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02 Jun 2026, 08:32
Kudos
Bookmarks
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.
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
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.
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
02 Jun 2026, 08:32
Kudos
Bookmarks
Official Solution:
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.
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
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”.)
Answer: C
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.
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
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”.)
Answer: C
