Bill Nye explains the basics of heredity in this video clip. Gregor Mendel was a monk who was curious about science. He conducted a series of experiments using generations of pea plants that were the first genetic experiments.
What traits in humans correspond to dominance / recessive rules (controlled by single gene). This is highly generalized, most traits are result of multiple genes, so it's not like there's only two alleles, one recessive, and one dominant.