I concur that the answer is Genetics
By using the pea plants he was able to see the different traits they had based on the cross pollination that he did. How did the offspring of the plants look like the adult plants and how did they look different. Did one plant have dominate traits while the other had heterozygous traits or did they have recessive traits. These are the questions Mendel wanted to answer when experimenting with the pea plants.
hope this helped and good luck!!