Co-dominance
கூட்டு ஓங்குதன்மை
Do not confuse Co-dominance with Incomplete Dominance. In Incomplete Dominance, the phenotype is a blend; in Co-dominance, both alleles express themselves fully and independently. This distinction is a high-yield trap in NEET genetics questions.
The idea
Co-dominance occurs when the F1 generation shows traits of both parents simultaneously. In human ABO blood grouping, the alleles IA and IB are both dominant. When both are present in the genotype, they produce both A and B types of sugar on the surface of red blood cells, resulting in AB blood type.
Why it matters
It exists when both alleles in a heterozygote are fully expressed, ensuring that both gene products contribute to the phenotype.
NEET lens
NEET focuses on the ABO blood group as the primary example of multiple alleles and co-dominance. Memorize that IA and IB are co-dominant, while i is recessive. Expect calculation-based questions involving parent-offspring blood type crosses. Do not waste time on complex definitions; focus on the expression of both alleles.
Clinical significance
ABO blood group incompatibility is a critical clinical reality. If a patient with type AB blood receives a transfusion of type A or B, the immune system recognizes the 'foreign' sugar and triggers a severe hemolytic reaction. Precise identification of co-dominant antigens is mandatory for safe clinical practice.
Introduction
Co-dominance is a mode of inheritance where both alleles in a heterozygote are fully and simultaneously expressed in the phenotype, with neither allele being dominant over the other — the classic example being ABO blood groups.
Core concept
Mechanism: Both alleles produce functional proteins that are both detectable in the phenotype. Neither masks the other. Example 1: ABO blood groups: IA allele: produces A antigen. IB allele: produces B antigen. IAIB (heterozygote): produces BOTH A and B antigens → Blood Group AB. Both alleles fully expressed → co-dominant. i allele (neither A nor B antigen): recessive to both IA and IB. So: IAIA or IAi → Group A. IBIB or IBi → Group B. IAIB → Group AB (co-dominance). ii → Group O. Example 2: MN blood groups: LM allele → M antigen. LN allele → N antigen. LMLN → both M and N antigens expressed on RBC → MN blood group. Example 3: Roan coat in Shorthorn cattle: red coat (RR) + white coat (WW) → roan (RW) — both red and white hairs present. Distinguish from incomplete dominance: incomplete dominance = blended phenotype. Co-dominance = both original phenotypes expressed simultaneously.
Go deeper
Molecular basis: each allele codes for a different glycosyltransferase enzyme. IA: N-acetylgalactosamine added to H antigen → A antigen. IB: galactose added to H antigen → B antigen. IAIB: both enzymes present → both A and B antigens present on surface. i allele: no functional enzyme (loss of function) → only H antigen remains → O blood group.
From Class 10
In simple dominance, one allele masks the other. But in co-dominance, both alleles are expressed equally in the heterozygote — neither is dominant over the other.