Determination of sex in humans was a puzzle until a woman who was a cytological researcher uncovered the reason. Nettie Stevens was born in Vermont on July 7, 1861, just as the Civil War started to get serious (by the end of the month the Union would taste its first defeat at the First Battle of Bull Run.) Her father was a carpenter, but did well enough that he was able to send his children to college. Nettie was a prodigy and finished four years of school at Westfield Normal School in two. In a time when women were not considered bright enough to compete with men in art, science or literature, she stood out as a competent teacher and researcher.
She finished her B.A. at Stanford in 1899, after a stint of teaching, and followed that with an M.A. in 1900! She continued her studies in cytology at Bryn Mawr College. She also studied marine biology abroad at the Naples Biological Station and Helgoland. She worked for a time at Theodor Boveri's lab at the Zoological Institute at Würzburg, Germany. She was much influenced by Thomas Hunt Morgan, the head of the Biology Department at Bryn Mawr College, as well as his predecessor Edmund Beecher Wilson. In 1903 she finished her Ph.D. from Bryn Mawr. In her studies at Bryn Mawr she discovered that in some animals the sexes had slightly different chromosomes. Her studies were based on mealworms, which have x and y chromosomes and she concluded that the male thus determined the sex of the progeny as males were xy and females xx. This led to the discovery of the chromosomal differences between the sexes in humans; in essence she discovered one of the functions of the y and x chromosomes in many organisms, including man. We now know that sex determination can vary within the animal kingdom, with some animals having zw in females and zz in males (birds) and in others (notably many of the Hymenoptera) the female is diploid and the male is haploid and thus lack sex chromosomes.
She died of breast cancer at the age of 50 in 1912. Morgan, who had praised her to the skies as the very best graduate student he had ever encountered, wrote a somewhat denigrating obituary implying that she was only a technician and not a real scientist.
Internet References:
Nettie Maria Stevens http://www.dnaftb.org/...
Nettie Stevens: A Discoverer of Sex Chromosomes http://www.nature.com/...
Nettie Stevens http://en.wikipedia.org/...