1. Multiple Alleles: There are three possible alleles for the ABO blood group: A, B, and O. This means that each individual inherits one allele from their mother and one from their father, but there are more than two possible options for each allele.

2. Codominance: When both A and B alleles are present, both are expressed, resulting in the AB blood type. This means that neither allele is dominant over the other, and both are expressed equally. The O allele is recessive to both A and B, meaning it is only expressed if an individual inherits two O alleles.

These two inheritance patterns together create the four main blood types: A, B, AB, and O.