Dominance was first described and named by Gregor Mendel in his well-known studies of pea plants. In modern terms, Mendel's Law of Dominance describes the interaction between the two alleles of a gene in a given individual such that the dominant allele masks the influence of the recessive allele on the individual's phenotype. In addition to completely dominant and recessive, alleles may be incompletely dominant—resulting in an intermediate phenotype—or co-dominant—resulting in the expression of both alleles simultaneously.