Which of the following terms is a result of two alleles that combine but neither dominant?

The correct term for a situation where two alleles combine but neither is dominant is codominant trait. In codominance, both alleles express themselves fully and independently without masking the presence of the other. This means that an organism with one allele for blood type A and another for blood type B will express both antigens on the surface of its red blood cells, resulting in a blood type of AB. This phenomenon clearly illustrates the concept of codominance, where both traits are equally expressed.

In contrast, heterozygous traits simply refer to the presence of two different alleles for a single trait, without implying any specific interaction between them. Homogeneous traits (often referred to as homozygous traits) involve two identical alleles, and recessive traits are those that only manifest in the absence of a dominant allele. Thus, these terms do not capture the unique phenomenon of both alleles being equally expressed as codominance does.