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Will use Fruit Flies for our example

Learn how Chi-Square Test helps determine if genetic data follows expected ratios. Step-by-step explanation with practical example calculations included.

andreaj
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Will use Fruit Flies for our example

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  1. Will use Fruit Flies for our example • R = Red eyes Rr X Rr • r = White eyes

  2. The Chi-Square Test Used to confirm whether a set of data follows a specific probability distribution. IE…how likely is it that deviations from expected results are due solely to chance?

  3. Goodness-of-fit test • Uses a sample to test whether a frequency distribution fits the predicted distribution • Example: Used to answer the following ques • Can we conclude that the 3:1 ratio is true based on the observation of _____(#) of fruit flies.

  4. Observed vs. Expected Frequencies • The chi-square test basically compares the observed (O) and the expected (E) frequencies to determine whether there is a statistically significant difference • Observed frequency is the # of flies seen for each phenotype • Expected frequency is the # of flies expected for each penotype • Example: if cross is a monohybrid one, you would expect a 3:1 phenotypic ratio. Therefore, if you observed 100 flies, 75 of them should have the dominant phenotype, while 25 of them should have the recessive phenotype

  5. Chi-square statistic • Use the following equation:

  6. Determining the Critical Chi-Square Score • Use an already constructed table to see where your Chi-square value falls. • Which row you look in depends on the number of degrees of freedom. • The number of degrees of freedom is equal to the number of phenotypic categories minus 1. • Monohybrid… • Dihybrid… • Sex-linked…

  7. Chi-square Values and Probabilities • 2 ways to analyze • if X2 is less than number in box, good fit… • If P value is greater than .05, than good fit Means your data did not happen by chance.

  8. Let’s do an example problem • A cross between two fruit flies that are heterozygous for red eyes produced the following offspring: • 260 Red eyes • 178 White eyes • Calculate X2 for this experiment

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