- A Drosophila mutant (line A) with vestigial wings is isolated in a laboratory. The vestigial wing phenotype was observed to be recessive and mapped to gene ‘X’. Three other laboratories also isolated vestigial mutants, called line B, C and D. In order to test if the mutation in lines B-D also mapped to gene the following crosses were made and phenotype of F1 progeny observed.
| Cross | F1 progeny (wing morphology |
| A X B | vestigial |
| A X B | Vestigial |
| A X D | Normoal |
| B X C | Vestigial |
| B X D | Normal |
| C X D | Normal |
Based on the above identify the line(s) which is most likely NOT to have a mutation in gene
(1) Both lines B and C
(2) Line C only
(3) Line D only
(4) Both lines B and D
Drosophila Vestigial Wings Mutation Analysis and Crosses
Question Data (Reconstructed)
[translate:vestigial wings], recessive mutation mapped to gene X.
Other [translate:vestigial mutants]: lines B, C, D (unknown whether in gene X).
Crosses and F1 wing phenotype:
- A × B → Vestigial (listed twice, same result)
- A × D → Normal
- B × C → Vestigial
- B × D → Normal
- C × D → Normal
Goal: identify which line(s) is/are most likely NOT mutated in gene X.
Genetic Logic of the Crosses
Assumptions:
- Wild-type allele at gene X = X⁺ (normal wings).
- Recessive mutant allele at gene X = x (vestigial).
- A is known: A = x/x.
Interpretations:
- A × B → all F1 vestigial: B is very likely x/x at gene X since all offspring show mutant phenotype.
- A × D → all F1 normal: D provides at least one wild-type X⁺ allele, so D is most likely NOT mutated in gene X.
- B × C → all F1 vestigial: C also likely x/x at gene X as F1 progeny show mutant phenotype.
- B × D → all F1 normal: D complements B, indicating D is wild-type at gene X.
- C × D → all F1 normal: D complements C, consistent with D not mutated at gene X.
Final Inference
A, B, and C behave as recessive mutants in gene X (fail to complement each other). Line D consistently complements all, so it is most likely NOT mutant in gene X.
Therefore, Line D only is most likely NOT mutated in gene X.
Why Each Option is Right or Wrong
- Option 1: Both lines B and C – Incorrect. B and C fail to complement A and each other, so both are mutated in gene X.
- Option 2: Line C only – Incorrect. C behaves like B, mutated in gene X.
- Option 3: Line D only – Correct. D complements all mutants, so not mutated in gene X.
- Option 4: Both lines B and D – Incorrect. B is mutant, D is not.
Introduction
In [translate:Drosophila genetics], complementation tests help determine if similar mutant phenotypes arise from mutations in the same gene. This problem uses vestigial wing mutants from lines A, B, C, and D to identify which line is not mutated in gene X based on F1 wing phenotypes from specific crosses.
Stepwise Solution
- Known info: Line A has vestigial wings due to recessive mutation at gene X (x/x). Lines B, C, D also have vestigial wings, gene status unknown.
- Crosses with A: A × B and A × C produce vestigial F1 implying B and C carry mutation in X. A × D produce normal F1 implying D is wild-type at X.
- Crosses among B, C, D: B × C produce vestigial F1 (both mutated at X). B × D and C × D produce normal F1 (D complements mutation).
- Final conclusion: A, B, C mutated at gene X; D not mutated at gene X.
