Final answer:
The phenomenon whereby a dominant allele shows different effects in heterozygous and homozygous individuals is called incomplete dominance. Incomplete dominance refers to an intermediate phenotype being expressed in heterozygotes, as opposed to complete dominance where a dominant allele masks the recessive allele in heterozygotes.
Step-by-step explanation:
When a dominant allele has different effects between heterozygous individuals and homozygous individuals, the phenomenon is called incomplete dominance. This situation arises when the heterozygote expresses a phenotype that is intermediate between the two homozygous phenotypes. For example, if we consider flower color, a cross between a plant with red flowers (homozygous dominant) and a plant with white flowers (homozygous recessive) results in offspring with pink flowers (heterozygous), which is an intermediate phenotype. Unlike in complete dominance where the dominant allele completely masks the presence of the recessive allele in heterozygotes, in incomplete dominance, the phenotype is somewhere between the two.
In the case of codominance, another exception to Mendelian inheritance, both alleles are simultaneously expressed in the heterozygote. An example of codominance is the human ABO blood type, where individuals with the genotype IAIB express both A and B antigens on their red blood cells.