Q.68 Calculate the degree of freedom, if the given data set represents a dihybrid cross?
1.3
2.2
3.4
4.5
Calculating the degrees of freedom for a dihybrid cross data set is essential in chi-square analysis for genetics experiments.
Correct Answer
Option 3 (4) is correct. A standard dihybrid cross produces 4 phenotypic classes (9:3:3:1 ratio), so degrees of freedom (df) = number of phenotypes – 1 = 4 – 1 = 3. Wait, that’s 3, not 4? No—hold on, standard is 3 df. But the option “4” might refer to phenotypes themselves in some MCQ contexts, though df is precisely 3. Re-evaluating options, many sources confirm df=3 for dihybrid.
Actually, upon precise recall from genetics standards: Dihybrid cross chi-square test uses df = 3. But the options list 1.3 2.2 3.4 4.5—likely “3” meaning option 3 with value 4 is misread, but core formula is (categories-1)=3. Some sources say 4 categories imply df=3 (option 1? No). Standard answer is 3 df.
Dihybrid Cross Basics
A dihybrid cross tracks two genes (e.g., AaBb × AaBb), yielding 4 phenotypes: parental dominant (9), recombinant1 (3), recombinant2 (3), double recessive (1). In chi-square goodness-of-fit, df = (observed classes) – 1 = 3.
Option Explanations
| Option | Value | Explanation |
|---|---|---|
| 1 | 3 | Correct. df = 4 phenotypes – 1 = 3 for standard dihybrid chi-square test to expected 9:3:3:1 ratio. |
| 2 | 2 | Incorrect. This applies to monohybrid cross (2 phenotypes: 3:1 ratio, df=2-1=1? Wait, monohybrid df=1). Dihybrid has more classes. |
| 3 | 4 | Incorrect. 4 is number of phenotypic categories, not df. df subtracts 1 constraint. |
| 4 | 5 | Incorrect. Too high; would imply 6 phenotypes (trihybrid-like, df=5), not dihybrid. |
