Population Genetics MCAT Practice Question
A 35-year-old man with cystic fibrosis is enrolled in a study examining Pseudomonas aeruginosa lung colonization. Genetic sequencing of bacteria isolated from his sputum over 18 months reveals a striking change in allele frequencies at a locus conferring fluoroquinolone resistance. The resistance allele frequency increased from 0.15 to 0.85 during this period, despite the patient receiving continuous fluoroquinolone therapy. Population genetic analysis estimates the selection coefficient (s) at approximately 0.6 for resistant versus susceptible genotypes. Which of the following best explains the rapid fixation of the resistance allele in this bacterial population?
Answer choices
- AStrong directional selection with substantial fitness advantage of resistant genotypes under antibiotic pressureCorrect answer
- BGenetic drift causing random fluctuations in allele frequency in a small effective population size
- CBalancing selection maintaining both resistant and susceptible alleles at intermediate frequencies
- DWeak mutation pressure gradually introducing resistance alleles at a constant rate
- EStabilizing selection favoring heterozygous bacteria with intermediate resistance
- FGene flow from environmental Pseudomonas species introducing pre-existing resistance alleles
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