Final answer:
In a trihybrid cross between two heterozygous individuals for all three traits, 8 degrees of freedom should be used to calculate the chi-square value.
Step-by-step explanation:
In a trihybrid cross between two individuals who are heterozygous for all three traits, there are a total of 8 degrees of freedom that should be used to calculate the chi-square value. The degrees of freedom for a chi-square test can be calculated using the formula (number of categories - 1). In this case, there are 3 traits, so there are 3 categories for each trait. Therefore, the total number of categories is 3 x 3 = 9. Subtracting 1 gives us 8 degrees of freedom. So, the correct answer is 8.