Q.48 Two independent non-segregating recessive mutants ( 𝒎𝟏 and 𝒎𝟐 ) display similar defects in petal

formation. When they were crossed with each other ( 𝒎𝟏 × 𝒎𝟐 ), all the F1 plants developed normal

petals. In view of this observation, which of the following conclusions is CORRECT?

(A) Mutations in both 𝑚𝑙 and 𝑚2 are in the same gene

(B) Mutations in both 𝑚𝑙 and 𝑚2 are in two separate genes

(C) Inheritance is non-Mendelian

(D) None of the above

The correct answer is (B) Mutations in both 𝑚₁ and 𝑚₂ are in two separate genes.

This result from crossing two recessive mutants with the same phenotype demonstrates complementation, a key genetic test.

Option Analysis

(A) Same Gene

If both mutations were in the same gene, the F1 plants would inherit two mutant alleles (m1/m2), showing defective petals since no functional copy exists. Normal petals in F1 rule this out.

(B) Separate Genes (Correct)

Each parent is homozygous recessive for different genes (e.g., m1 m1 M2 M2 × M1 M1 m2 m2). F1 gets one wild-type allele per gene (M1 m1 M2 m2), restoring function via complementation.

(C) Non-Mendelian

The pattern follows Mendelian diploid inheritance with recessive traits and complementation, not cytoplasmic or other non-Mendelian mechanisms.

(D) None

Option (B) fits perfectly, so this is incorrect.

Complementation Test Overview

Homozygous recessive mutants with identical phenotypes are crossed. Wild-type F1 indicates different genes; mutant F1 indicates the same gene. “Non-segregating” means true-breeding homozygotes.

Two independent non-segregating recessive mutants m1 and m2 show similar petal formation defects, but their cross (m1 × m2) produces all F1 plants with normal petals. This classic complementation test scenario from CSIR NET Life Sciences reveals mutations in separate genes.

Why F1 Shows Normal Petals

In genetics, the complementation test determines if mutations causing the same recessive phenotype are allelic (same gene) or non-allelic (different genes). Here, parents are homozygous: one m1 m1 (wild-type at gene 2), the other m2 m2 (wild-type at gene 1). F1 heterozygotes carry one functional allele per gene, complementing defects for wild-type petals.

Detailed Option Breakdown

• (A) Same gene: F1 would be mutant/mutant, defective petals—no complementation. Incorrect.

• (B) Separate genes: Matches observation—correct via complementation.

• (C) Non-Mendelian: Follows Mendelian rules. Incorrect.

• (D) None: (B) is valid. Incorrect.

CSIR NET Relevance

This tests gene mapping and epistasis understanding, key for molecular biology/genetics sections. Practice similar: white flower crosses yielding red F1.