04BIS10110152013RecombinLect4

Lrodriguez 52 interpreting probabilities from a chi

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Unformatted text preview: Gene Expression, R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 50 Another Method for Calculating χ2 Another χ2 = Class BC bc Bc bC O 770 716 401 318 2205 Σ (0 - E)2 E E* (O - E)2 551 551 551 551 551 551 551 47961 27225 27225 22500 22500 54289 54289 (O - E)2 E 87.0 49.4 40.8 98.5 χ2 = 276.5 BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 51 Interpreting the Chi-square Test of Linkage χ2 value of 276.5, a table is used that relates the χ2 values to the probability (p) that the deviations between the probability that observed and expected progeny are due to chance only (see χ2 Table). ­ Before selecting the appropriate probability, we must Before first choose the “degree of freedom” or df for this or df experiment. experiment ­ Degrees of Freedom is a measure of the number of Degrees categories that are independent of each other, typically n - 1 where n is the total number of categories. In this experiment, there are 4 progeny categories, or a df = 3. df ­ To interpret the meaning of a To BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 52 Interpreting Probabilities from a Chi Square (χ2) Table Critical Values of the χ 2 Distribution Critical BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 53 Interpreting Probabilities from a Chi Square (χ2) Table ­ From the table of χ2 From values, at 3 degrees of freedom, 276.5 is large and gives a probability (p) of 0.005 or, 276.5 of 0.5%. ­ This means that the probability that the deviations This between observed and expected progeny are due to chance only, is 0.5%. ­ Therefore, we must reject our hypothesis that the B Therefore, and C loci are unlinked. Conversely, we must consider that B and C are linked. consider ­ The χ2 test does not prove that B and C are linked The — only that our hypothesis does not fit the data well. χ2 is a measure of goodness of fit. goodness BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013 BIS101­001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 54 Chi Square (χ2) Table Critical Values of the χ 2 Distribution Critical Hypotheses that deviations are due to chance alone are rejected for are probabilities less probabilities than 5% or 0.05 BIS101­001, Spring 2013—Genes and Gene Expression,...
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This note was uploaded on 02/12/2014 for the course BIS 101 taught by Professor Simonchan during the Spring '08 term at UC Davis.

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