Gregor Mendel is credited with being the first person to carry out any systematic work on genetics. He carried out a number of experiments in the 1850s and 60s on inheritance in pea plants.
He crossed red flowering plants with white flowering plants, tall ones with short ones and yellow pods with green pods, amongst others.
In each case he was looking at the inheritance of a single characteristic and in each case he grew hundreds of plants.
Whatever characteristic he chose, he always obtained similar results.
His findings for the size of the plants
He started with pure breeding parent plants and crossed tall plants with short plants.

When he grew the seeds from this cross all the plants in the first generation (the F1) were tall.