44200 12Chapter 12 •Genomics44200 GRIFFITHS FREEM Ch-12 First Pages Sen 12-10-2003 p 12Application FileSIMPLE-SEQUENCE LENGTH POLYMORPHISMSWelearned in Chapter 4 that one category of repetitiveDNA has repeats of a very simple short sequence. DNAmarkers based on variable numbers of short-sequencerepeats are collectively called simple-sequence lengthpolymorphisms (SSLPs).SSLPs have two basic advantages over RFLPs. First,an RFLP usually has only 1 or 2 “alleles,” or morphs, in apedigree or population under study. In contrast, SSLPscommonly have multiple alleles, and as many as 15 alle-les have been found for an SSLP locus. As a consequence,sometimes 4 alleles (2 from each parent) can be trackedin a pedigree. Two types of SSLPs are now routinely usedin genomics—minisatellite and microsatellite markers.Minisatellite markersMinisatellite markersare based onvariation of the number of VNTRs (variable numbertandem repeats). The VNTR loci in humans are from 1-
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