Why might a resistance gene be present but not expressed, leading to a susceptible phenotype?

• A. The gene may be transcriptionally silent or require certain conditions or regulatory elements to express.
• B. The detection assay always detects the gene irrespective of expression.
• C. The gene is mutated to nonfunctional form.
• D. The gene's presence guarantees resistance regardless of expression.

Answer: (B)

The key idea is that having a resistance gene is not the same as expressing it. For a resistance mechanism to protect the cell, the gene must be turned on, transcribed, and translated into the functional product. If regulatory elements aren’t active, or the promoter is silent, the gene sits idle and no resistant protein is made. Environmental conditions, regulators, or repressors can keep expression off even though the gene is present, so the organism appears susceptible. This distinction between gene presence and gene expression is what explains a present gene not producing resistance. The fact that a DNA test can detect the gene regardless of whether it is expressed doesn’t contradict this, but it doesn’t explain the observed phenotype unless the gene is actually expressed.