The present invention provides assays systems and methods for detection of genetic variants in a sample, including copy number variation and single nucleotide polymorphisms. The invention preferably employs the technique of tandem ligation—, e.g., the ligation of two or more fixed sequence oligonucleotides and one or more bridging oligonucleotides complementary to a region between the fixed sequence oligonucleotides—combined with detection of levels of particular genomic regions using array hybridization.