LabBench Activity

Using the Chi-Square Critical Values Table

The chi-square critical values table provides two values that you need to calculate chi-square:

• Degrees of freedom. This number is one less than the total number of classes of offspring in a cross. In a monohybrid cross, such as our Case 1, there are two classes of offspring (red eyes and sepia eyes). Therefore, there is just one degree of freedom. In a dihybrid cross, there are four possible classes of offspring, so there are three degrees of freedom.
• Probability. The probability value (p) is the probability that a deviation as great as or greater than each chi-square value would occur simply by chance. Many biologists agree that deviations having a chance probability greater than 0.05 (5%) are not statistically significant. Therefore, when you calculate chi-square you should consult the table for the p value in the 0.05 row.

Use the critical values table here to do the problems below. *

 Degrees of Freedom (df) Probability (p) 1 2 3 4 5 0.05 3.84 5.99 7.82 9.49 11.1 0.01 6.64 9.21 11.3 13.2 15.1 0.001 10.8 13.8 16.3 18.5 20.5

1. Determine the degrees of freedom. This is the number of categories (red eyes or sepia eyes) minus one. For the data in Case 1, the number of degrees of freedom is

2. Find the probability (p) value for 1 degree of freedom in the 0.05 row.