The difficulties of Gregor Mendel was that he found out what happened to the pea plants (etc) but when he told the scientists, they didn't believe him since he was a priest. This was what was difficult.
Gregor Mendel took two different colored pea plants: one had traits for white plants and the other had traits for a red plant. When Mendel cross bred the two plants, a plant with traits for a pink plant grew. This is how Mendel contributed to the understanding of inherited traits.