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Lecture08S10 - BIS101/Engebrecht Lecture08 Announcements...

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BIS101/Engebrecht 4/15/10 Lecture08 Announcements: The first midterm is next Thursday, April 22. In addition to regular office hours, I will hold a review session on Tuesday April 20 at 7-9pm in Wellman 2. Today we continued our discussion of gene mapping. Tuesday we saw that you could map two genes relatively to each other by performing crosses and examining the phenotype of the progeny. However, using the 2point cross, with two genes, provides limited information. Today we will perform a 3point cross that will map three genes relative to each other. This is where you look at 3 genes segregating (and presumably linked) at a time. Please see the Handout07 I posted on the web, I will not recreate the crosses here. I recommended the following steps to determine gene order and distance between three genes in such a cross: 1) Determine the parental classes, these will be the most frequent classes (and have the same configuration of markers as the parents, if you know that information). 2) Determine the double cross over classes (DCO). These will be the least frequent classes. By examining the DCO classes you can determine gene order. This is because the gene that is unlinked from the original marker configuration, must be in the middle. You should draw out the chromosomes and convince yourself of this! I suggested that it may help you to rewrite the F2 progeny in the handout with the same gene order as we have now established. 3) Now we want to determine distances between the genes. This is just like what we saw for the 2 point crosses; however, we need to include in our recombination frequencies, the DCOs. In the example, you need to determine which recombinant classes represent crossovers between BA (those where A and B become unlinked) and which represent crossovers between AC (those where A and C become unlinked). You then add all of the recombinant classes (including DCO) and determine the distances between both intervals (please see Handout07 ). I find this approach the easiest but your book suggests another one – I recommend using what ever makes the most sense to you. For this second approach, you would calculate map distance from each gene pair: AB, AC and CB. The largest distance tells you the outside most genes.
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