17. The results of a complementation test for five independent mutants (1 to 5) are summarized below:

‘+’ represents complementation;
‘O’ represents non-complementation.
Based on the above, which one of the following conclusion is correct?
(1) There are two cistrons. Mutations 1, 2 and 3 belong to one cistron; while 4 and 5 belong to a second cistron.
(2) There is a single cistron. Mutations 1, 2 and 3 can recombine out from 4 and 5.
(3) Each mutation represents a cistron.
(4) There are two linkage groups. 1, 2, 3 comprise onegroup while 4 and 5 comprise the second group.

Understanding the matrix

• “0” = non‑complementation → mutations are in the same cistron (same gene).

• “+” = complementation → mutations in different cistrons (different genes).

From the table:

• 1 vs 2 = 0, 1 vs 3 = 0, 2 vs 3 = 0 → mutants 1, 2, 3 are in the same cistron.

• 4 vs 5 = 0 → mutants 4 and 5 are in another same cistron.

• 1, 2, 3 each give “+” with 4 and 5 → these two sets are in different cistrons.

So there are exactly two cistrons (genes):

• Cistron A: mutants 1, 2, 3.

• Cistron B: mutants 4, 5.

Option-wise explanation

1. There are two cistrons… – correct

• Matches the grouping above: 1–3 in one cistron, 4–5 in another.

2. There is a single cistron… – incorrect

• If there were one cistron, all mutant pairs would show 0 (no complementation). Here 1–3 complement 4–5, so at least two genes are involved.

3. Each mutation represents a cistron – incorrect

• This would imply five cistrons, but non‑complementation among 1–3 and between 4 and 5 clearly groups them into only two functional units.

4. There are two linkage groups… – incorrect

• Complementation tests classify mutations into cistrons (functional genes), not linkage groups (chromosomal locations). The matrix tells about allelism/complementation, not physical linkage.

Thus, the complementation data support conclusion (1): there are two cistrons—mutations 1, 2, 3 in one and 4, 5 in the other.