Final Answer:
In a dihybrid cross between heterozygous individuals, the chi-square test for independence can use 3 degrees of freedom.
Step-by-step explanation:
The degrees of freedom (df) in a chi-square test for independence are calculated using the formula df = (r - 1) × (c - 1), where "r" is the number of rows and "c" is the number of columns in the contingency table. For a dihybrid cross, there are four phenotypic classes, which can be represented as a 4x4 contingency table.
Applying the formula, df = (4 - 1) × (4 - 1) = 3 × 3 = 9, we determine that there are 9 degrees of freedom. However, in the context of a dihybrid cross, we need to consider that the phenotypic classes are determined independently for two different traits. Therefore, the degrees of freedom are divided by the number of traits, resulting in 9 / 2 = 4.5. Since degrees of freedom must be whole numbers, we round down to the nearest whole number, and thus, we use 4 degrees of freedom in the chi-square test for this dihybrid cross.
Understanding the appropriate degrees of freedom is crucial in the chi-square test as it affects the critical value used to determine statistical significance. Using an incorrect degrees of freedom value can lead to inaccurate interpretations of the test results. In this case, with 4 degrees of freedom, researchers can confidently apply the chi-square test to assess the independence of the observed and expected phenotypic ratios in the dihybrid cross.