Final answer:
With a chi-square value of 6.4 and assuming a p-value of 0.05, one would compare the calculated value to the critical value from the chi-square distribution table. If the degrees of freedom are 2, the critical chi-square value is approximately 5.99, and since 6.4 exceeds this value, the null hypothesis of recessive epistasis would be rejected.
Step-by-step explanation:
When conducting a chi-square test to investigate genetic crosses that result in three phenotypes, and testing a null hypothesis of recessive epistasis, it's important to compare the calculated chi-square value to the critical value in a chi-square distribution table at a specified degree of freedom (usually number of phenotypes minus 1).
If your calculated chi-square value is 6.4 and you are assuming a p-value of 0.05, you would look at the chi-square distribution table provided, for example, in the AP Biology Exam. For a typical genetics problem where the degrees of freedom might be 2 (since there are three phenotypes), the critical chi-square value at 0.05 p-value is approximately 5.99.
Since 6.4 exceeds this, you would reject the null hypothesis as your calculated value falls into the rejection region, suggesting that the observed ratios significantly differ from those expected under recessive epistasis.