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Today is the birthday of Nettie Maria Stevens (1861-1912), whose discoveries helped researchers understand chromosomal sex determination. In the early part of the 20th century, many biologists believed that as an embryo develops, factors that a baby is exposed to during gestation (such as the mother’s diet) influence whether the baby will be male or female. But Stevens and her colleagues helped demonstrate that the chromosomes we inherit determine sex, and that this happens at the moment of fertilization, not over the course of development. Stevens worked with organisms like aphids and mealworms, studying their reproductive cells. Her contemporaries (like E.B. Wilson) also helped show that chromosomes determine sex. But Wilson at first proposed that males have one less chromosome than females (X,O for males; X,X for females). Stevens, on the other hand, correctly showed that in most species (including humans) females have two full X chromosomes (X,X) and males have one X chromosome and one smaller Y chromosome (X,Y).

Gregor Mendel’s research on inheritance was rediscovered around 1900, so the work of Stevens and her contemporaries – including Thomas Hunt Morgan – helped establish the basic mechanisms of inheritance that Mendel had observed. You can learn more about Mendel’s work, see photos of Mendel among his fellow monks, and peer into Thomas Hunt Morgan’s fly laboratory by visiting the interactive timeline exhibit in the DNAtrium. Find out more about the Ó³»­´«Ã½â€™s DNAtrium or check out  in this Boston Globe post.