Final answer:
An allele that affects the phenotype in both homozygous and heterozygous state is often a dominant allele, although expressions like codominance and incomplete dominance also exist, which result in unique phenotypic presentations.
Step-by-step explanation:
An allele that affects the phenotype whether it is in a homozygous or heterozygous state is typically described as a dominant allele. In a dominant-recessive pattern, the dominant allele will express the trait in both homozygous dominant (AA) and heterozygous (Aa) genotypes, while the recessive allele (a) is only expressed in the homozygous recessive (aa) genotype. However, there are situations in which different patterns of inheritance can be observed, such as codominance and incomplete dominance.
Codominance is a situation where both alleles in the heterozygote (Aa) are equally expressed in the phenotype, as seen with human blood types A and B. Incomplete dominance is where the heterozygote (Aa) displays a phenotype that is intermediate between the two homozygous phenotypes. For example, a cross between red and white snapdragons results in pink flowers, showcasing incomplete dominance.