We've known that offspring inherit various traits from their parents since (at least) Aristotle. In The Elements of Plant Hybridization, Gregor Mendel treats that fact as common knowledge. Clearly, we don't credit Mendel with discovering it. Accordingly, it seems to me that Mendel's only notable discovery was the discovery of the existence of dominant and recessive genes. However, that discovery doesn't seem to justify the title 'founder of modern genetics', which Wikipedia, and other sources, use to refer to him.
Why is Mendel the big deal that he is?