A classic example of dominance is the inheritance of seed shape (pea shape) in peas. Peas may be round (associated with allele "R") or wrinkled (associated with allele "r"). In this case, three combinations of alleles (genotypes) are possible: "RR" and "rr" are homozygous and "Rr" is heterozygous. The "RR" individuals have round peas and the "rr" individuals have wrinkled peas. In "Rr" individuals the "R" allele masks the presence of the "r" allele, so these individuals also have round peas. Thus, allele "R" is completely dominant to allele "r", and allele "r" is recessive to allele "R". Incomplete dominance (also called "partial dominance", "semi-dominance" or "intermediate inheritance") occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes. For example, the snapdragon flower color is homozygous for either red or white. When the red homozygous flower is paired with the white homozygous flower, the result yields a pink snapdragon flower. The pink snapdragon is the result of incomplete dominance. A similar type of incomplete dominance is found in the four o'clock plant wherein pink color is produced when true-bred parents of white and red flowers are crossed. In quantitative genetics, where phenotypes are measured and treated numerically, if a heterozygote's phenotype is exactly between (numerically) that of the two homozygotes, the phenotype is said to exhibit "no dominance" at all, i.e. dominance exists only when the heterozygote's phenotype measure lies closer to one homozygote than the other. When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Pink:White). See partial dominance hypothesis. Co-dominance occurs when the contributions of both alleles are visible in the phenotype. For example, in the ABO blood group system, chemical modifications to a glycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other ("IA", "IB") and dominant over the recessive "i" at the ABO locus. The "IA" and "IB" alleles produce different modifications.