BICD 100 - Week 2 - BICD 100 Week 2 Handout Dihybrid...

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BICD 100 Week 2 Handout: Dihybrid Crosses / Chi Square Advice : You should be familiar with the Mendelian phenotypic ratio for an F1 dihybrid cross ( 9 A-B- : 3 A-bb : 3 aaB- : 1 aabb) - however, don’t worry about memorizing the phenotypic ratios for the non-Mendelian genetic interaction cases (synergistic, epistasis, incomplete dominance, redundant, etc.) - if you are presented with a phenotypic ratio that deviates from 9:3:3:1, think about how it might have occurred. This may help in finding a strategy to solving the problem. Examples: Phenotypic ratio of 15 : 1 This can occur if 9 A-B- : 3 A-bb : 3 aaB- are grouped together (9+3+3 = 15) - How can these 3 classes give the same phenotype? A and B could be redundant genes Phenotypic ratio of 12 : 3 : 1 This can occur if 9 A-B- : 3 A-bb are grouped together (9+3 = 12) - How can these 2 classes give the same phenotype? A could be dominant epistatic to B ** refer to Midterm Question #7 and Additional Problem 7. for practice on this** For Chi Square test , think about flipping a coin: - if someone flips a coin 10 times, and gets 5 heads and 5 tails, would you be surprised? (no) - what about 6 heads and 4 tails? (probably not) - what about 10 heads and 0 tails? (pretty weird!) - now, if someone flips a coin 100 times and gets 100 heads and 0 tails, you would think something’s really messed up about that coin (or the person tossing it)! - this “weirdness factor” is what Chi Square is testing: if you have a model (such as a coin being equally likely to land heads vs. tails), what is the probability that your data really fit the model? In the last case, the probability would be very low (way below the P = 0.05 cutoff in a Chi Square test), so you would reject the model. Perhaps the coin is not identical on both sides - there could be a magnet on one side, etc. - try setting up Chi Square tests with different coin flip scenerios to get the concept * for genetics problems, instead of a coin toss, the model involves alleles segregating into gametes (which is essentially like flipping a coin, given the principles of segregation / independent assortment)!
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