Which of the following defines co-dominance in genetics?

Co-dominance occurs when both alleles in a heterozygous organism are fully expressed, resulting in a phenotype that showcases both traits distinctly and equally, rather than blending them into a single new trait. This means that neither allele masks the other, allowing both to contribute to the organism's phenotype in a way that is visually evident. A classic example of co-dominance can be seen in certain blood types, such as AB blood type, where both A and B alleles are expressed in the phenotype, showing characteristics of both.

In contrast, blending of parental phenotypes describes a different genetic scenario, which is not reflective of co-dominance. Complete dominance occurs when one allele overshadows another, suppressing its expression entirely, unlike the scenario in co-dominance where both traits are visible. Lastly, the expression of only recessive traits would imply the absence of any dominant allele expression, which is not applicable to co-dominance, where both alleles are active in defining the phenotype.