What did Mendel cross pollination of pea plants prove

That pea plants have both male and female reproductive organs. As a result, they can either self - pollinate themselves or cross-pollinate with another plant. In his experiments, Mendel was able to selectively cross-pollinate purebred plants with particular traits and observe the outcome over many generations.

That you can predict the outcome of two chromosomes mixing depending on whether or not the chromosomes are heterozygous or homozygous. For example an PPxPP cross would have a 100% chance of being purple while a ppxpp cross would have a 100% chance of being white