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Understanding Chi-Square Analysis in Genetics Experiments

Learn how Chi-square analysis determines if differences between observed and expected results are due to randomness or other factors in genetics experiments. Explore calculations, degrees of freedom, hypothesis evaluation, and practical application. See a detailed example analysis of corn grain genetics experiment results.

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Understanding Chi-Square Analysis in Genetics Experiments

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  1. Chi Square and Corn

  2. What does Chi-square analysis tell us? If the difference between your observed results and expected results are due to random chance or not. EX –flip a coin enough and you would expect to get heads 50% of time and tails 50% of time. If this number is way off even after 1,00 tosses then something else besides randomness is occurring

  3. How’s it works Calculation to use is X2 = ∑ (observed – expected)2 expected Table to set up

  4. How it works continued • Determine degrees of freedom – it’s one less than the number of possible outcomes Ex – expect 4 different phenotypes, degrees of freedom is 3

  5. What are you looking for? • p = .05 is the cut off for accepting or rejecting you hypothesis that the difference is due to random events • p more than .05 means that your hypothesis may be supported • p less than .05 means hypothesis not supported

  6. What? • p is probability of an event occurring due to randomness If p = .50 then the difference between observed and expected results is due to random events 50% of time

  7. How it works • Once you have your degrees of freedom and Chi square sum, go to the distribution table

  8. You explain • If p = .05 then what does that mean? the difference between observed and expected results is due to random events only 5% of time so – something else must be going on! Your hypothesis can’t be supported b/c something else besides random events is playing a role in the outcome

  9. What are you looking for? • p = .05 is the cut off for accepting or rejecting you hypothesis that the difference is due to random events • p more than .05 means that your hypothesis may be supported • p less than .05 means hypothesis not supported

  10. Try it • P = Purple • p = Yellow • S = Smooth PpSs x PpSs • s = Shrunken What are the expected results? Hypothesis? 9:3:3:1 due to independent assort of alleles

  11. Results analysis • Observed results: An ear of corn has a total of 381 grains, including 216 Purple & Smooth, 79 Purple & Shrunken, 65 Yellow & Smooth, and 21 Yellow & Shrunken. • Expected?

  12. Table, Chi Square and Degrees of Freedom Df: SUM of Chi Squares:

  13. Hypothesis supported?

  14. Try this one • Same hypothesis: independent assort therefore 9:3:3:1 outcome • Observed: An ear of corn has a total of 389 grains, including 219 Purple & Smooth, 70 Yellow & Smooth, and 100 white.

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