He began with the observation that most living creatures had differences among them, and most likely wondered why this was so. So he set up the experiment of cross-breeding pea plants with the hypothesis that this may be a result of different genes (which I don't think he called genes) pairing. And so he conducted his experiments, recorded his observations throughout these experiments, and came to a conclusion after meticulous scrutinizing, that there was a genetic factor. He came up with similar results in later tests of the same nature in order to further prove his point. And so he laid the groundwork for modern genetics.