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Question #45597
The Question
A geneticist compares the molecular basis of 'white' phenotypes in two different biological systems: a white-flowered Antirrhinum majus (snapdragon) and a white colony of E. coli containing a non-recombinant pUC8 plasmid, grown on an agar plate containing X-gal. Sequencing reveals that the E. coli plasmid has a severe point mutation in the lacZ promoter region. Which of the following statements accurately contrasts the genetic principles governing these two white phenotypes?
Options
- A.
The white flower represents a quantitative dosage effect where insufficient enzyme is produced, whereas the white colony represents insertional inactivation of the lacZ structural gene.
- B.
The white flower results from homozygous null alleles failing to synthesize a functional pigment-producing enzyme, while the white colony results from the failure of RNA polymerase to bind the promoter, preventing transcription of the β-galactosidase gene.
- C.
Both phenotypes represent a qualitative loss of function where the structural gene is completely intact but the substrate (precursor pigment or X-gal) is absent from the cellular environment.
- D.
The white flower is due to the absence of a dominant epistatic gene required for anthocyanin production, whereas the white colony is due to the competitive inhibition of β-galactosidase by X-gal.
Check Your Answer
Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Comparative_Analysis
Sharpen your ability to evaluate multiple scenarios and distinguish between competing potential answers.
Concepts Behind This Question
Beta-galactosidase|Lac Z gene|Phenotypic expression
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students reflexively associate 'white colonies' in biotechnology exclusively with 'insertional inactivation' (recombinants).
This question forces them to recognize that a promoter mutation in a non-recombinant also yields a white phenotype by preventing transcription.
It then contrasts this qualitative, binary biotech readout with the molecular basis of a classic Mendelian/Non-Mendelian null allele (white snapdragon), testing deep comprehension of gene expression mechanisms.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Principles of Inheritance and Variation - Class 12 |
| Concept | Incomplete Dominance Pheno-Geno |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |