AAMC FL 2 Q 30

[Neplina94]

Researchers have noted that chloramphenicol (a commonly used antibiotic) is becoming less effective in treating typhoid fever. The best explanation for this observation would be selection:

ANS: for chloramphenicol resistance in populations of S typhi.

(note: according to the passage, S. Typhi is the causative agent of typhoid fever.)

Can someone please explain to me the rationale behind the answer, noteably, the difference between selecting for/against? What exactly does this mean? Thank you!

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[aldol16]

Well you'll have to give more information than that. Antibiotic resistance is a huge problem in medicine and you should understand how that occurs and what that means.

 

[Neplina94]

Well you'll have to give more information than that. Antibiotic resistance is a huge problem in medicine and you should understand how that occurs and what that means.

Im sorry, I thought maybe someone would recognise the problem. I uploaded the passage (with relevant section highlighted) as well as the answer choices. Specifically, would you be able to differentiate selecting "for and against" in the context of the question/passage?

 

[aldol16]

Well, A and B can be immediately ruled out because selection for and against chloramphenicol doesn't make sense. Selection is for resistance genes. Selection always has to be for or against some biological substrate, i.e. within the organism. Selection typically is used in the context "selection for gene X" or "selection for trait X." In this case, the trait is resistance.

 

[Neplina94]

Well, A and B can be immediately ruled out because selection for and against chloramphenicol doesn't make sense. Selection is for resistance genes. Selection always has to be for or against some biological substrate, i.e. within the organism. Selection typically is used in the context "selection for gene X" or "selection for trait X." In this case, the trait is resistance.

Thank you for your response. I understand selecting for a gene, however what does it mean to select "against" a gene. I couldnt find proper explanations online. Thank you

 

[Neplina94]

just makin

Well, A and B can be immediately ruled out because selection for and against chloramphenicol doesn't make sense. Selection is for resistance genes. Selection always has to be for or against some biological substrate, i.e. within the organism. Selection typically is used in the context "selection for gene X" or "selection for trait X." In this case, the trait is resistance.

making sure i understand here, if bacteria select for chlorom.. resistance, this means they are resistant to the bacteria and will multiply. So is selecting against the exact opposite?

 

[Matthew9Thirtyfive]

just makin

making sure i understand here, if bacteria select for chlorom.. resistance, this means they are resistant to the bacteria and will multiply. So is selecting against the exact opposite?

In this situation, selecting against chloramphenicol resistance makes no sense. The way selection for resistance works is that you have a group of bacteria exposed to an antibiotic. Some of the bacteria in the population have a trait that makes them resistant to the antibiotic. So the antibiotic kills all the bacteria that are not resistant, leaving only the ones that are--therefore, it is selecting for antibiotic-resistant bacteria. You don't select for bacteria that are not resistant to the antibiotic, because the ones that aren't resistant get killed lol.

 

[Matthew9Thirtyfive]

Thank you for your response. I understand selecting for a gene, however what does it mean to select "against" a gene. I couldnt find proper explanations online. Thank you

Selecting for or against a trait occurs when something happens that causes a certain genotype or phenotype to be selected for or against. For example, with the finches on the Galapagos islands, there was a huge drought that destroyed a lot of the plants that left seeds for the birds. So there were multiple phenotypes of these birds, some with big beaks and some with medium beaks and some with small beaks. So there were only a couple types of seeds available during the drought, and they were all huge. So the birds with the big beaks thrived while the smaller-beaked birds died out because they couldn't break the seeds open. So the big beaks were selected for, while the small beaks were selected against.

 

[aldol16]

Thank you for your response. I understand selecting for a gene, however what does it mean to select "against" a gene. I couldnt find proper explanations online. Thank you

if bacteria select for chlorom.. resistance, this means they are resistant to the bacteria and will multiply. So is selecting against the exact opposite?

Yes, basically. Selecting against a gene means that there is negative selection pressure, or a reason why that gene's product is unfavorable. Remember that in order for selection to occur, there must be a genotype-phenotype relationship. In other words, the gene codes for something that is then expressed and has a functional purpose - it is that functional purpose that is selected for or against. For example, say there was a gene (genotype) for bright orange hair (phenotype). Bright orange hair tends to attract predators in the forest, where everything else is green or brown. Bright orange haired animals get eaten at a much higher rate than other animals. We would thus say there is negative selection against the bright orange hair gene, because it leads an unfavorable trait. In evolution, unfavorable = gets you killed at a higher rate than others.

 

[Neplina94]

Great explanations! Thanks guys