Blending occurs as a result of incomplete dominance, meaning that both alleles for a trait are dominant. In order for blending to take place, there must be two different alleles for a trait; the organism must be hybrid, or heterozygous. This results in a third phenotype, which is a "blend" of the two traits. If the organism was homozygous, the alleles would be the same, so they would either be both recessive or dominant, no incomplete dominance would occur, and there would be no "blended" phenotype since both alleles were the same.
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