A selectable marker is used to:

Options

A. help in eliminating the non-transformants, so that the transformants can be regenerated
B. identify the gene for a desired trait in an alien organism
C. select a suitable vector for transformation in a spe- cific crop
D. mark a gene on a chromosome for isolation using restriction enzyme

Answer

### Step-by-Step Solution:

1. **Understanding Selectable Markers**: A selectable marker is a gene that is introduced into an organism's DNA to help identify which cells have successfully taken up a foreign DNA segment.

2. **Function of Selectable Markers**: The primary function of a selectable marker is to eliminate non-transformants (cells that did not take up the foreign DNA). This is crucial in genetic engineering, where only the cells that have been successfully transformed are desired.

3. **Mechanism of Action**: Selectable markers often confer resistance to certain antibiotics or toxins. When the cells are exposed to these agents, only those that have incorporated the selectable marker (and thus the foreign DNA) will survive, while the non-transformants will die off.

4. **Application**: By using a selectable marker, researchers can easily identify and isolate the transformants from the non-transformants. This allows for the regeneration of the desired transformants that carry the gene of interest.

5. **Conclusion**: Therefore, the correct answer to the question is that a selectable marker is used to help in eliminating the non-transformants so that the transformants can be regenerated.