Many variations in the DNA exist among the genes and noncoding DNA regions of different individuals. Such variants of a DNA sequence are called alleles. A DNA region for which several alleles exist is said to be polymorphic—that is, it exists in many forms. The identification of these variants has revolutionized the study of genetics because it allows researchers to study the inheritance of the alleles and to associate specific alleles with the presence of certain diseases. Several types of polymorphisms are commonly used for genetic analyses, including microsatellite markers and single-nucleotide polymorphisms (SNPs). Microsatellite markers are DNA sequences in which short motifs of two, three, or four nucleotides are repeated several times, with the number of repetitions varying from person to person (see figure 1C). Microsatellite markers are typically found in noncoding DNA regions. SNPs consist of the exchange of single nucleotides in the DNA (see figure 1C) and can be found both in coding and noncoding DNA regions. If SNPs occur within a gene or within a DNA region controlling the activity of a gene, they can result
