Fig. 10. A DNA polymorphism detected by a restriction enzyme. Individual A is a homozygote for one allele; individual B is a homozygote for the other allele. Individual A has a central Msp 1 site, CCGG, whereas individual B has lost the site due to a single base change of C to T, changing the recognized CCGG sequence to CTGG. On electrophoresis individual A has bands 1 and 2, while individual B has a single band, band 3, the size of which is the sum of the sizes of bands 1 and 2. Offspring of A and B are heterozygotes and have one of each of the alleles and all three bands. On Southern blot analysis with a probe indicated by the black bar, only bands 1 and 3 are detected. Band 2 does not contain sequences to which the probe can hybridize. Thus, on autoradiography (radiolabeled band) we will detect only band 1 in individual A, only band 3 in individual B, and bands 1 and 3 in individual C.