Final answer:
The heterozygote phenotype in incomplete dominance, such as that seen in snapdragons with pink flowers, appears as an intermediate between two parent phenotypes, while in codominance, such as the human ABO blood type, both alleles are expressed equally without blending.
Step-by-step explanation:
When considering heterozygote phenotypes, it is essential to differentiate between incomplete dominance and codominance. In incomplete dominance, a heterozygote exhibits a phenotype that is intermediate between the phenotypes of both homozygous parents. For example, in the case of snapdragons, a cross between a pure red flower (CRCR) and a pure white flower (CWCW) results in a pink flower (CRCW), which is neither completely red nor white but an intermediate pink color. On the other hand, codominance occurs when a heterozygote expresses both alleles simultaneously and equally. A classic instance is the human ABO blood grouping system, where an individual with genotype AB exhibits both Type A and Type B antigens in their blood cells. Hence, a person with AB blood type expresses the phenotypes of both alleles without blending. The sickle cell trait is another example where a heterozygous person (Ss) will have some misshapen sickle cells and some normal cells, demonstrating simultaneous expression of both recessive and dominant allelic traits.