666Chapter 20 •Quantitative Geneticstogether, and the different marker genotypes in the F2orbackcross will have different average phenotypes for thequantitative character.Quantitative linkage analysisThe localization of QTLs to small regions within chro-mosomes requires the presence of closely spaced markerloci along the chromosome. Moreover, it must be possi-ble to create parental lines that differ from each other inthe alleles carried at these marker loci. With the adventof molecular techniques that can detect genetic poly-morphism at the DNA level, a very high density of vari-ant loci has been discovered along the chromosomes ofall species. Especially useful are restriction fragmentlength polymorphisms (RFLPs), tandem repeats, and sin-gle nucleotide polymorphisms (SNPs) in DNA. Suchpolymorphisms are so common that any two lines se-lected for a difference in quantitative traits are also sureto differ from each other at some known molecularmarker loci spaced a few crossover units from eachother along each chromosome.An experimental protocol for localizing genes,shown in Figure 20-16, uses groups of individuals thatdiffer markedly in the quantitative character of interestas well as at marker loci. These groups may be created
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.