Mendel discovered that an organism receives two forms of a gene called allele from each parent. This alleles combine to form the genotype of that organism, which in turn determines the phenotype.
Mendel crossed purebreeding plants that produced only tall and short plants respectively. He got an all tall F1 offspring. He then self-crossed the F1 offsprings to get a phenotypic ratio of 3:1 in the F2 offsprings. He discovered that the tall allele was dominant over the short allele for the height gene in pea plants. This is because the tall alleles masked the expression of the short allele in the F1 generation making them all exhibit tall traits.
The F1 are regarded as hybrids or heterozygotes because they were a mixture or combination of two different alleles. This mixture was confirmed in the F2 offsprings that produced 3 tall plants and 1 short plant, showing that the recessive allele was still present but masked in the heterozygotic combination.
