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?
Answer
Independent assortment. Mendel showed that his genetic markers for different traits, or phenotypes are transmitted randomly to the F1 generation. This was before the concept of linkage was discovered, so fortunately he selected unlinked genes.
Segregation of alleles. The dominant and recessive alleles segregate away from each other when the germ cells are created, therefore each parent has two copies of every Gene.
Distinguishing phenotype from genotype in a predictable and testable framework.
The test cross. It was only careful record keeping and back crosses that revealed the recessive copies of mutant alleles in heterozygotes.
One description of his contributions is available on Wikipedia. Other sites include MendelWeb, Learn Genetics,
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