Final answer:
An additive allele contributes to the phenotype in a cumulative manner, while a non-additive allele does not. Non-additive alleles may interact with other alleles or environmental factors to produce unique phenotypes.
Step-by-step explanation:
An additive allele and a non-additive allele are two types of alleles that interact differently to produce phenotypes in organisms. In genetics, an additive allele is one that contributes in a cumulative manner to the phenotype. Each copy of the additive allele adds to the overall effect. For example, if a gene controls flower color and the additive allele contributes to the production of pigments, then more copies of the additive allele will result in darker coloration.
On the other hand, a non-additive allele does not contribute in a cumulative manner to the phenotype. The presence of one or more non-additive alleles can lead to a different phenotype than expected based on the traditional dominant/recessive relationship. Non-additive alleles may interact with other alleles or environmental factors to produce unique phenotypes. For example, in the case of incomplete dominance, a non-additive allele may result in an intermediate phenotype between the two homozygous genotypes.