Wednesday, September 18, 2013
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!
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
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
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