Final answer:
The phenotype of heterozygotes may be altered due to incomplete dominance, where a blend of traits is exhibited, or codominance, where both alleles are fully expressed, such as in the ABO blood group system.
Step-by-step explanation:
The phenotype of heterozygotes may be affected because of different patterns of gene expression such as incomplete dominance and codominance. In the case of incomplete dominance, a heterozygote (e.g. Rr) exhibits an intermediate phenotype between the two homozygous phenotypes (RR - dominant, rr - recessive). This results in a blending of traits, such as pink flowers in snapdragons when red and white flowering plants are crossed. On the other hand, codominance occurs when both alleles in the heterozygote are fully expressed, as seen in the human ABO blood group system. An individual with genotype AB expresses both A and B antigens on red blood cells. Hence, the genetic complexity of alleles and their interactions in heterozygotes can influence the resulting phenotype.