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(Waddington 1942). His concepts were formulated long before the
discovery of the TF control process, even before the structure of DNA
was known. In Waddington
s thinking, this term was needed to describe
how genes interacted with their cellular environment to produce a
phenotype. Waddington knew that the DNA could not function without
the cellular machinery. However, his somewhat complicated view of
developmental landscapes and canalizations (channeled or directed)
(Waddington 1953) could not be ascribed to any known molecular
mechanism. Epigenetics eventually became associated with heritable
changes in phenotype that did not always follow Mendelian rules.
However, in keeping with Waddington
'
s concept of epigenetics in the
context of development, in some sense epigenetics never had, and still
does not have, a precise de
'
ects the desperate
attempts to use epigenetics to describe all aspects of heritability that do
not strictly conform to Mendelian rules. If we maintain that the discrete
sequence of DNA is the only basis of genetics, then epigenetics has to
cover a lot of phenomena. Indeed, many authors began to de
nition. This certainly re
ne epi-
genetics as the genetics (DNA sequence) guided processes that form the
mechanisms that direct development, a concept that connected epige-
netics to genetics and was strongly
first promoted by David L. Nanney
(Nanney 1958).
Epigenetics gradually was narrowed to describe interactions of genes
controlling the expression of other genes during development to allow
the genotype to manifest as a phenotype. By the 1990s, this imple-
mentation process was de
ned simply as the control of gene expres-
sion, but it also raised many questions, mainly because of the early
work of Robin Holliday (Holliday and Pugh 1975) and others, espe-
cially Riggs (1975) on DNA methylation. More recently reiterated
Waddington
is view that epigenetics is a causal mechanism through
which the genome produces a phenotype, and Gottschling (Allis et al.
2007) restated David L. Nanney
'
spositionthatepigeneticsistheway
non-DNA information in the cell works with the DNA sequence to
produce different phenotypes (Nanney 1958). Both of these subtly
different but related concepts are captured by Russo et al. (1996) in
their book where epigenetics is de
'
ned as the study of mitotically and/
or meiotically heritable changes in gene function that do not involve
changes in DNA sequence and more recently with the same de
nition
by Felsenfeld in the topic
Epigenetics
(Allis et al. 2007). Note that
this de
nition includes the possibility that epigenetic (non-DNA
sequence) information can pass meiotically through germ cells to
the next generation.
