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This month the scientific community celebrated the 155th birthday of the late Nettie Stevens.

 Nettie Maria Stevens, born July 7 1861, discovered that male sperm are the key to determining the gender of offspring.

In a time when career choices for women were limited to teaching, nursing or secretarial work, Nettie strove to defy the norm and realise her passion: scientific research. A passion which made her one of the most prolific researchers of the late 19th century, she published around 40 papers in her field in 11 years.

Nettie was a biologist, with a particular interest in gender determination. She studied mealworms, examining their sperm and ova (egg cells) under a microscope. She discovered that where the female’s eggs only carry X chromosomes, the male’s sperm carry both X and Y. Nettie was the first to realise the significance of this, that the gender of offspring is determined by fertilisation of the egg by the sperm.

Nettie worked as a teacher to earn the money she needed to pay for her further education – her Bachelor’s and Master’s degrees at Stanford University, and her PhD at Bryn Mawr College in Pennsylvania.

It was widely believed that the gender of offspring was either determined by the mother, or by other environmental factors. This meant that Nettie’s theory was not accepted by her peers in education or science at first, but she wasn’t the only one who made the discovery.

Edmund Wilson, a zoology professor at Columbia University, made the same discovery as Nettie in his research. Though Nettie is credited with making the theoretical leap, which has since been proven true. Both developed theories on sex determination by chromosomes at the same time, and so both are credited with this ground-breaking discovery.

Nettie’s prolific research career was over too soon. She died of breast cancer on May 4 1912, at the age of 50.
Her substantial contribution to embryology and genetics will never be forgotten.