In 1910, von Dungern and Hirszfeld hypothesized that ABO blood groups are characteristics that are inheritable from generation to generation. Later, in 1924, Bernstein proposed the one genetic locus–three allelic model, which explained the mode of inheritance of the ABO blood groups. He postulated that there were three alleles (A, B, and O) at the single genetic locus of ABO. He assumed that A and B alleles are co-dominant against the recessive O allele. Based on the combination of two alleles, there are six genotypes of AA, AO, BB, BO, AB, and OO, which result in 4 phenotypes of A, B, AB, and O. Several examples of inheritance from parents of certain genotypes are also shown. For example, from the parents of AA and AO, only the children with AA or AO genotypes are possible. Similarly, from the parents of AB and O (OO genotype) phenotypes, only the children with A (AO) or B (BO) phenotypes are possible, and no children with AB or O phenotypes are expected (See the cis-AB cases for exception).