoverer of the sex chromosomes. He was working on the same topics as Nettie and even published similar results around the same time. But Wilson had worked on a species where the make had one chromosome less than the female, which is not common in nature.
However, it is generally believed that Wilson obtained the same results as Nettie. And further owing to his higher reputation and his paper being published before Nettie, he is credited with the discovery. Even though their papers were similar, it was Nettie’s discoveries that presented a stronger and eventually the correct conclusion. Unfortunately, nine years after completing her PhD, she died of breast cancer on May 4, 1912.