Most famous for her discovery of the chromosomal difference that determines whether an animal is male or female, Nettie Stevens is one of the first American women actually recognized for her contribution to science!
She was born on this day in 1861. She studied at Westfield Normal School in Massachusetts, graduating at the top of her class, and she earned B.A. And M.A. Degrees from Stanford. Finally, at Bryn Mawr, Stevens studied insects and was able to identify the Y chromosome in a species of mealworms. She completed her PhD at Bryn Mawr in 1903.
You might know that most mammals have two sex chromosomes. Those individuals who are male (men and boys) have one X and one Y chromosome. Females (women and girls) have two X chromosomes. Some insects such as fruit flies also have X and Y chromosomes. Many animals do not have sex chromosomes, and in many of these creatures sex is determined by the environment. For example, if an egg is kept at one particular temperature, the developing creature becomes male; at another temperature, the creature develops into a female. Some animals have both male and female gametes and are either born male, later becoming female if necessary, or vice versa. Clownfish like Nemo fall into this category. Many animals and most plants don't have separate sexes at all.
Older students might like to read a Smithsonian article on sex determination.