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The NIH "Roadmap Epigenomics Project", ongoing as of 2016, uses the following definition: "For purposes of this program, epigenetics refers to both heritable changes in gene activity and expression (in the progeny of cells or of individuals) and also stable, long-term alterations in the transcriptional potential of a cell that are not necessarily heritable." In 2008, a consensus definition of the epigenetic trait, "stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence", was made at a Cold Spring Harbor meeting.
The similarity of the word to "genetics" has generated many parallel usages.
The term "epigenetic" has also been used in developmental psychology to describe psychological development as the result of an ongoing, bi-directional interchange between heredity and the environment.
The microstructure (not code) of DNA itself or the associated chromatin proteins may be modified, causing activation or silencing.
This mechanism enables differentiated cells in a multicellular organism to express only the genes that are necessary for their own activity.
The more recent usage of the word in science has a stricter definition.
It is, as defined by Arthur Riggs and colleagues, "the study of mitotically and/or meiotically heritable changes in gene function that cannot be explained by changes in DNA sequence." The term "epigenetics", however, has been used to describe processes which have not been demonstrated to be heritable such as some forms of histone modification; there are therefore attempts to redefine it in broader terms that would avoid the constraints of requiring heritability.
The "epigenome" is a parallel to the word "genome", referring to the overall epigenetic state of a cell, and epigenomics refers to more global analyses of epigenetic changes across the entire genome.
The phrase "genetic code" has also been adapted—the "epigenetic code" has been used to describe the set of epigenetic features that create different phenotypes in different cells.Such effects on cellular and physiological phenotypic traits may result from external or environmental factors, or be part of normal developmental program.The standard definition of epigenetics requires these alterations to be heritable, either in the progeny of cells or of organisms.The consensus definition now requires a trait to be heritable for it to be considered epigenetic.A consensus definition of the concept of epigenetic trait as "stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence" was formulated at a Cold Spring Harbor meeting in 2008, From the generic meaning, and the associated adjective epigenetic, C. Waddington coined the term epigenetics in 1942 as pertaining to epigenesis, in parallel to Valentin Haecker's 'phenogenetics' (Phänogenetik).Another variation, probabilistic epigenesis, was presented by Gilbert Gottlieb in 2003.Tags: Adult Dating, affair dating, sex dating