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Definition of Epigenetics
Epigenetics (as in "epigenetic landscape") was
coined by C. H. Waddington in 1942 as a portmanteau of the words
genetics and epigenesis. Epigenesis is an old word which has more
recently been used (see preformationism for historical background)
to describe the differentiation of cells from their initial
totipotent state in embryonic development. When Waddington coined
the term the physical nature of genes and their role in heredity was
not known; he used it as a conceptual model of how genes might
interact with their surroundings to produce a phenotype.
Robin Holliday defined epigenetics as "the study of the mechanisms
of temporal and spatial control of gene activity during the
development of complex organisms." Thus epigenetic can be used to
describe anything other than DNA sequence that influences the
development of an organism.
The modern usage of the word in scientific discourse is more narrow,
referring to heritable traits (over rounds of cell division and
sometimes transgenerationally) that do not involve changes to the
underlying DNA sequence. The Greek prefix epi- in epigenetics
implies features that are "on top of" or "in addition to" genetics;
thus epigenetic traits exist on top of or in addition to the
traditional molecular basis for inheritance.
The similarity of the word to "genetics" has generated many parallel
usages. The "epigenome" is a parallel to the word "genome", and
refers to the overall epigenetic state of a cell. 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. Taken to its extreme, the
"epigenetic code" could represent the total state of the cell, with
the position of each molecule accounted for in an epigenomic map, a
diagrammatic representation of the gene expression, DNA methylation
and histone modification status of a particular genomic region. More
typically, the term is used in reference to systematic efforts to
measure specific, relevant forms of epigenetic information such as
the histone code or DNA methylation patterns.
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