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Epigenetic inheritance occurs in the development of multicellular organisms: dividing fibroblasts for instance give rise to new fibroblasts even though their genome is identical to that of all other cells. Epigenetic transmission of traits also occurs from one generation to the next in some organisms, though it is comparatively rare. It was first observed in maize.

1 Epigenetic inheritance systems

Epigenetic inheritance systems (EISs) allow cells of different phenotype but identical genotype to transmit their phenotype to their offspring, even when the phenotype-inducing stimuli are absent, as is often the case. Jablonka et al. (ref. 2), name three types of EISs that may play a role in what has become known as cell memory.

1. Steady-state systems. Some metabolic patterns are self-perpetuating. Sometimes a gene, after being turned on, transcribes a product (either directly or indirectly) that maintains the activity of that gene. Descendants of the cell in which the gene was turned on will inherit this activity, even if the original stimulus for gene-activation is no longer present. Also, diffusion of the gene's product to other cells can make the (heritable) characteristic spread.
2. Structural inheritance systems. In ciliates such as Tetrahymena and Paramecium, genetically identical cells show heritable differences in the patterns of ciliary rows on their cell surface. Experimentally altered patterns can be transmitted to daughter cells. It seems existing structures act as templates for new structures. The mechanisms of such inheritance are unclear, but reasons exist to assume that multicellular organisms also use existing cell structures to assemble new ones. (ref. 3)
3. Chromatin-marking systems. Proteins or chemical groups that are attached to DNA and modify its activity are called chromatin marks. These marks are copied with the DNA. For example, several cytosinesCytosine is one of the 5 main nitrogenous bases used in storing and transporting genetic information within a cell. It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached (an amine group at position 4 and a keto gro in eukaryoticEukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. They include the animals, plants, and fungi, which are mostly multicellular, as well as various other groups called protists, many of which DNA are methylated ( 5-methylcytosine5-methylcytosine is the methylated form of cytosine. The methyl group is attached to carbon 5, and alters its properties. 5-methylcytosine is formed by the action of DNA methyltransferase on CpG sites. It can undergo spontaneous deamination to form thymin). The number and pattern of such methylated cytosinesCytosine is one of the 5 main nitrogenous bases used in storing and transporting genetic information within a cell. It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached (an amine group at position 4 and a keto gro influences the functional state of the gene: low levels of methylation correspond to high potential activity while high levels correspond to low activity. While there are random changes in the methylation pattern, there are also very specific ones, induced by environmental factors. After DNA replicationDNA polymerase (green) binds to one strand of the DNA. It moves along the strand, using it as a template for assembling a leading strand (red) of nucleotides and reforming a double helix. Then a second DNA polymerase molecule (also green) is used to bind, maintenance DNA methyltransferase make sure the methylation pattern of the parental DNA is copied to the daughter strand.