After a hepadnavirus inserts itself into a chromosome

Question

After a hepadnavirus inserts itself into a chromosome of an animal, fragments of the virus are passed on to all of that animal's descendants. A hepadnavirus fragment is present in a chromosome of the zebra finch and in precisely the same location in a corresponding chromosome of the dark-eyed junco. The fact that these two bird species diverged from each other about 25 million years ago therefore means that the hepadnavirus is at least 25 million years old.

Which one of the following, if true, most strengthens the argument?

(A) Viruses can affect the evolution of an organism and can thereby influence the likelihood of their diverging into two species.

(B) The chromosomes of the zebra finch and the dark-eyed junco contain fragments of no virus other than the hepadnavirus.

(C) When a virus inserts itself into an animal's chromosome, the insertion occurs at a random spot.

(D) Many bird species other than the zebra finch and the dark-eyed junco contain fragments of the hepadnavirus.

(E) The presence of a hepadnavirus in an animal species does not affect the likelihood of that species' survival.

Source: LSAT

GMATNinja · Accepted Answer

We are told that "a hepadnavirus fragment is present in a chromosome of the zebra finch and  in precisely the same location  in a corresponding chromosome of the dark-eyed junco."

But what if the virus always inserts itself in the same location whenever it infects an animal? If that were the case, the fact that the virus is present in the same location in two animals would not be strong evidence that the virus must have entered that location before the two animals diverged. In that case, the virus could have entered the two animals independently at any point in time.

Choice (C) tells us that the location is random, which means it would be highly unlikely that the virus entered the two animals independently. This suggests that the virus likely entered that location before the two animals diverged 25 million years. This, in turn, supports the conclusion that the virus is at least 25 million years old.

Choice (A), on the other hand, does not give us any reason to believe that the virus likely entered that location before the two animals diverged. This statement is much too general to suggest that the specific hepadnavirus discussed in the passage is what caused the animals to diverge. There could have been numerous other viruses affecting the organism more than 25 million years ago, and we have no idea whether this specific hepadnavirus was one of them.

Choice (C) is the best answer.

broall · Answer

The chromosomes of the zebra finch and the dark-eyed junco contain fragments of the hepadnavirus at the same location. What if that virus inserts itself into the same location of an animal's chromosome? Then we can't conclude that that virus is at least 25 million years old because it is possible that the hepadnavirus has inserted itself into these two kinds of birds. Hence C strengthens the argument.

arvind910619 · Answer

I am totally confused by the answer to this question 
I thought the answer is A 
Experts please explain the answer as the argument does not talks about the pattern of insertion of the chromosome.

ma14292 · Answer

The answer is C because, given that the virus appears in a random chromosome, the chances of finding it in the same spots in two different species is very very very low unless the two species were merged into ONE.

adkikani · Answer

GMATNinja   broall

Do we not have to strengthen this conclusion?

Is not B strengthening the argument by eliminating against other possibilities and protecting against
potential weakeners? Let me know your thoughts.

WR,
Arpit.

arvind910619 · Answer

Hi GMATNinja

Thanks for the explanation though i had to read it twice to understand fully.
I find LSAT CR to much harder than GMAT CR.

Regards,
Arvind

GMATNinja · Answer

Agreed! In theory, GMAT OG questions range in difficulty from 200 to 800, but I'd argue that the LSAT questions range in difficulty from 500 to 850. Or something like that. So they're great practice if you need an extra challenge -- and if you can nail the LSAT stuff, the GMAT questions will eventually start to feel easy. Hopefully.

arvind910619 · Answer

Indeed i have made it point to practice LSAT question to have a healthy challenge

abrakadabra21 · Answer

Conclusion :- Virus H is at least 25 million years old. 
Premise:-

1> Found in is present in a chromosome of the zebra finch and in precisely the same location in a corresponding chromosome of the dark-eyed junco.
2> Two birds were diverged 25 million years before. 
3> fragments of the virus are passed on to all of that animal's descendants.

So if H is found at the same location in two birds who belonged to same species, logical conclusion is - H is 25 years old otherwise they wouldn't have found it in both birds.

Further, "Precisely same location" carries some significance for this conclusion.

This conclusion can be weakened :- 
1. if we can show that in both birds, it happened recently. (imagine you have a grandfather. if you have X attribute and he has X attribute, may be you got it from your grandfather, but what if, you both acquired it recently) right?

(A) Viruses can affect the evolution of an organism and can thereby influence the likelihood of their diverging into two species. :- 
I think this would have been a better strengthener if it is written in this manner :- 
Only H Viruses affect the evolution of an organism and thereby influence the likelihood of their diverging into two species :- In this case, we are 100% sure that virus was there 25 million years before and it was the reason that divergence happen.

(C)When a virus inserts itself into an animal's chromosome, the insertion occurs at a random spot.
If it is not true then it weakens our conclusion. if it occurs randomly, then why H is found at the same location for both birds. They both have acquired this from the same descendants. otherwise if it is a recent time, for zebra finch it could have been at X location and for DEJ it could have been another Y location.  
It strengthen our conclusion.

GMATNinja · Answer

We are only told that fragments of a  hepadnavirus  are passed on to all descendants once inserted into an animal's chromosome. We are not told what happens when a chromosome is infected with any other kind of virus. For example, perhaps no other virus passes fragments on to an animal's descendants.

This information does not tell us whether the hepadnavirus entered those animals before or after they diverged and, thus, does not tell us anything about the age of the hepadnavirus.

Choice (B) neither strengthens nor weakens the argument.

stne · Answer

What if in its random insertion it inserted itself into the same spot that it went into 25 years ago?
This could give us a false proof of its age.

Lets say the virus went into the dark-eyed junco chromosome 10 years later but it went into the same spot as it had in the zebra finch , this would certainly not strengthen the argument.

option C doesn't say the virus infecting the same spot twice is highly unlikely.If it said so then it could certainly imply that both the species got infected at the same time 25 years ago.

option C falls apart if the virus in its random insertion it coincidentally went into the same spot as it had 25 years ago.

Sidraman57 · Answer

After a hepadnavirus inserts itself into a chromosome of an animal, fragments of the virus are passed on to all of that animal's descendants. A hepadnavirus fragment is present in a chromosome of the zebra finch and in precisely the same location in a corresponding chromosome of the dark-eyed junco. The fact that these two bird species diverged from each other about 25 million years ago therefore means that the hepadnavirus is at least 25 million years old.

Which one of the following, if true, most strengthens the argument?

(C) When a virus inserts itself into an animal's chromosome, the insertion occurs at a random spot.
talks about being in precisely the same location just as in the passage.

Kushchokhani · Answer

stne Your contention seems to be true. However, choice C seems better than choice A on the basis of specific is preferred over general. Choice A is very general and may not cover this specific virus. On the contrary, choice C states "a virus" which covers this specific virus as well. Also, please refer to this post which somewhat explains our contrary contention. https://gmatclub.com/forum/after-a-hepa ... l#p1922724

BBianca · Answer

I don't understand how C is strengthening the argument here. As a matter of fact, I thought it weakened the conclusion because as I can see, the speaker says that the both birds diverged from each other and we know this because they have the virus in the same spot. So, ,how exactly does insertion of the virus in a random spot help?

unraveled · Answer

Refer the passage again and notice the sequence. I'm no expert but would try best to explain to help, though i faltered in this one.

First, fragments are passed on to next generations. Then it gives an info about ZF and DEJ birds wherein the H virus fragment is present in chromosome of the two birds at the same location.  (Now, here one must have raised his/her eyebrows for why such an info(location) is given at this point.)  
Then there's this conclusion as in the two birds are related, diverging 25M years ago and thus H virus is 25M years old. How is that possible? 
So, you get the jump that the conclusion makes here. We need to just find that jump. I am more comfortable with POE approach than choosing C.

D and E are plain wrong for irrelevancy. A is wrong for two red text nowhere elaborate or even suggest slightly about how old the virus is. B is wrong for similar logic but by different factor. Again how other virus fragments help find age of the H virus. Here itself you can choose C.

C gives a point blank info that is hard to digest for how it is at all helping us. But if the spot of insertion(different from location of fragment presence) is not same, does it weaken, may be. Whether same location or not the vital point is H virus fragment is passed on to next generations, thus giving us a hint that Virus is also somewhat 25M years old.

HTH.

GMATNinja · Answer

We actually don't know that the two species diverged  because  they have the virus in the same spot. We just know that the two species diverged at some point, and also have the virus in the same spot.

In the passage, the author attempts to set up a timeline of events: first, an animal got infected by a hepadnavirus. Then, the animal's descendants diverged into two distinct species. Because the species diverged about 25 million years ago, the author concludes that the virus must be at least that old.

Notice that there's a gap in this argument: how do we know that the virus infected the animal  before  it diverged into two species? Isn't it possible that the virus infected both species separately, after they diverged?

(C) helps us to close this gap:
 
We know from the passage that the two species are infected in "in precisely the same location" within their respective chromosomes. (C) tells us that this point of infection is random. The odds of two species randomly getting infected in the exact same spot seem pretty low. So, the information in (C) strengthens the argument that the animal got infected  before  it diverged into two species.

(C) is the correct answer.

I hope that helps!

VerbalBot · Answer

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After a hepadnavirus inserts itself into a chromosome

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