Fig. 12. Heteroduplex analysis: Detection of single-base variations, either mutations or polymorphisms. Suppose that an individual is a heterozygote for a single-base change, A to G. To detect the heterozygous state, we can use several different techniques, two of which are illustrated here. Assume that we amplify the region bounding the base change. By heating the resulting DNAs, we will obtain four different double-stranded DNAs; two of the four will be perfect matches, and the two others will be so-called heteroduplexes. The heteroduplexes contain single base–pair mismatches, and no hydrogen bonding occurs at the mismatch. Simple electrophoresis in some conditions allows us to resolve the perfectly base-paired DNAs from the heteroduplexes. This is illustrated on the left. The nonbase-paired heteroduplex bases are more susceptible to strand modification by reagents frequently used in Maxam and Gilbert sequencing. These weaken the DNA, and it is selectively broken at that spot by piperidine. The broken strands have very different mobilities on a urea acrylamide (sequencing) gel from the those of the homoduplexes. All possible substitutions can be detected with this method, although it requires extra steps.