Lecture 9 Outline - spineless ( ss ) recessive to normal (...

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Bio 352 1 of 2 LECTURE 9 : Linkage and Recombination in Eukaryotes I Today’s topics : 1. Biases from multiple crossovers 2. Thee-point testcross Biases of the 2-point testcross Distance between linked genes – may act as if unlinked. Double crossovers – underestimate genetic distance Example: 250 meiotic events produce 1000 chromatids in gametes Actual distance = 10 cM Real data: 50 crossovers BUT there are 5 DCs so total of 45 meioses have crossovers. # crossovers # (parental cells) meiosis events 0 205 1 40 2 5 # nonrecombinant chromosomes produced: (205*4) + (40*2) + (5*2) = 910 # recomb chromosomes produced; (40*2) + (5*2) = 90 Estimated map distance = 9.0 cM but actual distance = 10 cM The three-point testcross involves three genes studied simultaneously . Example from Drosophila : Figure 7.14 1. eye color: scarlet ( st ) recessive to red ( st + ) 2. body color: ebony ( e ) recessive to gray ( e + ) 3. bristle size:
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Unformatted text preview: spineless ( ss ) recessive to normal ( ss + ) 4. All three loci are on the 3 rd chromosome 5. Gene order assigned arbitrarily. Any dominant phenotypes in the offspring result from a dominant allele inherited from the triple heterozygote ("TH"). 1. Which progeny received a nonrecombinant chromosome from TH? 2. Which progeny received a double recombinant chromosome from TH? Bio 352 2 of 2 3. The remaining 4 phenotypes received a chromosome with one crossover. Determine gene order : Which gene is in the middle? 1. Draw 3 different TH chromosomes each with a different locus in the middle. 2. Double crossover each one and see which produces double crossover progeny observed. Mapping the three genes 1. Calculate distance between st and ss by counting all the recombinants between these two loci. 4. Calculate distance between ss and e . 5. Construct the map. Next Time : EXAM 1 (Good luck!)...
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This note was uploaded on 05/07/2011 for the course BIOLOGY 352 taught by Professor Townsend during the Spring '08 term at San Diego State.

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Lecture 9 Outline - spineless ( ss ) recessive to normal (...

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