Gregor Mendel was probably the first to study intensively how genetic traits were passed on from parents to offspring. Mendel studied peas and noticed that from one set of parents, there is a possibility of producing four different kinds of offspring. Below are two study guides on Mendel and his peas.
You may want to also look into one or each of the first acknowledged geneticists after Mendel; since their own work and their re-discovery of his work is what propelled his findings into acceptance. Their names are: Hugo de Vries, Carl Correns and Erich von Tschermak.