Question

Match the LIST-I with LIST-II

LIST-I Gene Interaction	LIST-II  Dihybrid ratio for a single character
A. Duplicate dominant epistasis	I. 9:7
B. Duplicate recessive epistasis	II. 15:1
C. Recessive epistasis	III. 9:3:4
D. Dominant & Recessive Epistasis	IV. 13:3

Choose the correct answer from the options given below:

• A. A-I, B-II, C - III, D - IV
• B. A-II, B-I, C - III, D - IV
• C. A-I, B-II, C - IV, D - III
• D. A-II, B-I, C - IV, D - III

Hint:

To quickly derive these ratios, start with the standard 9:3:3:1. For "duplicate recessive," you need both dominant genes, so only the '9' group is different (9:7). For "duplicate dominant," you need only one dominant gene, so only the '1' group (double recessive) is different (15:1). For "recessive epistasis," the double recessive 'aa' group masks the 'B' gene, so group '3' (aaB_) and '1' (aabb) are combined (9:3:4).

Answer 1

The Correct Option is B

Step 1: Concept Overview:
This question assesses understanding of modified Mendelian dihybrid ratios resulting from gene interactions like epistasis. Epistasis occurs when one gene masks or modifies the effect of another at a different locus, altering the standard 9:3:3:1 dihybrid ratio.
Step 2: Detailed Explanation:
Match each gene interaction type with its phenotypic ratio:

A. Duplicate dominant epistasis: A dominant allele at either locus (A_B_, A_bb, or aaB_) yields the same dominant phenotype. Only aabb produces the recessive phenotype, resulting in a (9+3+3) : 1, or 15:1 ratio. Therefore, A matches with II.

B. Duplicate recessive epistasis (Complementary genes): Dominant alleles at both loci (A_B_) are required for the dominant phenotype. All other genotypes (A_bb, aaB_, aabb) produce the recessive phenotype, resulting in a 9 : (3+3+1), or 9:7 ratio. Thus, B matches with I.

C. Recessive epistasis: A recessive genotype at one locus (aa) masks alleles at another (B/b). A_B_ and A_bb produce two phenotypes, while aaB_ and aabb produce a third, resulting in a 9 : 3 : (3+1), or 9:3:4 ratio. Thus, C matches with III.

D. Dominant & Recessive Epistasis (or Dominant Inhibitory Epistasis): The term is somewhat ambiguous, but the 13:3 ratio signifies an interaction where a dominant allele at one locus (A) and the recessive genotype at that locus combined with a recessive genotype at the second locus (bb) produce the same phenotype. The alternative phenotype requires the dominant allele at the second locus (B) but only if the first locus is not dominant (aaB_). A more common name is Dominant Inhibitory Epistasis where a dominant allele at one locus suppresses the expression of the other locus. This produces a (9+3+1) : 3, or 13:3 ratio. Therefore, D matches with IV.

Step 3: Solution:
The correct matching is A-II, B-I, C-III, D-IV, corresponding to option (B).