Which inheritance pattern is characterized by both alleles contributing equally to the phenotype?

The inheritance pattern characterized by both alleles contributing equally to the phenotype is known as co-dominance. In co-dominance, neither allele is dominant or recessive, and when both are present in a heterozygote, they manifest visibly and distinctly in the phenotype. A classic example of co-dominance can be observed in certain blood types, where both A and B alleles can be expressed simultaneously, resulting in an AB blood type rather than a blend or one overpowering the other.

In contrast, complete dominance occurs when one allele completely masks the effect of another allele, so only the dominant trait is expressed in the phenotype. Incomplete dominance refers to a situation where the phenotype of a heterozygous individual is a blend of the two alleles, rather than both being expressed distinctively as in co-dominance. Multiple alleles refer to the presence of more than two alleles for a particular gene in a population, but this concept does not specifically address the expression of phenotypes in heterozygous individuals.