"Epigenetics" is the (relatively) new buzzword. Old-fashioned genetics is boring so if you want to convince people (and grant agencies) you're on the frontlines of research you have to say you're working on epigenetics. Even better, you can tell them that you are on the verge of overthrowing Darwinism and bringing back Jean-Baptiste Lamarck.But you need to be careful if you adopt this strategy. Don't let anyone pin you down by defining "epigenetics." It's best to leave it as ambiguous as possible so you can adopt the Humpty-Dumpty strategy.1 Sarah C.P. Williams made that mistake a few years ago and incurred the wrath of Mark Ptashne [Core Misconcept: Epigenetics].
Now Carrie Deans and Keith A. Maggert are giving it a try and they are far more successful (Deans and Maggert, 2015). I won't bore you with the history of the term "epigenetics" but, if you're interested, this paper gives a nice summary. The authors end up with two distinct definitions of "epigenetics" that they label the Waddington definition (after Conrad Waddington) and the Holliday definition (after Robin Holliday). The Waddington definition is essentially equivalent to saying that epigenetics is the regulation of gene expression. It's a common definition
[Jablonka's definition of epigenetics] but one that's not particularly useful. It means that calico cats are prime examples of epigenetics [Epigenetics and the Calico Cat], but so is regulation of the lac operon.
The Holliday definition incorporates the idea of heritability that is not based on DNA sequence. A concise definition along these lines was proposed by Wu and Morris (2001),
[Epigenetics is] the study of changes in gene function that are mitotically and/or meiotically heritable and that do not entail a change in DNA sequence.This doesn't seem to be any better than previous definitions so Deans and Maggert propose their own definition,
"... the study of phenomena and mechanisms that cause chromosome-bound, heritable, changes to gene expression that are not dependent on changes to DNA sequence."
We feel that this definition makes a strong distinction between gene regulation (Waddington's definition) and epigenetic inheritance (Holliday's definition), and also emphasizes that epigenetic phenomena must deal exclusively with chromosome-bound changes.
The Dean and Maggert definition focuses attention on modification of DNA (e.g. methylation) and modification of histones (chromatin) that are passed from one cell to two daughter cells. That's where the action is in terms of the debate over the importance of epigenetics.
Methylation is trivial. Following semi-conservative DNA replication the new DNA strand will be hemi-methyalated because the old strand will still have a methyl group but the newly synhtesized strand will not. Hemi-methylated sites are the substrates for methylases so the site will be rapidly converted to a fully methylated site. This phenomenon was fully characterized almost 40 years ago [Restriction, Modification, and Epigenetics]. There's no mystery about the inheritance of DNA modifications and no threat to evolutionary theory.
Histone modifications are never inherited through sperm because the chromatin is restructured during spermatogenesis. Modifications that are present in the oocyte can be passed down to the egg cell because some of the histones remain bound to DNA and pass from cell to cell during mitosis/meiosis. The only difference between this and inheritance of lac repressors is that the histones remain bound to the DNA at specific sites while the repressor molecules are released during DNA replication and re-bind to the lac operator in the daughter cells [Repression of the lac Operon].
Some people think this overthrows modern evolutionary theory.
1. “When I use a word,” Humpty Dumpty said, in rather a scornful tone, “it means just what I choose it to mean—neither more nor less.” “The question is,” said Alice, “whether you can make words mean so many different things.” “The question is,” said Humpty Dumpty, “which is to be master—that’s all.” Lewis Carroll (Charles L. Dodgson) Through the Looking-Glass (1872)
Deans, C., and Maggert, K.A. (2015) What do you mean,“epigenetic”? Genetics, 199:887-896. [doi: 10.1534/genetics.114.173492]