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BIO 325 September 20th Lecture Notes

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BIO 325
Sanghamitra Mohanty

9/20 Lecture Wednesday, September 18, 2013 3:19 PM Three point mapping - the parent must be heterozygous for all three genes under consideration  DCOs doesn't happen frequently -- numbers are lower than expected Noncrossover F2 phenotypes - occur in the greatest proportion of offspring Double-crossover phenotypes - occur in the smallest proportion Reciprocal classes - F2 phenotypes complement each other The distance between two genes in a three-point cross is equal to the percentage of all detectable exchanges occurring between them and includes all single and double crossovers! Figure 5-10 SCO = pr - v = 22.3% = 22.3% 22.3 map units SCO = pr - bm = 43.4% = 43.4% 43.4 map units Expected DCO = 0.223 x 0.434 = 0.0968 ---> 9.7% Observed DCO = 7.8% *this phenomenon shows interference (I) = 1 - C Coefficient of coincidence = 7.8/9.7 = 0.8041 *since this is less than 1 and positive, then that means the crossover is positive interference -  Oppositely, negative interference means that one crossover event increases the frequencies of other crossovers Coefficient of coincidence (C)- observed number of DCOs divided by the expected number of DCOs The expected frequency of multiple exchanges between two genes can be predicted from the distance between them! Do not pay attention to figure 5.12, it's not in the book anymore! When two genes are close together, the accuracy of mapping is HIGH! ----- as the distance between them increases, the accuracy of mapping DECREASES! This is called POISSON DISTRIBUTION (don't need to know any more details about this) Lod score analysis - relies on probability calculati
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