Learning Outcomes
i. Understand the concept of co-dominance in genetics.
ii. Explain the ABO blood group system and identify the genotypes associated with each blood type.
iii. Recognize how co-dominance is exhibited in individuals with type AB blood.
i. Co-dominance in Genetics
Co-dominance occurs when two different alleles at a locus are both fully expressed in a heterozygous individual. Unlike complete dominance, where the dominant allele masks the recessive one, co-dominant alleles are both fully manifested in the phenotype.
ii. The ABO Blood Group System
The ABO blood group system illustrates co-dominance perfectly. The system is determined by the presence or absence of two antigens (A and B) on the surface of red blood cells, which are encoded by three alleles: IA, IB, and i. The IA allele codes for the presence of the A antigen, the IB allele for the B antigen, and the i allele produces no antigen.
The possible genotypes and phenotypes for the ABO blood group system are:
Type A: Genotype IAIA or IAi (where IA is completely dominant over i)
Type B: Genotype IBIB or IBi (where IB is completely dominant over i)
Type AB: Genotype IAIB (exhibits co-dominance, both antigens are expressed)
Type O: Genotype ii (neither antigen is produced)
iii. Co-dominance in Type AB Blood
Individuals with type AB blood have both IA and IB alleles. The IA allele produces the A antigen, and the IB allele produces the B antigen, and both are expressed equally on the surface of red blood cells. This results in a unique phenotype where neither allele is recessive, and therefore, both A and B antigens are fully and simultaneously expressed.
The co-dominant nature of the IA and IB alleles allows for the AB blood type to be considered the universal recipient in blood transfusions, as it can accept red blood cells from any ABO blood type without causing an adverse immune reaction.
In conclusion, co-dominance is a form of inheritance where two different alleles are both fully expressed in a heterozygous individual's phenotype, and the ABO blood type system is a classic example of how this genetic principle operates in nature. Understanding co-dominance helps to explain the genetic complexity behind human traits and the inheritance patterns seen in populations.
