In this family-based method, studies assess the likelihood that a genetic marker that is close to the causative genetic variant of functional importance is shared more often than by chance alone by sets of affected relatives [11-17]. The more genetic variants of common ancestral origin (i.e. the same allele inherited by each offspring from the same parent) that affected relatives share, the more evidence in support of linkage. Usually the causative genetic variant is unknown and evenly spaced markers are typed along the length of the genome (including the autosomes and sex chromosomes) and a test of linkage is carried out. The resulting likelihood of odds (LOD) score reflects evidence in favor of linkage, on a logarithmic scale. These LOD scores (particularly when they exceed 3.0[18]) imply that a segment of the genome is shared frequently by affected sets of relatives.
