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lecture15 - BioEE 278 Evolutionary Biology Fall 2007...

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BioEE 278 Fall 2007 Evolutionary Biology Lecture 15 TWO OR MORE LOCI: RECOMBINATION AND LINKAGE DISEQUILIBRIUM Reading: Ch. 9, pp. 205-206; Ch. 13, pp. 303-304 A. Genes and their effects on the phenotype do not exist in isolation 1. Epistasis: the effect of the genotype at locus A locus on the phenotype depends on genotype at locus B (or C,D, etc.). a. example: coat color in Labrador retrievers 2. Linkage: two or more loci are on the same chromosome a. simple model of two loci, each with two alleles (A, a and B, b), four possible gametes (or haplotypes): AB, ab, Ab, aB b. let g AB , g ab , g Ab , g aB be the frequencies of the four gametes, let p and q be the frequencies of A and a, m and n be the frequencies of B and b c. if loci are on different chromosomes, independent assortment of chromosomes during meiosis leads to random association of alleles at loci. Frequencies of the four genotype are g AB =pm, g ab =qn, g Ab =pn, g aB =qm d. if loci are on linked, chromosomes (and loci) do not assort independently e. define coefficient of linkage disequilibrium as D= g
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