A method for differentiating between alleles of a locus in a sample, wherein the sample is a maternal sample comprising both maternal and fetal DNA, introducing to the sample two sets of fixed sequence oligonucleotides, wherein the two sets of fixed sequence oligonucleotides are provided for a single locus having a possible polymorphism, wherein each set comprises: (i) a first fixed sequence oligonucleotide comprising a sequence complementary to one allele of the locus, a label binding region, a capture region, a universal primer region and two restriction enzyme recognition sites, wherein the label binding regions comprise sequences that are different for the sets of fixed sequence oligonucleotides to allow differential labeling of fixed sequence oligonucleotides associated with each different allele, and wherein the capture regions are the same for both sets of fixed sequence oligonucleotides to allow for competitive hybridization to the capture features of a hybridization array; and (ii) a second fixed sequence oligonucleotide comprising a sequence complementary to the selected locus and a universal primer region; allowing the sets of fixed sequence oligonucleotides to hybridise to the locus; adding a bridging oligonucleotide to each set and hybridizing adjacently between the hybridized fixed sequence oligonucleotides; ligating the oligonucleotides to create ligation products; introducing universal primers to the ligation products, which universal primers bind to universal primer regions, to create amplicons comprising label binding regions capture regions and restriction enzyme recognition sites; introducing one or more restriction enzymes to the amplicons, and dually cleaving the amplicons at the restriction enzyme recognition sites to create cleaved amplicons each comprising a label binding region and a capture region; introducing the cleaved amplicons to a hybridization array comprising a plurality of capture probes where the capture regions of the cleaved amplicons competitively hybridize to capture probes; introducing the hybridized cleaved amplicons to labeled oligonucleotides which bind to complementary sequences on the cleaved amplicons; detecting the labeled oligonucleotides to differentiate between the different alleles of the locus. (i) a first fixed sequence oligonucleotide comprising a sequence complementary to one allele of the locus, a label binding region, a capture region, a universal primer region and two restriction enzyme recognition sites, wherein the label binding regions comprise sequences that are different for the sets of fixed sequence oligonucleotides to allow differential labeling of fixed sequence oligonucleotides associated with each different allele, and wherein the capture regions are the same for both sets of fixed sequence oligonucleotides to allow for competitive hybridization to the capture features of a hybridization array; and a sequence complementary to one allele of the locus, a label binding region, a capture region, a universal primer region and two restriction enzyme recognition sites, wherein the label binding regions comprise sequences that are different for the sets of fixed sequence oligonucleotides to allow differential labeling of fixed sequence oligonucleotides associated with each different allele, and wherein the capture regions are the same for both sets of fixed sequence oligonucleotides to allow for competitive hybridization to the capture features of a hybridization array; and (ii) a second fixed sequence oligonucleotide comprising a sequence complementary to the selected locus and a universal primer region;