A method for determination of zygosity. Three primers are used, two of which are allele-specific and one of which matches the complementary strand of both alleles. Allele specificity is achieved by designing mismatches to the unwanted allele at the 3' end of the primer. However, precisely because the primers must be chosen to generate amplicons of characteristic size, they will have different efficiencies of amplification, which will cloud the interpretation of results.