Co-dominance typically involves how many alleles?

Co-dominance involves multiple alleles, specifically two alleles that can be expressed equally in the phenotype of a heterozygous individual. In this genetic scenario, both alleles contribute to the organism's observable traits without one being dominant over the other. This results in a phenotype that displays characteristics of both alleles simultaneously. A classic example of co-dominance is seen in the ABO blood group system, where individuals with the genotype IAIB express type AB blood, showcasing both A and B antigens on the surface of red blood cells.

Understanding that co-dominance requires at least two alleles is essential, as it distinguishes it from other forms of inheritance like simple dominance, where one allele masks the expression of another. The presence of more than one allele allows for the unique phenotypic outcomes typical of co-dominance, reinforcing the concept that both alleles actively influence the trait being expressed.