A young woman in the mid-1900s couldn’t take an at-home pregnancy test. Instead, she sent a vial of urine to a clinic, where a technician would, of all things, inject it into a frog, and hormones in the urine would cause the frog to lay eggs. This frog-based test was far faster, easier, and cleaner than any pregnancy test before, and it shifted power for family planning from doctors to women themselves.

After scientists had a handle on how many chromosomes humans have, other researchers began exploring whether certain ailments might be caused by chromosomal abnormalities. To this end, a French cardiologist discovered that Down syndrome was caused by the presence of an extra chromosome in humans. But a colleague stole credit for her work, and the battle over their legacies continues to this day, in part because the colleague is on track to become a certified Catholic saint.

It seems like a simple question: how many chromosomes do human beings have? But getting an accurate count proved surprisingly hard for much of last century. In fact, virtually every textbook once cited an incorrect number, until in 1956, a fiery Indonesian scientist finally determined the true count—and had to battle his boss over who would receive credit for this legacy-making discovery.