Epigenetics & EpigenomicsTraditional genetics attributes human characteristics to a simple arithmetical combination of inheritable traits from unchanging genes. As a result, genetic mutations and recombinations have driven most descriptions of how traits are handed down from one generation to another.

The discovery and understanding of DNA, and the role of non-coding (junk) DNA, reveals a more complex — and subtle — situation. Today, scientists know that heritable changes in gene function can occur without a change in the DNA sequence. Called epigenetics, this insight has further changed the way researchers think about heredity. Epigenetics bridges the gap between nature and nurture.

Both epigenetics and epigenomics — the genomewide distribution of epigenetic changes — are related to many other topics requiring a thorough understanding of all aspects of genetics. The latter includes aging, agriculture, cloning, evolution, sexual differentiation, species conservation, stem cells, and synthetic biology.

---

There are more than 200 different cell types in the human body; each cell contains the same genetic information and can, in theory, synthesize the same proteins. However, each cell type is unique and synthesizes a specific set of proteins. Nerve cells synthesize proteins that are necessary for generating nerve cells, muscle cells synthesize those necessary for building muscle fibers, etc.

This specialization takes place during early embryonic development and continues throughout a person's life. Cells exercise control over their own development using a mechanism called epigenetic regulation, which “opens” or "closes" the DNA structure. Differences in protein synthesis result from the activation and inactivation of genes.

This is fundamental to all animals, humans, and plants (eukaryotic cells). It is involved in tissue regeneration and the preservation of stem cells and DNA.

Epigenetic processes are natural and essential to many organism functions, but disruptions can result in major adverse health and behavioral effects. Variations in epigenetic gene activity regulation are causally connected in human beings to disruptions in early embryonic development and serious diseases.

The cell has to condense two meters of DNA inside a 1/100 millimeter diameter body. During the condensation process, the cell mechanism determines which genes activate. A special group of proteins, called the histones, plays a central part during this process.

The DNA is wound around the histones — which also determine the DNA structure — during condensation. They attach a number of complex and relatively unknown combinations of small chemical modifications under the influence of different enzymes. This opens and closes parts of the DNA structure to regulate gene activation — specific for each of our distinct cell types.

---

Most epigenetic modifications are erased with each new generation, during gametogenesis and after fertilization. Recent reports suggest that some epigenetic changes may endure in at least four subsequent generations of organisms. If reproducible, the findings could suggest some interesting new approaches. Other studies have found that epigenetic effects occur not just in the womb, but over the full course of a human life span.
Imprinted genes don't rely on the traditional laws of Mendelian genetics, which describe the inheritance of traits as either dominant or recessive. In Mendelian genetics, both parental copies are equally likely to contribute to the outcome. The impact of an imprinted gene copy, however, depends only on which parent it was inherited from. For some imprinted genes, the cell only uses the copy from the mother to make proteins, and for others only that from the father.

In the mid 1980s, scientists studying mice discovered that normal development requires the inheritance of genetic material from both a male and a female. The resulting variances changed, depending on the material's origin.

One hypothesis has it that imprinting regulates embryonic growth. Maternally-expressed imprinted genes usually suppress growth, while those from the male parent usually enhance growth, ensuring continuation of the father's genes.

This is important for a species in which a single litter of offspring can result from the contributions of more than one male. However, the mother, interested in her own health maintenance (biologically speaking), "fights" the paternal genes and limits the size of the embryo or fetus.

CitationTransgenerational Epigenetic Inheritance: Prevalence, Mechanisms, and Implications for the Study of Heredity and Evolution. Eva Jablonka and Gal Raz. Quarterly Review of Biology 2009; 84(2): 131-176. 0033-5770/2009/8402-0001
Download PDF
Abstract

This review describes new developments in the study of transgenerational epigenetic inheritance, a component of epigenetics. We start by examining the basic concepts of the field and the mechanisms that underlie epigenetic inheritance. We present a comprehensive review of transgenerational cellular epigenetic inheritance among different taxa in the form of a table, and discuss the data contained therein. The analysis of these data shows that epigenetic inheritance is ubiquitous and suggests lines of research that go beyond present approaches to the subject. We conclude by exploring some of the consequences of epigenetic inheritance for the study of evolution, while also pointing to the importance of recognizing and understanding epigenetic inheritance for practical and theoretical issues in biology.

Keywords: cell memory, epigenetics, induced heritable variations, lamarckism, microevolution, macroevolution.