What combination of alleles exists in snapdragon plants that show partial dominance?

In snapdragon plants, partial dominance occurs when the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygous parents. This means that if one parent has red flowers and another has white flowers, the heterozygous offspring will exhibit a blend of both traits, such as pink flowers.

The presence of two different alleles, specifically one for red and one for white, leads to the expression of a mixed phenotype, which is characteristic of partial dominance. Therefore, being heterozygous for both red and white directly results in this intermediate trait, making it the correct answer.

The other options represent scenarios that do not embody the concept of partial dominance. For instance, being homozygous for one color or being purebred for either white or yellow would not produce an intermediate phenotype, thus failing to illustrate the partial dominance seen in heterozygous plants. In contrast, two identical dominant alleles would lead to complete dominance, where the offspring would exhibit the phenotype of the dominant allele only without any blending.