Epigenetics is the study of the changes in phenotype brought about by modification of genetic expression rather than through changes in the actual structural information found within the DNA i.e. genetic mutations.
Although the individual organisms within a species share the same essential blueprint imbedded within the DNA, they express individual phenotypes. In addition, complex traits and diseases cannot be fully explained via differences in genotype. This suggests that developmental and environmental factors that are unique to the individual play an important role in determining the terminal phenotype.
The kinds of chemical modifications that are associated with epigenetics are DNA methylation and histone modification. Histones are the family proteins that are intimately associated with DNA and play an important role in genetic expression. Other factors that have been implicated in epigenetics are nonocoding RNAs and nucleosome location.
Since much current genetic research is focused on the role of epigenetics in determining phenotypic characteristics, there has been considerable confusion as to what constitutes epigenetics. An epigenetic system needs to meet the following criteria:
Prions – infectious proteins – meet these criteria since they perpetuate themselves through altered protein folding states and may, in fact, serve as indicators of environmental stressors. Prions certainly alter phenotype as exemplified by the diseases they produce – Creutzfeldt-Jakob disease (CJD) being an example.
Some metazoans – metazoans encompass all animals advanced enough to have differentiated tissue - undergo genome-wide reprogramming of DNA methylation and histone modifications during gametogenesis and embryogenesis as a way of clearing those epigenetic changes that were introduced by environmental factors during the life of the individual. Furthermore, there is evidence that small noncoding RNAs may serve as tags for marking deleterious sequences within the DNA. This Reprogramming may play a critical role in cell differentiation, and has been linked to pluripotency in both gametes and zygotes.
The field of epigenetics is undergoing rapid expansion; the implication of the critical role epigenetic processes play in the development of the individual is just beginning to be understood.