Because recombination frequency is usually proportional to the distance between loci, recombination frequencies can be used to create genetic maps. Recombination frequencies tend to underestimate map distances, especially over long distances, double crossovers may be genetically indistinguishable from non-recombinants. Three-point crosses can be used to determine the order and map distance between of three loci, and can correct for some of the double crossovers between the two outer loci. linkage complete (absolute) repulsion recombination linkage map units (mu) independent assortment incomplete (partial) centiMorgans (cM) crossover linkage genetic map recombinant genotype syntenic conserved synteny parental genotype synapsis double-crossover unlinked coupling (cis) three-point cross recombination frequency configuration (RF) repulsion(trans) configuration 7.1 Compare recombination and a) What does this ratio assume about crossover. How are these similar? the linkage between the two loci in How are they different? the dihybrid cross? b) What ratio would be expected if 7.2 Explain why it usually necessary the loci were completely linked? Be to start with pure-breeding lines sure to consider every possible when measuring genetic linkage by configuration of alleles in the the methods presented in this dihybrids. chapter. 7.5 Given a dihybrid with the 7.3 If you knew that a that genotype CcEe: affected earlobe shape was tightly a) If the alleles are in coupling (cis) linked to a locus that affected configuration, what will be the susceptibility to cardiovascular genotypes of the parental and disease human, under what recombinant progeny from a test circumstances would this information cross? be clinically useful? b) If the alleles are in repulsion (trans) configuration, what will be the 7.4 In a previous chapter, we said a genotypes of the parental and 9:3:3:1 phenotypic ratio was expected recombinant progeny from a test among the progeny of a dihybrid cross? cross, in absence of gene interaction.
Because recombination frequency is usually proportional to the distance between loci, recombination frequencies can be used to create genetic maps. Recombination frequencies tend to underestimate map distances, especially over long distances, double crossovers may be genetically indistinguishable from non-recombinants. Three-point crosses can be used to determine the order and map distance between of three loci, and can correct for some of the double crossovers between the two outer loci. linkage recombination independent assortment crossover recombinant genotype parental genotype unlinked recombination frequency (RF) complete (absolute) linkage incomplete (partial) linkage syntenic synapsis coupling (cis) configuration repulsion(trans) configuration repulsion map units (mu) centiMorgans (cM) genetic map conserved synteny double-crossover three-point cross 7.1 Compare recombination and crossover. How are these similar? How are they different? 7.2 Explain why it usually necessary to start with pure-breeding lines when measuring genetic linkage by the methods presented in this chapter. 7.3 If you knew that a that affected earlobe shape was tightly linked to a locus that affected susceptibility to cardiovascular disease human, under what circumstances would this information be clinically useful? 7.4 In a previous chapter, we said a 9:3:3:1 phenotypic ratio was expected among the progeny of a dihybrid cross, in absence of gene interaction. a) What does this ratio assume about the linkage between the two loci in the dihybrid cross? b) What ratio would be expected if the loci were completely linked? Be sure to consider every possible configuration of alleles in the dihybrids. 7.5 Given a dihybrid with the genotype CcEe: a) If the alleles are in coupling (cis) configuration, what will be the genotypes of the parental and recombinant progeny from a test cross? b) If the alleles are in repulsion (trans) configuration, what will be the genotypes of the parental and recombinant progeny from a test cross?
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