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Scott_Handout1-key - 2/1/2010 MCB 104; Handout #1 1....

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2/1/2010 MCB 104; Handout #1 1. Barbara McClintock demonstrated that genes are distributed linearly on chromosomes by demonstrating that recombination between traits correlated with recombination between knobs—physical features of maize chromosomes. Why did McClintock use these knobs and not just genes with a clear marker phenotype? The point of the experiment was to show that recombination frequency was an effect of the physical orientation of genes on chromosomes. Knobs—physical features of the chromosome—behaved just like genes in recombination experiments. An experiment that only used genes would not have connected the physical map of the chromosome to the recombination map of genes. 2. Recombination mapping is a great way to identify the approximate location of genes. Here are the actual results of a testcross I did to ascertain the location of a mutation in a newly-discovered Drosophila gene called cp1: f - cp1 - pw - ♀ x f - cp1 - pw - f + cp1 + pw + Relevant Genotype: Number of flies : f - cp1 - pw - / f - cp1 - pw - 1743 f - cp1 - pw + / f - cp1 - pw - 12 f - cp1 + pw + / f - cp1 - pw - 132 f - cp1 + pw - / f - cp1 - pw - 0 f + cp1 + pw + / f - cp1 - pw - 1742 f + cp1 + pw - / f - cp1 - pw - 11 f + cp1 - pw - / f - cp1 - pw - 150 f + cp1 - pw + / f - cp1 - pw - 1 2A. How far apart are f and cp1? How far apart are cp1 and pw? How far apart are f and pw? How can it be proved that cp1 is in-between f and pw? Be able to convert your recombination frequency units into centimorgans and map units. f-cp1 = (132+0+150+1)/3791 = 7.5mu = 7.5cm = 7.5% recombination cp1-pw = (12+0+11+1)/3791 = 0.6mu = 0.6cm = 0.6% recombination cp1 must be in the middle—compare the double recombinants to the parentals: f - cp1 + pw - compared to f - cp1 - pw - and f + cp1 - pw + compared to f + cp1 + pw + . cp1 is the one gene that differs, so it must be in the middle, while f and pw must flank it. 2B. Would this recombination mapping have worked if the mother had been homozygous for cp1 1 ? Would this recombination mapping have worked if f and cp1 were more than 50 centimorgans apart? What if f and cp1 had been 40 centimorgans apart, and cp1 and pw had been 20 centimorgans apart? Would not have worked if f and cp1 had been more that 50cm apart.
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Scott_Handout1-key - 2/1/2010 MCB 104; Handout #1 1....

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