What type of inheritance results in both traits being expressed in the offspring?

Codominance is the type of inheritance where both alleles in a heterozygous condition are fully expressed, resulting in offspring that display both traits simultaneously. This means that neither allele is dominant over the other, and as a result, the phenotype of the offspring exhibits characteristics of both parents distinctly. A classic example of codominance can be observed in the ABO blood group system, where individuals with one A allele and one B allele will express both antigens on the surface of their red blood cells, demonstrating both traits at the same time.

In contrast, complete dominance occurs when one allele completely masks the effect of another allele, leading to a phenotype that reflects only the dominant trait. Incomplete dominance leads to a blending of traits, where the resulting phenotype is a mixture of the two parental traits rather than full expression of both, which distinguishes it from codominance. Simple dominance is often synonymous with complete dominance, reinforcing the idea that only one trait gets expressed in the phenotype. Thus, codominance specifically describes the scenario where both traits are clearly manifested in the offspring, making it the correct choice.