Step 1: Define co-dominance.
Co-dominance is the full and equal expression of two alleles in a heterozygote, without any blending.
Step 2: Classic example.
A prime example is the human ABO blood group system, where the IA and IB alleles are co-dominant, resulting in the AB blood group.
Step 3: Check options.
- Co-dominance: Correct; both alleles are expressed.
- Partial dominance: Incorrect; the heterozygote displays an intermediate phenotype.
- Pseudo dominance: Incorrect; this occurs when a recessive allele is expressed due to the deletion of a dominant allele.
- Over dominance: Incorrect; the heterozygote expresses a trait more strongly than either homozygote.
Step 4: Conclusion.
Therefore, when both alleles of a gene are fully expressed in heterozygotes, the condition is termed Co-dominance.