Agriculture Reference
In-Depth Information
III. EPIGENETICS
A. Epigenetics: The Softer Side of Genetics
Despite the inability to
nition,
the study of these phenomena has led to some very important concepts
about the relationship between genotype and phenotype. Thus, although
the use of the term epigenetics has had this history of various meanings
differing both subtly and signi
“
pin down
”
epigenetics with a precise de
cantly, we now seem to have reached a
broad consensus on understanding that the molecular basis of epige-
netics includes any and all alterations in the mitotic (somatic) and
sometimes meiotic (gametic) heritable characteristics of DNA and chro-
matin and indeed the cell, except the DNA sequence itself (Russo et al.
1996; Chan et al. 2005; Grant-Downton and Dickinson 2006; Richards
2006; Allis et al. 2007; Youngson and Whitelaw 2008). This seemingly
clear de
nition can be seen to be actually a collection of several
phenomena that are lumped together under the label of epigenetics
(Table 1.1).
Because transgenerational inheritance of epigenetic (non-DNA
sequence) traits may be non-Mendelian in the pattern of sexual progeny
produced and usually does not persist through many generations, it has
also been referred to as soft genetics by Ernst Mayr (Mayr and Provine
Table 1.1.
Types of Mendelian and non-Mendelian inheritance.
Type
Description
I
Spontaneous changes in DNA sequence inherited through mitosis and meiosis
(classical Mendelian)
II
Changes in DNA sequence not directed by the environment inherited by mitosis
(antibody genes in adaptive immune system)
III
Changes in DNA sequence directed by the environment inherited through mitosis
and meiosis (transposon activation and silencing) (yeast mating type) (DNA
ampli
cation)
IV
Non-DNA sequence changes not directed by environment inherited through mitosis
(many RNA pol-II mediated expression changes and other non-DNA sequence
changes mediated by chemical modi
cation of DNA and chromatin such as
imprinting and likely paramutation)
V
Non-DNA sequence changes directed by environment inherited through mitosis
(vernalization)
VI
Non-DNA sequence changes not directed by environment inherited through mitosis
and meiosis global imprinting
VII
Non-DNA sequence changes directed by environment inherited through mitosis
and meiosis (somaclonal variation, cortical, cell structure, glucose tolerance in
the Dutch Hungary Winter episode).
