The invention provides a method for genotyping interfering polymorphic loci in a target polynucleotide, such as a strand of genomic DNA, in a multiplex hybridization-based assay. The invention also provides nucleic acid standards for validating the performance of such hybridization-based assays. In one aspect, the method of the invention is carried out by providing for each interfering polymorphic locus one or more probes so that at least one probe is capable of forming a perfectly match duplex at the locus regardless of the characteristic sequence of an adjacent polymorphism.