Step 1 - Calculating What Is Expected

Figure 3. A select F1 tomato plant from a cross between a resistant parent and a susceptible parent. Notice the resulting F1 progeny are showing resistance to bacterial spot. Photo credit: David Francis, The Ohio State University.

Following along with our tomato bacterial spot case, let’s walk through the steps in calculating a chi-square analysis. In the first step, we need to figure out what phenotypic classes are expected and the number of individual plants in each class. Based upon our observations in the F1 where all showed resistance, we decided to test the hypothesis that resistance is controlled by one dominant gene. Therefore, for the 197 plants in our F2 generation of selfing the F1s from the  6.8068 x OH88119 cross, we expected 147.75 tomato plants to be resistant to bacterial spot and 49.25 to be susceptible.  

Below is a tutorial video clip further explaining where these expected numbers came from if it is not clear to you.