How did Mendel arrive at his discoveries?

There are few historical records concerning Gregor Johann Mendel and his work, so theories abound concerning his motivation. These theories range from Fisher's view that Mendel was testing a fully formed previous theory of inheritance to Olby's view that Mendel was not interested in inheritance at all, whereas textbooks often state his motivation was to understand inheritance. In this Perspective, we review current ideas about how Mendel arrived at his discoveries and then discuss an alternative scenario based on recently discovered historical sources that support the suggestion that Mendel's fundamental research on the inheritance of traits emerged from an applied plant breeding program. Mendel recognized the importance of the new cell theory; understanding of the formation of reproductive cells and the process of fertilization explained his segregation ratios. This interest was probably encouraged by his friendship with Johann Nave, whose untimely death preceded Mendel's first 1865 lecture by a few months. This year is the 200th anniversary of Mendel's birth, presenting a timely opportunity to revisit the events in his life that led him to undertake his seminal research. We review existing ideas on how Mendel made his discoveries, before presenting more recent evidence.