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What causes a helix in nature to appear with either a

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What causes a helix in nature to appear with either a  [#permalink]

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New post 05 May 2018, 12:01
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What causes a helix in nature to appear with either a dextral (right-handed, or clockwise) twist or a sinistral (left-handed, or counterclockwise) twist is one of the most intriguing puzzles in the science of form. Most spiral-shaped snail species are predominantly dextral. But at one time, handedness (twist direction of the shell) was equally distributed within some snail species that have become predominantly dextral or, in a few species, predominantly sinistral. What mechanisms control handedness and keep left-handedness rare?

It would seem unlikely that evolution should discriminate against sinistral snails if sinistral and dextral snails are exact mirror images, for any disadvantage that a sinistral twist in itself could confer on its possessor is almost inconceivable. But left- and right-handed snails are not actually true mirror images of one another. Their shapes are noticeably different. Sinistral rarity might, then, be a consequence of possible disadvantages conferred by these other concomitant structural features. In addition, perhaps left- and right-handed snails cannot mate with each other, having incompatible twist directions. Presumably an individual of the rarer form would have relative difficulty in finding a mate of the same hand, thus keeping the rare form rare or creating geographically separated right- and left-handed populations.

But this evolutionary mechanism combining dissymmetry, anatomy, and chance does not provide an adequate explanation of why right-handedness should have become predominant. It does not explain, for example, why the infrequent unions between snails of opposing hands produce fewer offspring of the rarer than the commoner form in species where each parent contributes equally to handedness. Nor does it explain why, in a species where one parent determines handedness, a brood is not exclusively right- or left-handed when the offspring would have the same genetic predisposition. In the European pond snail Lymnaea peregra, a predominantly dextral species whose handedness is maternally determined, a brood might be expected to be exclusively right- or left-handed—and this often occurs. However, some broods possess a few snails of the opposing hand, and in predominantly sinistral broods, the incidence of dextrality is surprisingly high.

Here, the evolutionary theory must defer to a theory based on an explicit developmental mechanism that can favor either right- or left-handedness. In the case of Lymnaea peregra, studies indicate that a dextral gene is expressed during egg formation; i.e., before egg fertilization, the gene produces a protein, found in the cytoplasm of the egg, that controls the pattern of cell division and thus handedness. In experiments, an injection of cytoplasm from dextral eggs changes the pattern of sinistral eggs, but an injection from sinistral eggs does not influence dextral eggs. One explanation for the differing effects is that all Lymnaea peregra eggs begin left-handed but most switch to being right-handed. Thus the path to a solution to the puzzle of handedness in all snails appears to be as twisted as the helix itself.
Which of the following would serve as an example of “concomitant structural features” (line 13) that might disadvantage a snail of the rarer form?

A) shell and body that are an exact mirror image of a snail of the commoner form
B) A smaller population of the snails of the rarer form
C) A chip or fracture in the shell caused by an object falling on it
D) A pattern on the shell that better camouflages it
E) A smaller shell opening that restricts mobility and ingestion relative to that of a snail of the commoner form


Spoiler: :: OA
E



The second paragraph of the passage is primarily concerned with offering possible reasons why

A) it is unlikely that evolutionary mechanisms could discriminate against sinistral snails
B) sinistrality is relatively uncommon among snail species
C) dextral and sinistral populations of a snail species tend to intermingle
D) a theory based on a developmental mechanism inadequately accounts for the predominance of dextrality across snail species
E) dextral snails breed more readily than sinistral snails, even within predominantly sinistral populations


Spoiler: :: OA
B



Which of the following accurately describes the relationship between the evolutionary and developmental theories discussed in the passage?

A) Although the two theories reach the same conclusion, each is based on different assumptions.
B) They present contradictory explanations of the same phenomenon.
C) The second theory accounts for certain phenomena that the first cannot explain.
D) The second theory demonstrates why the first is valid only for very unusual, special cases.
E) They are identical and interchangeable in that the second theory merely restates the first in less technical terms.


Spoiler: :: OA
C



It can be inferred from the passage that a predominantly sinistral snail species might stay predominantly sinistral for each of the following reasons EXCEPT for

A) a developmental mechanism that affects the cell-division pattern of snails
B) structural features that advantage dextral snails of the species
C) a relatively small number of snails of the same hand for dextral snails of the species to mate with
D) anatomical incompatibility that prevents mating between snails of opposing hands within the species
E) geographic separation of sinistral and dextral populations


Spoiler: :: OA
B


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Re: What causes a helix in nature to appear with either a  [#permalink]

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New post 05 May 2018, 12:16
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Got slaughtered !
Source of the passage?

Good for excersing brain muscles. Oh sorry, power lifting. Haha.
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New post 05 May 2018, 12:31
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New post 05 May 2018, 12:39
carcass wrote:
Even myself had hard time to keep track. If this might solace you :-)

Haha. Did you get the question(s) right?
Hats off if u did !
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New post 05 May 2018, 14:01
Around 9.5 mins. Got the 4th one wrong. I was stuck between B and C. Couldn't find support for either, so thought of marking C [since only Sinister rarity is mentioned].

Thanx.
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New post 05 May 2018, 21:18
Tough one!!

took 9 mins.

Got all correct except 1.

carcass, could you please help with q1 ?
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Re: What causes a helix in nature to appear with either a  [#permalink]

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New post 05 May 2018, 21:41
Easy passage Tough questions. Took 9 mins 50sec
1 wrong. Looking forward to its explanation.
Q4. asks us to find an answer choice which cannot be a reason for Sin rarity. Option B talks about dextral snails and thus provides no explanation for rarity. Option C is reworded and can be a reason as it is mentioned in the para 2.
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New post 06 May 2018, 07:14
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But left- and right-handed snails are not actually true mirror images of one another. Their shapes are noticeably different.


The key is in this sentence.

A) shell and body that are an exact mirror image of a snail of the commoner form

They are NOT exact mirror, indeed

B) A smaller population of the snails of the rarer form

Nothing is mentioned about small population


C) A chip or fracture in the shell caused by an object falling on it

Nothing is mentioned about this
D) A pattern on the shell that better camouflages it

No mention about camouflages


E) A smaller shell opening that restricts mobility and ingestion relative to that of a snail of the commoner form

Their shape is different but if we do have something that gets close them in terms of evolution-shape, then we do have an example

Hope now is clear.

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New post 07 May 2018, 03:06
Would anyone please explain question 3, question 4 ( need all options explanation) and for question 2 ( Only elimination process of options A and E ) ? Thanks in advance .
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New post 11 May 2018, 13:17
Took me 10:30.

Reading the passage took me 6:30 byitself. The questions took me 4 minutes. The last question I got wrong. I found myself re-reading parts of the passage and then ran out time (I replicated real life exam pressure).

Tip for all, get familiar with a strategy to handle this particular type of scientific passages. I took the gmat yesterday and halfway through my verbal I got really long scientific passage similar to this one (even more convoluted). It was about amphibians' skin and how it helps them breathe and the blood flow and all that. The questions seemed challenging too ( I am sure time pressure and nervousness played a role). What I am doing now, is coming up with a new habitual pattern to tackle these kinds of passages and deal with questions that ask about detail.
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New post 12 May 2018, 16:36
Possible Explanation:
1.Which of the following would serve as an example of “concomitant structural features” (line 13) that might disadvantage a snail of the rarer form?
According to passage question:
Their shapes are noticeably different. Sinistral rarity might, then, be a consequence of possible disadvantages conferred by these other concomitant structural features.
The answer focuses on shape:
ANS: D states something about shape

2.The second paragraph of the passage is primarily concerned with offering possible reasons why
Function question:
Explain why the rarity
ANS: B

3. Which of the following accurately describes the relationship between the evolutionary and developmental theories discussed in the passage?
According to passage
But this evolutionary mechanism combining dissymmetry, anatomy, and chance does not provide an adequate explanation of why right-handedness should have become predominant.
Here, the evolutionary theory must defer to a theory based on an explicit developmental mechanism that can favor either right- or left-handedness
ANS: C

4.It can be inferred from the passage that a predominantly sinistral snail species might stay predominantly sinistral for each of the following reasons EXCEPT for
A. P4: Here, the evolutionary theory must defer to a theory based on an explicit developmental mechanism that can favor either right- or left-handedness.
B. NOT STATED
C. P2: Presumably an individual of the rarer form would have relative difficulty in finding a mate of the same hand, thus keeping the rare form rare or creating geographically separated right- and left-handed populations.
P3: It does not explain, for example, why the infrequent unions between snails of opposing hands produce fewer offspring of the rarer than the commoner form in species where each parent contributes equally to handedness.
It states that union between opposing shapes is possible.

D. P2: In addition, perhaps left- and right-handed snails cannot mate with each other, having incompatible twist directions.
E. P2: Presumably an individual of the rarer form would have relative difficulty in finding a mate of the same hand, thus keeping the rare form rare or creating geographically separated right- and left-handed populations.
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New post 13 May 2018, 00:41
Can anyone explain the answer of question no 4. My answer was E as the passage gave example of European lake also. Could not infer how B is correct
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New post 13 May 2018, 14:01
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ruchik789 wrote:
Can anyone explain the answer of question no 4. My answer was E as the passage gave example of European lake also. Could not infer how B is correct


It can be inferred from the passage that a predominantly sinistral snail species might stay predominantly sinistral for each of the following reasons EXCEPT for

A) a developmental mechanism that affects the cell-division pattern of snails
B) structural features that advantage dextral snails of the species
C) a relatively small number of snails of the same hand for dextral snails of the species to mate with
D) anatomical incompatibility that prevents mating between snails of opposing hands within the species
E) geographic separation of sinistral and dextral populations

You basically have to find the wrong answer (because it says all correct EXCEPT).

A) Mentioned in the last paragraph
B) Not mentioned in the passage. It's alluded that it may be a reason and then kinda dismissed
C) Mentioned end of 2nd paragraph or 3rd I think
D) Mentioned end of 2nd paragraph or 3rd I think
E) End of paragraph mentioned

B is the only one not mentioned and therefore is not true which makes it correct.
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New post 13 May 2018, 20:14
oasis90 wrote:
ruchik789 wrote:
Can anyone explain the answer of question no 4. My answer was E as the passage gave example of European lake also. Could not infer how B is correct


It can be inferred from the passage that a predominantly sinistral snail species might stay predominantly sinistral for each of the following reasons EXCEPT for

A) a developmental mechanism that affects the cell-division pattern of snails
B) structural features that advantage dextral snails of the species
C) a relatively small number of snails of the same hand for dextral snails of the species to mate with
D) anatomical incompatibility that prevents mating between snails of opposing hands within the species
E) geographic separation of sinistral and dextral populations

You basically have to find the wrong answer (because it says all correct EXCEPT).

A) Mentioned in the last paragraph
B) Not mentioned in the passage. It's alluded that it may be a reason and then kinda dismissed
C) Mentioned end of 2nd paragraph or 3rd I think
D) Mentioned end of 2nd paragraph or 3rd I think
E) End of paragraph mentioned

B is the only one not mentioned and therefore is not true which makes it correct.


Thanks. It does make sense.
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Re: What causes a helix in nature to appear with either a  [#permalink]

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New post 13 Sep 2018, 05:24
Hi. Can you please explain Q2?
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Re: What causes a helix in nature to appear with either a &nbs [#permalink] 13 Sep 2018, 05:24
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