16. Four different mutant lines showing similar phenotype were identified from a genetic screen. When genetic crosses among these mutants were carried out, the first mutant was found to complement the second, third and lines. However, no other fourth mutant complementation groups do the four mutant lines belongs to?
(1) 1          (2) 2
(3) 3          (4) 4

Explanation

• A complementation group consists of mutants that fail to complement each other because mutations lie within the same gene (allelic).

• Mutations in different genes complement each other and produce wild-type progeny when crossed.

Given:

• Four mutants all show a similar phenotype.

• Mutant 1 complements mutants 2, 3, and 4.

• Mutants 2, 3, and 4 do not complement each other.

Interpretation:

• Mutant 1 belongs to a different gene (complementation group) since it complements the others.

• Mutants 2, 3, and 4 are in the same complementation group (same gene) because they fail to complement each other.

So:

• Total complementation groups = 2

  • Group 1: mutant 1 alone.

  • Group 2: mutants 2, 3 and 4.

Option-wise explanation

1. 1 – Incorrect; not all mutants belong to one group because mutant 1 complements the others.

2. 2 – Correct; one group contains mutant 1, the other group contains mutants 2, 3, 4.

3. 3 – No evidence of three groups since three mutants fail to complement each other.

4. 4 – No; four separate groups would require all mutants to fail complementing all others.

Therefore, the four mutants belong to two complementation groups (option 2).