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Difficulty: HardComparative Reading Analysis

Passage A
In recent decades, claims that epigenetic marks—chemical modifications to chromatin structure and DNA methylation that regulate gene expression without altering nucleotide sequences—can be transmitted across generations have generated intense debate. Proponents of transgenerational epigenetic inheritance assert that environmentally induced epigenetic alterations can persist across multiple generations, thereby providing a non-genetic substrate for evolutionary adaptation. However, methodological rigor warrants substantial skepticism regarding these claims in mammalian systems.

In mammals, the germline undergoes two comprehensive waves of epigenetic reprogramming: first during primordial germ cell development, and subsequently in the pre-implantation embryo. During these phases, the vast majority of parental methylation marks and chromatin states are actively erased and reset to ground state pluripotency. Consequently, putative demonstrations of transgenerational inheritance often conflate true intergenerational epigenetic stability with direct environmental exposure. If a gestating female is exposed to a toxin, the mother (F0), her fetus (F1), and the fetus's germ cells (F2) are all directly exposed to the environmental perturbation. True transgenerational inheritance can therefore only be established if an altered phenotype persists into the unexposed F3 generation, a standard that experimental studies rarely satisfy rigorously without confounding genetic variants or persistent behavioral transmission via maternal care.

Passage B
The classical Modern Synthesis posits that the nucleotide sequence of nuclear DNA is the exclusive carrier of heritable biological information in metazoans. However, emerging molecular evidence demonstrates that non-DNA regulatory components can bypass germline reprogramming and exert enduring regulatory influences across generations. While widespread DNA methylation erasure does occur during mammalian embryogenesis, specific genomic regions—notably certain retrotransposon sequences, imprinted loci, and heterochromatic domains—consistently evade complete epigenetic reset.

Moreover, the scope of epigenetic inheritance is not restricted to DNA methylation alone. Sperm and oocytes deliver complex repertoires of small non-coding RNAs (sncRNAs), such as microRNAs and transfer-RNA-derived fragments, as well as structurally preserved histone modifications, directly into the zygote. These non-genetic molecules orchestrate post-fertilization transcriptional cascades that can faithfully recapitulate ancestral physiological and behavioral phenotypes in subsequent generations. Far from being an experimental artifact or a rare exception confined to plants and nematodes, multi-generational epigenetic transmission represents a functional evolutionary mechanism that allows organisms to dynamically transmit phenotypic plasticity across rapid temporal horizons where standard DNA mutation and natural selection operate too slowly.

Based on the passages, which of the following statements express claims with which BOTH the author of Passage A and the author of Passage B would agree? (Select TWO correct answers)

  1. During standard mammalian development, extensive erasure and resetting of epigenetic marks takes place in the germline.Answer
  2. B
    Small non-coding RNAs delivered by gametes serve as the primary molecular vehicle for transgenerational phenotype transmission.
  3. Epigenetic modifications regulate gene expression without altering the underlying nucleotide sequence of DNA.Answer
  4. D
    Empirical evidence of multi-generational transmission in mammals is sufficiently robust to challenge classical gene-centric evolutionary theory.
  5. E
    Direct chemical exposure of a gestating female permanently alters the genetic code of the F3 generation through targeted DNA mutations.

Answer

The two statements that both authors would agree with are that extensive erasure and resetting of epigenetic marks takes place in the mammalian germline, and that epigenetic modifications regulate gene expression without altering the underlying nucleotide sequence of DNA.
Both authors explicitly affirm that extensive germline reprogramming and epigenetic erasure occur in mammals (Passage A notes 'the vast majority of parental methylation marks and chromatin states are actively erased' while Passage B confirms 'widespread DNA methylation erasure does occur during mammalian embryogenesis'). Additionally, both authors accept the baseline biological definition that epigenetic modifications regulate gene expression without altering nucleotide sequences.

Step-by-Step Solution

1
Analyze Passage A for core premises and explicit claims regarding epigenetic mechanisms and mammalian development.
Passage A explicitly defines epigenetic marks as modifications regulating gene expression without nucleotide sequence changes, notes two comprehensive waves of germline reprogramming/erasure in mammals, and argues that most claims of transgenerational inheritance are confounded by direct exposure.
Establishing Passage A's scope and factual commitments prevents misattributing unendorsed claims to the author.
2
Analyze Passage B for points of overlap and divergence with Passage A.
Passage B concedes that widespread DNA methylation erasure occurs in mammals, defines epigenetic inheritance as non-DNA information transmission, but argues that escapes from erasure and small non-coding RNAs enable genuine transgenerational evolutionary effects.
Comparing both texts identifies overlapping shared factual baselines versus divergent interpretive conclusions.
3
Evaluate candidate statements to identify two claims explicitly supported by the textual evidence in both passages.
Both authors agree on the foundational definition of epigenetic marks as non-nucleotide-altering regulators of expression, and both explicitly acknowledge that extensive germline erasure/reprogramming occurs during normal mammalian development.
Comparative reading questions require verifying mutual textual support rather than extrapolating agreement from silence or conflicting conclusions.

Key Concept

Identifying shared theoretical definitions and mutual factual concessions across paired comparative passages.
Estimated Time:3m 0s
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Comparative Reading Analysis

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