7 03 Fall 2006 Lecture 8 In the last lecture we discussed how to measure the distance between two genes on the X chromosome To do this we used the trick of looking only at male progeny so the genotype of the X chromosome could be scored directly since these flies only carry one copy of the X For autosomes we can have the same ability to score all recombinant classes by crossing to a homozygous recessive individual ie a test cross Consider the recessive traits vestigial wings and short bristles that are specified by two different genes on the same autosome non sex chromosome vg vg sh sh All F1 x vg sh The F1 flies are heterozygous for both genes so we are in position to see how often crossovers between these chromosomes occur in meiosis by doing a test cross Note that the progeny have distinct phenotypes vg sh vg vg x vg vg sh sh vg sh sh sh vg vg vg 458 442 47 53 sh The distance between vg and sh 100 x sh sh 100 1000 crossover classes 10 cM 7 03 Fall 2006 Now we ll do a second cross Note that the key is to set up a parent that is heterozygous at two loci sh sh x sh cn sh sh x sh cn sh sh cn cn sh sh cn cn cn sh cn cn cinnabar eyes cn cn 12 crossover classes 8 493 487 Distance between sh and cn 2 cM There are two possible orders We could resolve them by measuring the cn to vg distance which should be either 8 cM or 12 cM depending on the order However it s difficult in practice to get a statistically significant measurement that would cleanly distinguish between these possibilities A better way to find the order is to set up all three heterozygous markers at the same time and to look at the frequencies of the eight different gamete genotypes 7 03 Fall 2006 This is known as a 3 factor cross cn sh vg x cn sh vg cn sh vg cn sh vg 900 912 cn 2 sh vg 1 cn sh 75 vg 70 cn vg 18 22 sh These are all of the possible combinations One pair of these gamete classes must be the result of double crossovers This class will be very rare 0 1 x 0 02 2 x 10 3 By finding the rare class we have a qualitative test to determine gene order The double crossover classes for the two possible orders are cn vg or sh If this were the order these would be the rare classes sh vg or cn Since these are the rare classes we know this to be the order 7 03 Fall 2006 There is a simple system for evaluating 3 factor crosses 1 Group recombinant classes into reciprocal pairs 2 The most frequent pair is the parental classes 3 Derive the gene order from the least frequent pair which are the double crossover classes 4 The single crossover frequency for the two intervals can be obtained by adding the frequency of each of the single crossover class pairs to the frequency of the double crossover class pair In the present example the double crossovers are so rare that their inclusion doesn t matter sh cn vg
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