In the middle of the 19th century, an Augustinian monk named Gregor Mendel took up the question of biological inheritance with a series of experiments. Mendel had a naturally inquisitive mind and a profound love of nature. His scientific interests ranged from research on plants to meteorology and the theories of evolution. Being at a monastery in what is now the Czech Republic, Mendel started by crossbreeding different strains of garden peas than observing the characteristics of their offspring. Why choose peas? He did it, he said, for the fun of the thing. Mendel noticed that when he crossed a round pea seed with a wrinkled one, the offspring were round, not a mix of the two characteristics as he had expected. That money bred the round pea offspring, that's where the mix appeared, and the second generation had both round and wrinkled seeds. He continued to experiment, trying to understand what kind of biological mechanism would cause certain characteristics to disappear in the first generation, only to reappear in the second. Then one day, Mendel counted the number of peas in the second generation that had the wrinkled characteristic. The one quarter of the peas were wrinkled. What Mendel observed in his experiments were the biological phenomena we now refer to as dominance and segregation, only Mendel didn't know it yet. Still, his experiments produced a curious set of facts, which, as he said, forced themselves upon my notice. You see, no matter how he crossbred the various strains of peas, the hidden characteristics showed up, but only in one quarter of the second generation. For Mendel, here was the breakthrough. For the first time, he could demonstrate that the traits of successive generations were inherited in certain numerical ratios. In other words, there were fixed laws of nature that governed heredity. With this insight, Mendel made the first great discovery in the science of genetics. Such inherited characteristic must be decided by a pair of what he called factors. Each parent, he said, contributes one factor for each characteristic. Certain factors are dominant and others recessive, depending on the combination of the factors the offspring inherits. Mendel's factors are called genes. The term Mendelian trait is used to describe a characteristic caused by a single gene that sometimes reappears in one quarter of the offspring. That characteristic can be innocuous, such as freckles or the ability to curl your tongue. But it also can lead to serious illnesses, like cystic fibrosis or TASAX disease. Imagine all that from one man's work with the humble pea, and was experiments with another humble species that produced our next great discovery.