Figure 1.

Linkage analysis tracks alleles of polymorphisms through families to determine if an allele is linked to a phenotype. In this example, alleles of gene 1, 1_A, 1_B, 1_C, and 1_D, track with severity (black, severe; gray, mild), showing concordant genotypes between siblings with similar phenotypes (left pedigree) and discordant genotypes when phenotypes are dissimilar (right pedigree). In contrast, genotype and phenotype show no relationship at polymorphism 2. Association studies examine a population of unrelated individuals to determine if particular alleles of a polymorphism are found in different proportions, depending on the disease profile. In the example here, alleles 1_A and 1_B have equal frequencies in the population, but 1_A is much higher in the severely affected subjects (black) and 1_B higher in the mildly affected subset (gray).