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biol139-lecture20-2011

# Question if proposed genetic mechanism is correct

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Unformatted text preview: expected values by just chance or variation? or The Chi Square Test Conventional measure of “goodness-of-fit” between observed and expected values is called chi-square expected chi-square Indicates how often an experimentally observed deviation from a particular hypothesis will occur solely by chance from chance • takes into account importance of sample size sample • chi square test is always calculated with actual data - not % or proportions, requires LARGE sample size LARGE When geneticists want to determine if two genes When are linked, they actually test whether the data are consistent with a null hypothesis of no linkage consistent null no • large degree of phenotypic variability if linked • unlinked will assort independently to produce 50% parental and 50% recombinant in F2 50% If the chi square test shows that the observed If observed data differ significantly from what would be significantly expected with independent assortment, the null hypothesis is rejected -genes could be linked Applying The Chi Square Test Applying First: -state the genetic hypothesis in detail First: • (YyRr x yyrr) will yield an F2 ratio of 1:1:1:1 if genes assort independently -not linked Use the data obtained from a breeding experiment Use to determine: to the total number of offspring total the number of different classes of offspring (phenotypes) (phenotypes) and the number of offspring observed in each class number 568 progeny with 154 Y_R_:144 Y_rr:124 yyR_:146 yyrr 568 154 Total offspring = 568 568 Number of different classes of offspring = 4 Number observed in each class = 154:144:124:146 154:144:124:146 2. Calculate the number of offspring that would be expected for each class if the null hypothesis expected were correct (percentage predicted x total were number of offspring) number Expected values: (1/4 x 568): (1/4 x 568): (1/4 x 568): (1/4 x 568) = 142:142:142:142 The Chi Square Test The Deviation from predicted χ2 = Σ 3. Class (Number observed-Number expected)2 Number expected Y_R_ Y_R_ Number observed = 154 Number expected = 142 =Σ Y_rr Y_rr 1...
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