Statistical appendix 667 have not been very successful, although the marker seg-regation technique has been a success in Fnding loci whose mutations are responsible for single-gene disor-ders or for quantitative characters whose variation is strongly in±uenced by variation at one locus. ²or exam-ple, people vary in their ability to taste the substance phenylthiocarbamate (PTC). Some can detect quite low concentrations, whereas others can detect only high con-centrations or are unable to taste PTC at all. Linkage analysis using single nucleotide polymorphisms located a region on human chromsome 7q that accounted for about 75 percent of the variation in taste sensitivity. This chromosomal region was already known to contain sev-eral genes coding for bitter taste receptor proteins. When the DNA of these genes was sequenced, three amino acid polymorphisms in one of the genes were found to be strongly associated with the difference between the taster and the nontaster phenotypes. Percentage of QTLs identified
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