Disclosed are methods for establishing the compatibility between two blood types on the basis of cross-matching (under a designated rule of stringency) the minor blood group genotypes of recipient and prospective donors. To determine compatibility, the blood group genotypes are mapped to corresponding phenotypes according to the expression states associated with a set of underlying haplotypes, and compatibility is established by establishing the compatibility of blood types constructed as a combination of constituent phenotypes. The bit strings are matched, preferably using an algorithm expression. Where ambiguity in mapping genotypes to haplotypes exists, it can be reduced based on frequency of occurrence of the haplotypes in the sample population, or resolved by gametic phasing. Such reduction or resolution of ambiguity is particularly desirable where mismatches in the antigens expressed by the constituent haplotypes have greater clinical significance.