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One of the most common types of genetic variation detectable at the DNA level is called a single nucleotide polymorphism (SNP and pronounced “SNiP”). Fig. 4.1 illustrates a SNP. Recall that DNA is made up of billions of bases called nucleotides. When just one of them is mutated it may change from A to C, or G to T, or C to A, etc. When that happens, the DNA can exist in two forms at that site—essentially, it becomes polymorphic (derived from the Greek for “many forms”).

Fig. 4.1. Comparison of base sequences of DNA strands illustrating a single nucleotide polymorphism (SNP). A hypothetical sequence is shown for five horses. The bold letter denotes the presence of a SNP, a site at which two horses have a T and three horses have a C.

Other types of variation include rearrangements of DNA, such as inversions, duplications, and translocations from one site to another, as well as insertions and deletions of DNA, ranging from single bases to large sections with tens of thousands of bases.