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# After a hepadnavirus inserts itself into a chromosome

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04 Sep 2017, 02:50
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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
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04 Sep 2017, 07:50
ganand wrote:
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

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.
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05 Sep 2017, 07:04
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.
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09 Sep 2017, 07:47
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.
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10 Sep 2017, 08:52
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1
arvind910619 wrote:
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.

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.

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

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.

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

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.
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10 Sep 2017, 17:35
GMATNinja broall

Quote:
the hepadnavirus is at least 25 million years old

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.
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11 Sep 2017, 05:37
GMATNinja wrote:
arvind910619 wrote:
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.

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.

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

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.

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

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.

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
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11 Sep 2017, 08:08
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arvind910619 wrote:
I find LSAT CR to much harder than GMAT CR.

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.
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11 Sep 2017, 08:39
GMATNinja wrote:
arvind910619 wrote:
I find LSAT CR to much harder than GMAT CR.

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.

Indeed i have made it point to practice LSAT question to have a healthy challenge
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11 Sep 2017, 23:18
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.
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20 Sep 2017, 20:48
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.

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

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.
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20 Sep 2017, 23:10
arvind910619 wrote:
GMATNinja wrote:
arvind910619 wrote:
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.

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.

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

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.

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

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.

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

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.
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