Mendel allowed the first generation of plants to self pollinate. He found for every three dominant traits offspring, there was one recessive offspring. From that he knew the dominant alleles would take over.
He investigated two characteristics at a time, so he crossed plants with yellow round seeds and plants with green wrinkled seeds.
The plants phenotype determined if it was gonna be green or yellow. Its genotype determined whether it was gonna be wrinkled or not. He found that 1/4 of the offsprings were heterozygous and the other 3/4 were homozygous.