PS2_Key - BICD100 ` Prof Reinagel Fall 2005 Problem Set 2...

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BICD100 Prof. Reinagel Fall 2005 ` Problem Set 2 ANSWERS 1. Mendel’s Second Law states that in a dihybrid cross, traits segregate independently. Prove that if Mendel’s Second Law is true, you will get a 9:3:3:1 segregation ratio for traits in a dihybrid cross. To “prove” you need to show where the numbers come from (for example, using a Punnet Square or a Tree Diagram or a Probability Equation). Answer: A dihybrid cross is of the form AaBb x AaBb There are many ways to derive the phenotypic ratios, for example: construct the dihybrid Punnet square, Write the probabilities of each gamete type (AB Ab aB ab all have P= ¼) Multiply to get the P of each offspring type (each square is 1/16) Count the number of squares that have A_B_, A_bb, aaB_, and aabb 2. In Mendel’s dihybrid cross (p.15) he obtained the following data 315 round and yellow, 101 wrinkled and yellow, 108 round and green, 32 wrinkled and green. a) What is the observed ratio of the phenotypic classes? Answer: 315 R_Y_ / 556 = 56.65% x 16 = 9.1 101 rrY_ / 556 = 18.17% x 16 = 2.9 108 R_yy /556 = 19.42% x 16 = 3.1 32 rryy / 556 = 5.76% x 16 = 0.9 9.1 : 2.9 : 3.1 : 0.9 b) Perform a Chi Square test to determine the probability of getting data that deviated this much from the predicted ratios, if Mendel’s second law is true. Is the deviation statistically significant?
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