04BIS10110152013RecombinLect4

Recombinant map distances can be additive if double

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Unformatted text preview: v ). only the middle (ct) of the 3 loci switches chromosomes! only This fact can be used to establish the order of three linked This genes. genes. BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 23 Mapping Tips ­ Mapping studies always start with a test cross ­ Recombination Frequency (RF) increases with the Recombination distance between two loci distance ­ Double crossover events are more rare than single Double crossovers crossovers ­ Only the middle of three loci is changed in a double Only crossover crossover ­ An odd number of crossovers is need to produce An recombinant gametes. recombinant ­ Map distances can be additive if double crossovers Map and interference are accounted for. and BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 24 Gene Mapping Tips ­ Double crossover progeny are more rare than Double single crossovers. single q For example, if the probability of one cross over is For 0.1 and the probability of another single crossover is 0.15. Using the Product Rule, the probability of Product the both single crossovers (a double crossover) occurring together is the product of the two independent probabilities or, 0.1 x 0.15 = 0.015. Therefore, out of 1000 progeny, there will 100 progeny resulting from one crossover, 150 progeny from the other crossover, but only 15 progeny resulting from the double progeny. progeny BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 25 Relating RF to Genetic Distance Genetic Distance. Although there are special circumstances that make Although crossing over more or less likely along a given stretch of DNA (see interference below), the crossover frequency (or recombination frequency) can be used to approximate genetic distance based on the following assumptions: following 1% recombination = 1 crossover/100 postmeiotic events = 1 map unit ­ (In honor of TH Morgan, 1 map unit (m.u.) has been designated 1 centiMo...
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