How are most blood group systems inherited?

The inheritance of most blood group systems, including the ABO and Rh blood groups, is typically classified as autosomal codominant. In this mode of inheritance, both alleles at a locus contribute equally and visibly to the phenotype. For example, in the ABO blood group system, individuals can inherit one allele from each parent. If one parent contributes an A allele and the other a B allele, the resulting genotype expresses both traits, leading to the AB blood type.

This principle of codominance is crucial in understanding blood typing, as it explains why individuals with AB blood type can express both A and B antigens on the surface of their red blood cells. It contrasts with other inheritance patterns, such as recessive or dominant traits, where one allele may mask the expression of another. Therefore, the autosomal codominant inheritance pattern importantly illustrates how various blood type phenotypes can emerge based on genetic combinations.