All nonparametric linkage analyses are based on a concept called identity by descent (IBD) marker allele sharing (see figure 2). If siblings inherit the same marker allele from the same parent, the allele is called IBD. If the marker being tested is in close physical proximity to a gene influencing the disease or trait under study, then siblings who are similar for the trait would be expected to share more IBD marker alleles. Conversely, siblings who are dissimilar would be expected to exhibit fewer IBD marker alleles near the gene influencing the trait. More recently, nonparametric linkage methods have allowed the inclusion of more extended families beyond sibling pairs in the genetic analysis. Thus, more genetic information can be gained by studying more affected family members. Non-parametric linkage analyses can be used to study both traits that exist in an “either-or” fashion (i.e., qualitative traits, such as having or not having a disease) and traits that vary in severity along a continuum (i.e., quantitative traits, such as quantity or frequency of alcohol use).
