Final answer:
An allele that is expressed in the phenotype even in the presence of a recessive allele is called a dominant allele. It overshadows the recessive allele in heterozygous genotypes, resulting in the dominant trait being visible.
Step-by-step explanation:
An allele that expresses its phenotype even when heterozygous with a recessive allele is termed dominant. This means that in a heterozygous genotype, such as 'Aa', where 'A' is dominant and 'a' is recessive, the dominant allele's trait will be observed in the phenotype. In this scenario, 'A' would mask the effect of the recessive 'a' allele. For example, if we look at Mendel's pea plants, a heterozygous pea plant (Tt) will be tall because the dominant 'T' allele for height overshadows the recessive 't' allele. On the other hand, the recessive trait, such as dwarfism represented by 'tt', only appears in the phenotype when both alleles are recessive. In summary, dominant alleles manifest in the organism's traits whether they are in a homozygous ('AA') or a heterozygous ('Aa') state, while recessive alleles only show their effect when present in pairs without the dominant allele ('aa').