Presentation on theme: " R = Red eyesRr X Rr r = White eyes. Used to confirm whether a set of data follows a specific probability distribution. IE…how likely is it that deviations."— Presentation transcript:
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?
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.
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
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…
2 ways to analyze ◦ if X 2 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.
A cross between two fruit flies that are heterozygous for red eyes produced the following offspring: ◦ 260 Red eyes ◦ 178 White eyes Calculate X 2 for this experiment