Identity by descent (IBD) mapping generally uses single nucleotide polymorphism (SNP) arrays to survey known polymorphic sites throughout the genome of affected individuals and their parents and/or siblings, both affected and unaffected.  While these SNPs probably do not cause the disease, they provide valuable insight into the makeup of the genomes in question. A region of the genome is considered identical by descent if contiguous SNPs share the same genotype.  When comparing an affected individual to his/her affected sibling, all identical regions are recorded (ex. Shaded in red in above figure).  Given that an affected sibling and an unaffected sibling do not have the same disease phenotype, their DNA must by definition be different (barring the presence of a genetic or environmental modifier).  Thus, the IBD mapping results can be further supplemented by removing any regions that are identical in both affected individuals and unaffected siblings.  This is then repeated for multiple families, thus generating a small, overlapping fragment, which theoretically contains the disease gene.