What type of allele will only express a phenotype if the genotype is homozygous?

A recessive allele only expresses a phenotype when the organism has two copies of that allele, which is referred to as being homozygous for that trait. In a heterozygous genotype, where there is one dominant and one recessive allele, the dominant allele masks the effect of the recessive one, preventing the recessive phenotype from being expressed. This characteristic is essential to understanding inheritance patterns, as it defines how certain traits are passed down and expressed in offspring. In contrast, dominant alleles will show their effect even in a heterozygous state, codominant alleles express both traits simultaneously in a heterozygous state, and the term "multiple" refers to multiple alleles existing for a gene rather than the dominance or recessiveness of specific alleles.