2-9-11 Reading Notes

2-9-11 Reading Notes - Epistasis, Penetrance and...

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Epistasis, Penetrance and Expressivity Pgs. 239-249 Analyzing double mutants of random mutants To learn whether two genes interact, we need to assess the phenotype of the double mutant to see if it is other than the combination of both single mutations The double mutant is obtained by intercrossing. The F 1 generation is obtained as part of the complementation test so with the assumption that the complementation has been observed, suggesting different genes, the F 1 is selfed to obtain an F 2 . The F 2 generation should contain a double mutant. The double mutant may then be identified by looking at Mendelian ratios. o If a standard 9:3:3:1 ratio is obtained, the phenotype present in only 1/16 of the progeny represents the double mutant o In cases of gene interaction, the phenotype of the double mutant may not be distinct but will match that of one of the single mutants and the ratio will be 9:3:4 The 9:3:3:1 ratio : no gene interaction Two mutated genes do not interact and is a situation where the ratio is 9:3:3:1 Example: skin coloration in corn snakes o Natural coloration is black and orange camouflage o The phenotype is produced by two separate pigments both of which are under genetic control o One gene determines the gene pigment being orange or not. Another gene determines the black pigment and its alleles. These two genes are unlinked. o The natural pattern is produced by the genotype o+/-, b+/- and is black because it lacks orange pigment. o Double homozygous recessive is albino. If the presence of one mutant makes one pathway fail, the other pathway is still active, producing the other pigment color. Only when both mutants are present do both pathways fail, and no pigment of any color is produced. The 9:7 ratio: genes in the same pathway The F 2 ratio from the harebell cross shows both blue and white plants in a ratio of 9:7. The 9:7 is a modification of the dihybrid 9:3:3:1 ratio with the 3:3:1 ratio combined to make 7. A 9:7 ratio is possible if and only if the double mutant has the same phenotypes as the two single
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This note was uploaded on 06/27/2011 for the course BIOL 202 taught by Professor Kieber-hogan during the Spring '08 term at UNC.

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2-9-11 Reading Notes - Epistasis, Penetrance and...

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