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I know that epigenetics have capacity to affect and degrade the genes thereby inducing problems/illness/degradation in body functions.

Can they also make better genes or have positive impact on genes or body? If yes, can someone give some examples?

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First, let me qualify the idea of "problematic" epigenetic modifications by saying that the impact of a modification on an organism is often dependent on the environment. That is to say that outcome is dependent on the interaction of genetics (or epigenetics) and the environment in which the associated genes are expressed; e.g. a mutation or modification that confers an advantage in a nutrient-limiting environment may be detrimental when nutrients are plentiful.

Second, it is helpful to think of epigenetic modifications as reversible switches rather than entities that "degrade the genes". In fact, the etymology of "epigenetic" implies that such changes work at a level above that of the gene sequence, i.e. gene expression. Canonically, DNA methylation and histone modifications (acetylation and methylation) modify the expression of a gene by regulating the ability of RNA polymerase to access that gene, either by directly influencing transcription factor binding or modulating the associations of nucleosomes and DNA. Read up on facultative heterochromatin.

As for examples, I point you to a set of papers that show associations between human starvation, epigenetics, and disease / mortality outcomes across generations.

DNA methylation as a mediator of the association between prenatal adversity and risk factors for metabolic disease in adulthood

[DNA methylation] at six CpGs, including at previously [serum triglyceride]-associated CpGs at TXNIP and ABCG1, mediated the association between famine exposure and [serum triglyceride]. [DNA methylation] at these CpGs was likewise associated with the expression of genes implicated in cell growth and energy metabolism.

Note that the authors present caveats to their analyses in the discussion. The New York Times also covered this publication.

Paternal grandfather’s access to food predicts all-cause and cancer mortality in grandsons

This publication is essentially a replication of the published works concerning transgenerational epigenetics in the Överkalix cohort. They give a concise summary of how epigenetic changes caused by environmental stress may be heritable:

An intriguing aspect of these studies is the conjecture that an epigenetic pathway, carrying information across generations, may open up just before puberty, during the so-called slow growth period (SGP). Pre-puberty may be one of several “windows” for germline reprogramming in response to nutritional signals. A number of mechanisms for transmission across generations have been suggested, usually involving DNA methylation, chromatin formation or small noncoding RNAs. After fertilization, in the preimplantation embryo, epigenetic modifications acquired early in life are then usually erased, but not fully. Imprinting on specific loci may resist the post fertilization wave of reprogramming, eventually causing changes in offspring phenotype that are not driven by changes of the DNA sequence.

If you're looking for more examples, research genomic imprinting.

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