The distance between two molecular markers that are linked along the same chromosome can be determined by analyzing the outcomes of crosses. This can be done in humans by analyzing a family’s pedigree.However, the accuracy of linkage mapping using human pedigrees is fairly limited because the number of people in most families is relatively small. As an alternative, researchers can analyze a population of sperm, produced from a single male, and compute a map distance in this manner. As an example, let’s suppose a male is heterozygous for two polymorphic STSs. STS-1 exists in two sizes: 234 bp and 198 bp.STS-2 also exists in two sizes: 423 bp and 322 bp. A sample of sperm was collected from this man, and individual sperm were placed into 40 separate tubes. In other words, there was one sperm in each tube.Believe it or not, PCR is sensitive enough to allow analysis of DNA in a single sperm! Into each of the 40 tubes were added the primers that amplify STS-1 and STS-2, and then the samples were subjected to PCR and gel electrophoresis. The following results were obtained.

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