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**: Selectable markers are genes that are introduced into a cell along with the gene of interest. Their primary function is to allow for the identification and selection of cells that have successfully incorporated the desired DNA.

2. **Function of Selectable Markers**: The main role of selectable markers is to help in the identification and elimination of non-transformants (cells that did not take up the foreign DNA). This is crucial in experiments where only a small percentage of cells may be successfully transformed.

3. **Examples of Selectable Markers**: Common examples of selectable markers include antibiotic resistance genes. For instance, genes that confer resistance to antibiotics like ampicillin or kanamycin are often used. Only the cells that have taken up the plasmid containing the selectable marker will survive in the presence of the antibiotic.

4. **Process of Selection**: When the transformed cells are grown on a medium containing the antibiotic, only those cells that have successfully integrated the selectable marker (and thus the gene of interest) will survive. This allows researchers to easily identify and isolate the successful transformants.

5. **Conclusion**: Therefore, the correct function of a selectable marker is to help in eliminating known transformants so that only the desired transformants can be regenerated.

### Final Answer:
A selectable marker is used to help in eliminating known transformants so that transformants can be regenerated.

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