For much of the twentieth century, hereditary traits were understood

Question

For much of the twentieth century, hereditary traits were understood to be transmitted exclusively through the nucleotide sequences in DNA. While environmental conditions massively impact each individual’s capability to reach genetic potential, it was generally thought impossible for those impacts to be heritable. Although this simplistic model has provided decades of enormous real-world productivity, recent evidence from the emerging field of epigenetics has revealed complications that researchers are only beginning to comprehend.

Epigenetic marks are chemical changes to DNA or associated structures that alter gene expression without changing the DNA’s nucleotide sequence. The two most studied mechanisms are DNA methylation, in which methyl groups attach to cytosine bases and silence nearby genes, and histone modification, in which chemical tags on the proteins around which DNA is wound loosen or tighten the helical shape of the DNA molecule itself, making genes more or less accessible to transcription machinery. Crucially, some of these marks are  mitotically stable : they persist through cell division and thus can be transmitted into new cells—enabling the marks to persist longer than the cells’ regenerative cycle, which ranges from a few days for the epithelial cells lining the stomach up to decades for cells in the body’s largest bones.

The same quality also enables the transmission of epigenetic marks into sperm and egg cells, theoretically making possible the phenomenon of transgenerational epigenetic inheritance—the passing of epigenetic marks from parents on to their offspring through the reproductive process. Rodent studies have found altered gene expression patterns in the offspring of animals exposed to stressors such as toxins or nutrient deficiencies, even when the offspring have not been exposed to those stressors. In one well-known set of experiments, male mice fed a low-protein diet sired offspring with markedly altered hepatic gene expression, even though the offspring were raised on standard diets.

Skeptics caution that demonstrating true epigenetic inheritance requires ruling out alternative explanations, including behavioral transmission, microbiome transfer, and conventional genetic mutation. Whether such inheritance operates in humans remains altogether unknown, largely because the human genome has been shown to undergo extensive epigenetic reprogramming during the embryonic and early fetal stages—a process that would be expected to erase most parental marks.
  
 1 
 1. The primary purpose of the passage is to

(A) compare the relative impacts of two possible types of determining factors in genetic inheritance

(B) defend a well-established scientific model against a set of recently proposed theories that challenge it

(C) introduce and examine the current state of evidence for a possible refinement to a long-accepted scientific framework

(D) present recent research findings that upend the central principle of a widely accepted scientific theory

(E) present two possible alternative mechanisms for a widely investigated biological process, with examples of each in animals

2 
 2. If an epigenetic mark altering gene expression in epithelial cells of the stomach lining is  NOT  mitotically stable, then it can most clearly be inferred that this alteration

(A) can be passed to offspring only through sperm or egg cells

(B) comes about through either DNA methylation or histone modification

(C) may spread to cells outside the stomach

(D) will not persist for more than a few days without re-exposure to its cause

(E) will regenerate itself within that region of the body via a cyclical mechanism

3 
 3. The author most likely cites the low-protein diet experiment with male mice in order to

(A) demonstrate that transgenerational epigenetic effects can manifest in offspring without direct environmental exposure

(B) prove that dietary modifications can create heritable changes in the nucleotide sequences of DNA

(C) provide an example of the behavioral transmission of epigenetic modifications of gene expression

(D) suggest that the effects of significant fundamental lifestyle changes in a parent animal are likely to redound to its offspring

(E) support the claim that epigenetic marks can be passed from parent to offspring outside the reproductive process

4 
 4. The passage implies that epigenetic reprogramming of the human genome during the embryonic and early fetal stages is relevant to the debate over transgenerational inheritance because it

(A) cannot possibly co-occur in the same individual as transgenerational epigenetic inheritance

(B) confirms the presence of transgenerational epigenetic inheritance in human beings

(C) narrows the range of possible mechanisms of epigenetic modification to human genes

(D) raises the possibility that transgenerational epigenetic inheritance may be active in cases where it leaves no lasting evidence

(E) suggests that human genes are more amenable than rodent genes to epigenetic modification than that of rodents

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RonPurewal · Accepted Answer

Official Explanations:

1.

The primary purpose of the passage is to

(A) compare the relative impacts of two possible types of determining factors in genetic inheritance

(B) defend a well-established scientific model against a set of recently proposed theories that challenge it

(C) introduce and examine the current state of evidence for a possible refinement to a long-accepted scientific framework

(D) present recent research findings that upend the central principle of a widely accepted scientific theory

(E) present two possible alternative mechanisms for a widely investigated biological process, with examples of each in animals

For a passage as neatly divided into small paragraph units as this one, a paragraph-by-paragraph summary can be helpful in drilling down to the author’s primary purpose. Since the answer to Primary Purpose questions always starts with a verb, let’s start each of the paragraph summaries with a verb.

¶1: Lays out the long-accepted principle of how genetic heritabliity works (through the sequence of nucleotides in DNA); mentions new research to suggest it may be more complicated than this.

¶2: Describes the phenomenon of “epigenetic marks”, which can influence genetic traits (*gene expression”) without changing the DNA nucleotide sequence; explains how these marks can outlive the individual cells where they first appear.

¶3: Points out that epigenetic marks can also be passed down to offspring via sperm/eggs. Cites an animal study as potential supporting evidence.

¶4: Notes that studies like the one in ¶3 don’t PROVE the passing down of epigenetic marks across generations; mentions other possible explanations. Closes with a phenomenon that will make it difficult to rule out epigenetic inheritance in humans.

Now, compress this outline to the approximate length of the answer choices to this question:
Lays out a long-accepted theory; introduces a new factor that the theory may have to take into account; presents and analyzes evidence for that new factor. 
Choice C matches this summary nicely, so C is the correct answer.

INCORRECT ANSWERS:

(A) Although DNA nucleotide sequences and epigenetic marks can accurately be described as “two possible types of determining factors on genetic inheritance”, nowhere does the passage consider the “impacts” of these two factors in general—let alone compare them, as needed to satisfy “relative impacts” here.

(B) “Defending a well-established model” here would mean pushing back against the proposal to add epigenetic marks to our scientific understanding of genetic heritability, and presenting a case that the longstanding, simplistic model that considers only DNA nucleotide seuqences is still sufficient to account for all major findings to date. Nowhere in the passage does the author do this.

(D) To “upend” a principle—literally meaning to “knock it over”—is to disprove it or otherwise cause it to fall from general acceptance. The “central principle of a widely accepted scientific theory” here is the dependence of genetic heritability on DNA nucleotide sequences; the passage focuses on additional factors that ought to be considered alongside nucleotide sequences, but does not try to disprove or attack the significance of nucleotide sequences themselves.

(E) The only thing that qualifies as a “widely investigated biological process” here is genetic inheritance in general. While “two possible alternative mechanisms” could feasibly describe DNA nucleotide sequences and epigenetic marks, the author neither describes any of the mechanisms of nucleotide seuqencing nor gives any animal examples of it. (Alternatively, we could try to take epigenetic marking to be the “biological process” here, with DNA methylation and histone modification being the two alternative mechanisms. That interpretation also fails, however, because no examples are given for either of those two mechanisms. Furthermore, epigenetic marking—as a relatively new area of research—cannot be described as “widely investigated”.)

The correct answer is (C).

2.

If an epigenetic mark altering gene expression in epithelial cells of the stomach lining is NOT mitotically stable, then it can most clearly be inferred that this alteration

(A) can be passed to offspring only through sperm or egg cells

(B) comes about through either DNA methylation or histone modification

(C) may spread to cells outside the stomach

(D) will not persist for more than a few days without re-exposure to its cause

(E) will regenerate itself within that region of the body via a cyclical mechanism

This is an IMPLY/INFER/SUGGEST question, so we should expect the correct answer to restate factual content from the passage, but in a way that requires at least one logical step (e.g., turning “My brother is taller than me” into “I’m shorter than my brother”) from the original statement in the text.

Here we need to account for two different specifications in the prompt:
• not “mitotically stable”
• in epithelial cells located in stomach lining 
Let’s ‘collect’ what the passage says about these specifics one by one:

• Not mitotically stable: As described in the passage, mitotically stable epigenetic marks can be transferred to newly manufactured cells in the body. If an epigenetic mark is NOT mitotically stable, then, it can therefore be inferred that the mark will not survive the division process that creates new cells, and therefore will NOT be transferred to new cells from the cell(s) where it is currently found. (This is an example of the kind of “logical step” needed for IMPLY/INFER/SUGGEST questions.) So, a non-mitotically-stable mark will not be passed on from the cell where it is currently found.

• In the epithelial cells of stomach lining: The only thing mentioned in the passage about these cells is that they are replaced every few days.

Putting these two facts together, we can come up with the only statement that can be made with logical certainty about the kind of mark described in this problem: Since that mark is ‘stuck’ in a cell that gets turned over every few days, the genetic effect of the mark can last only a few days (unless it is ‘renewed’ in newer cells by another exposure to whatever caused it in the first place). That’s choice D.

INCORRECT ANSWERS:

(A) Since a non-mitotically-stable epigenetic mark cannot be passed on to other cells in general, it is extremely unlikely that such a mark could possibly be passed on to offspring by any means whatsoever.

(B) DNA methylation and histone modification are described in the text as “the two most studied mechanisms” of epigenetic marking—a statement that implies that there are other, less studied mechanisms as well—and nowhere in this passage does the author ever assign one of these mechanisms to any particular instance of epigenetic marking.

(C) Non-mitotically-stable epigenetic marks specifically CANNOT be passed on to cells that are newly manufactured by division, and the passage contains nothing to suggest the possibilty that epigenetic marks could spread to other body cells by any means other than the process of new cell creation by division.

(E) The passage mentions the “regeneration” of cells in the body of a living organism. The “regeneration” of epigenetic marks—which would presumably entail the disappearance and then reappearance of a mark and its effect on gene expression—is not mentioned anywhere.

The correct answer is (D).

3.

The author most likely cites the low-protein diet experiment with male mice in order to

(A) demonstrate that transgenerational epigenetic effects can manifest in offspring without direct environmental exposure

(B) prove that dietary modifications can create heritable changes in the nucleotide sequences of DNA

(C) provide an example of the behavioral transmission of epigenetic modifications of gene expression

(D) suggest that the effects of significant fundamental lifestyle changes in a parent animal are likely to redound to its offspring

(E) support the claim that epigenetic marks can be passed from parent to offspring outside the reproductive process

The cited experiment is mentioned immediately after the general remark that “rodent studies have found altered gene expression patterns in the offspring of animals exposed to stressors   even when the offspring have not been exposed to those stressors”, so it is meant to present a specific instance of that general observation—in which mice are the “rodent” in question, the low-protein diet given to the parent mice is the “stressor” that’s causally linked to epigenetic marks, and the altered hepatic gene expression is the “altered gene expression ... in offspring   have not been exposed to th  stresso ”.
To “demonstrate” a principle is to confirm it by providing a specific instance/example, so choice A describes the precise function of the specific instance/example called out here.

INCORRECT ANSWERS:

(B) The dietary modifications cited here are epigenetic factors that cause alterations in “gene expression”—i.e., that do not alter the genes (= nucleotide sequences in DNA) themselves.

(C) Although the passage does not supply a precise definition of “behavioral transmission” of epigenetic marks, any such phenomenon would at least have to involve imitation of relevant behaviors/actions by the offspring in which modified gene expression is eventually observed. Since the offspring in this instance did NOT consume the special low-protein diet associated with the epigenetic mark (= the only associated ‘behavior’ mentioned in the text), the findings of the mouse experiment cannot reasonably involve behavioral transmission.

(D) Not only is this statement far too general, but it entirely fails to mention the actual principle that this example is meant to illustrate—namely, the potential inheritance of epigenetic marks and the traits associated with the resultant gene expression

(E) No claims are made about whether the epigenetic mark associated with altered hepatic gene expression in this example was passed down via the reproductive process or via some other route (such as any of the three non-reproductive routes mentioned near the beginning of the last paragraph).

The correct answer is (A).

4.

The passage implies that epigenetic reprogramming of the human genome during the embryonic and early fetal stages is relevant to the debate over transgenerational inheritance because it

(A) cannot possibly co-occur in the same individual as transgenerational epigenetic inheritance

(B) confirms the presence of transgenerational epigenetic inheritance in human beings

(C) narrows the range of possible mechanisms of epigenetic modification to human genes

(D) raises the possibility that transgenerational epigenetic inheritance may be active in cases where it leaves no lasting evidence

(E) suggests that human genes are more amenable than rodent genes to epigenetic modification than that of rodents

Epigenetic reprogramming in human embryos is mentioned at the very end of the passage. The main significance of this phenomenon is laid out immediately after the author’s description of the phenomenon itself, in the modifier linked with a dash: “a process that wolud be expected to erase most parental marks”. In other words, IF indeed there is any passing down of epigenetic marks from human parents to their children, this “reprogramming” process will overwrite the majority of those marks very early in a baby’s gestation—leaving no evidence of the marks having been passed down in the first place. Choice D properly summarizes this stated significance.

INCORRECT ANSWERS:

(A) A process can only overwrite the results of another process if both of those processes DO co-occur, so the cases of epigenetic reprogramming in which the author is interested all directly contradict choice A.

(B) Quite the opposite: The reprogramming process, by overwriting the results of epigenetic inheritance, will make it much more difficult to confirm that any such inheritance has occurred even in cases where it actually has.

(C) The text contains nothing to suggest that the fetal epigenetic reprogramming process can narrow down the set of potential mechanisms for epigenetic marking. If anything, the reprogramming process—which destroys most evidence of whatever epigenetic marks may have been inherited—may even expand that set of potential mechanisms, by destroying evidence that would have narrowed it!

(E) The author does not consider any probabilities of ocurrence for epigenetic modifications—let alone compare those probabilities across species.

The correct answer is (D).

VibhuAnurag · Answer

Passage Map  
 Paragraph 1: Old model vs new complication
For much of the twentieth century, scientists believed hereditary traits were transmitted only through DNA nucleotide sequences.
Environmental conditions could affect how fully someone reached genetic potential, but these environmental effects were not thought to be heritable.
Then the passage introduces epigenetics as a complication to this older model.
Main idea of Paragraph 1:
Old view: inheritance = DNA sequence only.
New evidence: epigenetics may complicate this view.

Paragraph 2: What epigenetic marks are 
 Epigenetic marks are chemical changes that affect gene expression without changing the DNA sequence.
Two examples are given: 
 1. DNA methylation  : methyl groups attach to cytosine bases and silence nearby genes. 
 2. Histone modification  : chemical tags affect how tightly DNA is wound, making genes more or less accessible. 
 Then the key concept appears:
Some epigenetic marks are mitotically stable, meaning they persist through cell division.
So, if a cell divides, the mark can be passed into the new cells.
Main idea of Paragraph 2:
Epigenetic marks can affect gene expression and, if stable through cell division, can persist over time.

Paragraph 3: Possible inheritance across generations
Because some marks can persist, they may also be transmitted into sperm and egg cells.
This creates the possibility of transgenerational epigenetic inheritance: passing epigenetic marks from parents to offspring.
Rodent studies are cited as evidence. For example, male mice fed a low-protein diet had offspring with altered liver gene expression, even though the offspring were raised on standard diets.
Main idea of Paragraph 3:
Animal studies suggest that some epigenetic effects may pass from parents to offspring.

Paragraph 4: Skepticism and limits
Skeptics warn that proving true epigenetic inheritance requires ruling out other explanations: 
 behavioral transmission 
 microbiome transfer 
 ordinary genetic mutation 
 In humans, the phenomenon remains unknown because the human genome undergoes extensive epigenetic reprogramming during early development. This reprogramming would likely erase most parental epigenetic marks.
Main idea of Paragraph 4:
The evidence is promising but not conclusive, especially in humans.

Q1. The primary purpose of the passage is to  
 Correct Answer: C
C. introduce and examine the current state of evidence for a possible refinement to a long-accepted scientific framework
This is exactly what the passage does. The long-accepted framework is:
Inheritance happens through DNA nucleotide sequences.
The possible refinement is:
Some environmentally induced epigenetic marks may also be heritable.
The passage examines current evidence, especially rodent studies, and then discusses skepticism and uncertainty in humans.
 
 Why the other choices are wrong: 
 A. compare the relative impacts of two possible types of determining factors in genetic inheritance 
 This sounds like the passage compares DNA inheritance versus epigenetic inheritance in terms of relative importance. It does not. The passage introduces epigenetics as a possible complication, not as one side of a detailed comparison.

B. defend a well-established scientific model against a set of recently proposed theories that challenge it 
 The passage does not defend the old model. In fact, it says recent evidence has revealed complications in that model. 
 Wrong.

D. present recent research findings that upend the central principle of a widely accepted scientific theory 
 “Upend” is too strong.   The author does not say the old theory has been overturned. The author says researchers are only beginning to understand the complications and that human inheritance remains unknown.

E. present two possible alternative mechanisms for a widely investigated biological process, with examples of each in animals 
 The passage mentions two mechanisms: DNA methylation and histone modification. 
 But that is not the primary purpose of the passage. These mechanisms are background information used to explain epigenetics. 
 Also, the passage does not give animal examples of each mechanism separately.

Q2. If an epigenetic mark altering gene expression in epithelial cells of the stomach lining is NOT mitotically stable, then it can most clearly be inferred that this alteration  
 Correct Answer: D
The key sentence from the passage is:
Some epigenetic marks are mitotically stable: they persist through cell division and thus can be transmitted into new cells.
So, mitotically stable = persists through cell division
Therefore, not mitotically stable = does not persist through cell division
The passage also says epithelial cells lining the stomach regenerate in a few days.
So if the mark is in stomach epithelial cells and is not mitotically stable, it will not be passed into newly divided replacement cells. Therefore, without re-exposure to whatever caused the mark, it should disappear within that short regenerative cycle.
Hence:
D. will not persist for more than a few days without re-exposure to its cause

Why the other choices are wrong: 
 A. can be passed to offspring only through sperm or egg cells 
 This is about reproductive transmission. But the question asks about epithelial cells of the stomach lining.   Stomach epithelial cells are not sperm or egg cells.

B. comes about through either DNA methylation or histone modification 
 The passage says these are the two most studied mechanisms, not the only possible mechanisms.   Also, the question asks what follows from the mark being not mitotically stable. We cannot infer which mechanism caused it.

C. may spread to cells outside the stomach 
 Nothing in the passage suggests that a non-mitotically stable mark spreads to other cells. In fact, the opposite idea is more relevant. I it is not stable, it will not persist through cell division.

E. will regenerate itself within that region of the body via a cyclical mechanism 
 This is the opposite of what “not mitotically stable” suggests. 
 If the mark is not stable through cell division, it will not automatically regenerate in new cells.

Q3. The author most likely cites the low-protein diet experiment with male mice in order to  
 Correct Answer: A
In Paragraph 3, the author first states that transgenerational epigenetic inheritance is theoretically possible.
Then the author gives rodent studies as evidence.
The low-protein diet experiment is one such example:
Male mice were fed a low-protein diet. Their offspring showed altered hepatic gene expression, even though the offspring were raised on standard diets.
The key phrase is: even though the offspring were raised on standard diets
This means the offspring were not directly exposed to the same environmental condition.
So the example is used to show that epigenetic effects may appear in offspring without direct exposure.
Therefore:
A. demonstrate that transgenerational epigenetic effects can manifest in offspring without direct environmental exposure

Why the other choices are wrong: 
 B. prove that dietary modifications can create heritable changes in the nucleotide sequences of DNA 
 This directly contradicts the passage. 
 Epigenetic marks alter gene expression without changing DNA’s nucleotide sequence. 
 Also, the passage does not say the experiment proves anything conclusively.

C. provide an example of the behavioral transmission of epigenetic modifications of gene expression 
 The passage later says behavioral transmission is an alternative explanation that must be ruled out.   The low-protein diet example is presented as possible evidence of epigenetic inheritance, not as an example of behavioral transmission.

D. suggest that the effects of significant fundamental lifestyle changes in a parent animal are likely to redound to its offspring 
 This is too broad and vague.   The passage is not making a general lifestyle claim. It is specifically discussing altered gene expression in offspring after parental exposure to an environmental stressor.

E. support the claim that epigenetic marks can be passed from parent to offspring outside the reproductive process 
 The passage says transmission would happen through sperm and egg cells, i.e., through the reproductive process.   So “outside the reproductive process” is wrong.

Q4. The passage implies that epigenetic reprogramming of the human genome during the embryonic and early fetal stages is relevant to the debate over transgenerational inheritance because it  
 Correct Answer: D 
 D. raises the possibility that transgenerational epigenetic inheritance may be active in cases where it leaves no lasting evidence
The passage says reprogramming would erase most parental marks. That could make it difficult to find lasting evidence of inherited marks.
So D captures the idea that inheritance may be difficult to detect because evidence may be erased.

Why the other choices are wrong:  
 A. cannot possibly co-occur in the same individual as transgenerational epigenetic inheritance 
 Too extreme.   The passage says reprogramming would be expected to erase most parental marks, not all. So it does not say co-occurrence is impossible.

B. confirms the presence of transgenerational epigenetic inheritance in human beings 
 Opposite of the passage.   The passage says whether such inheritance operates in humans remains unknown.

C. narrows the range of possible mechanisms of epigenetic modification to human genes 
 The passage is not saying that reprogramming narrows the mechanisms of epigenetic modification.

E. suggests that human genes are more amenable than rodent genes to epigenetic modification than that of rodents 
 This is ungrammatical and unsupported.   The passage does not compare human and rodent genes in terms of being more amenable to modification.

Final Answers
Q1: C
Q2: D
Q3: A
Q4: D

For much of the twentieth century, hereditary traits were understood

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For much of the twentieth century, hereditary traits were understood

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