Linkage studies in pedigrees

In the first pedigree, the disease cosegregates with an A allele of the marker. In the second pedigree, the disease initially cosegregates with a B allele until there is a recombination, after which it cosegregates with a C allele.

Linkage can be used to map disease genes by typing polymorphic DNA markers to see if their alleles cosegregate with disease among related subjects. The strength of evidence in favour of linkage can be measured as the lod score which is the logarithm (base 10) of the likelihood ratio in favour of linkage. Traditionally a lod of 3 is taken as good evidence for linkage.