What does the term 'codominant' mean?

The term 'codominant' refers to a genetic scenario where both alleles in a heterozygous individual contribute equally and visibly to the phenotype. This means that when two different alleles are present, neither is recessive; instead, both traits can be seen simultaneously.

For instance, in the case of blood types, a person with one allele for type A and one for type B will express both traits, resulting in blood type AB. Here, both the A and B characteristics are fully represented in the phenotype, illustrating the concept of codominance. This distinguishes codominance from other genetic interactions, such as complete dominance, where one trait would completely mask the presence of another.

In contrast, the other options describe different genetic principles: one suggests dominance in which only one trait is visible, another implies that one trait might completely overshadow another, and the last option refers to environmental influences on trait expression, which does not pertain to the concept of codominance.