Co-dominance

Co-dominance is an inheritance pattern in which two different alleles of a gene are both fully expressed in a heterozygote. Instead of one allele being dominant over the other, the heterozygous individual shows both traits simultaneously.

• Classic example: human blood type AB. The A allele and B allele are co-dominant, so someone with one A and one B allele has type AB blood - expressing both A and B antigen traits at the same time.
• Do not confuse co-dominance with incomplete dominance. In co-dominance, both allele products are seen side by side (no blending). For instance, a red and white spotted flower or AB blood type shows co-dominance. In incomplete dominance, by contrast, alleles blend to produce an intermediate phenotype (like pink flowers from red and white parents).
• Genetically, co-dominant alleles are often denoted with two different capital letters (since neither is recessive). For example, the MN blood group alleles can be written as <em>L^M</em> and <em>L^N</em>, and an <em>L^ML^N</em> individual expresses both M and N antigens on red blood cells.

• If a question describes a heterozygote showing both phenotypes distinctly (e.g., an animal with patches of two colors, or a person with an AB blood type expressing two antigens), that's a hallmark of co-dominance.
• Exams often test the difference between co-dominance and other patterns. Recognize that co-dominant inheritance means no allele is masked: a heterozygote manifests both traits fully. For example, seeing "both alleles contribute equally to the phenotype" in a question stem indicates co-dominance.
• Blood type problems are common: e.g., a question might ask what offspring blood types are possible if one parent is type A (AO genotype) and the other is type B (BO). An AB blood type child in this scenario illustrates co-dominance (expressing both A and B).