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: Understanding the Concept:
This question tests the knowledge of modified Mendelian dihybrid ratios that result from different types of gene interactions, specifically epistasis. Epistasis occurs when the effect of one gene is masked or modified by another gene at a different locus. The standard dihybrid ratio is 9:3:3:1, which gets modified in epistasis.
Step 2: Detailed Explanation:
Let's match each type of gene interaction with its characteristic phenotypic ratio:

A. Duplicate dominant epistasis: The dominant allele at either of the two loci (A_B_, A_bb, or aaB_) produces the same dominant phenotype. Only the double recessive genotype (aabb) produces the recessive phenotype. The ratio becomes (9+3+3) : 1, which is 15:1. Thus, A matches with II.

B. Duplicate recessive epistasis (Complementary genes): The dominant alleles at both loci are required to produce the dominant phenotype (A_B_). All other genotypes (A_bb, aaB_, aabb) produce the recessive phenotype. The ratio becomes 9 : (3+3+1), which is 9:7. Thus, B matches with I.

C. Recessive epistasis: A recessive genotype at one locus (e.g., aa) masks the expression of alleles at another locus (B/b). The genotypes A_B_ and A_bb produce two different phenotypes, while aaB_ and aabb produce a third, single phenotype. The ratio becomes 9 : 3 : (3+1), which is 9:3:4. Thus, C matches with III.

D. Dominant \& Recessive Epistasis (or Dominant Inhibitory Epistasis): The question uses a slightly ambiguous term, but the ratio 13:3 is characteristic of a specific interaction where a dominant allele at one locus (A) and the recessive genotype at that same locus (aa) combined with the recessive genotype at the second locus (bb) produce the same phenotype. Essentially, the dominant allele at the second locus (B) is required for the alternative phenotype, but only when 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 leads to a (9+3+1) : 3 or 13:3 ratio. Thus, D matches with IV.

Step 3: Final Answer:
The correct set of matches is A-II, B-I, C-III, D-IV, which corresponds to option (B).