Transgenerational epigenetic inheritance is the transmission of epigenetic markers from one organism to the next (i.e., from parent to child) that affects the traits of offspring without altering the primary structure of DNA (i.e. the sequence of nucleotides)—in other words, epigenetically. The less precise term "epigenetic inheritance" may cover both cell–cell and organism–organism information transfer. Although these two levels of epigenetic inheritance are equivalent in unicellular organisms, they may have distinct mechanisms and evolutionary distinctions in multicellular organisms. Environmental factors can induce the epigenetic marks (epigenetic tags) for some epigenetically influenced traits. Some marks are heritable, leading some to consider that due to epigenetics, modern biology should no longer reject the inheritance of acquired characteristics (Lamarckism) as strongly as it once did. Epigenetic categories Four general categories of epigenetic modification are known: self-sustaining metabolic loops, in which an mRNA or protein product of a gene stimulates transcription of the gene; e.g. Wor1 gene in Candida albicans; structural templating in which structures are replicated using a template or scaffold structure on the parent; e.g. the orientation and architecture of cytoskeletal structures, cilia and flagella, prions, proteins that replicate by changing the structure of normal proteins to match their own; chromatin marks, in which methyl or acetyl groups bind to DNA nucleotides or histones thereby altering gene expression patterns; e.g. Lcyc gene in Linaria vulgaris described below; RNA silencing, in which small RNA strands interfere (RNAi) with the transcription of DNA or translation of mRNA; known only from a few studies, mostly in Caenorhabditis elegans. Inheritance of epigenetic marks Although there are various forms of inheriting epigenetic markers, inheritance of epigenetic markers can be summarized as the dissemination of epigenetic information by means of the germline. Furthermore, epigenetic variation typically takes one of four general forms, though there are other forms that have yet to be elucidated. Currently, self-sustaining feedback loops, spatial templating, chromatin marking, and RNA-mediated pathways modify epigenes of individual cells. Epigenetic variation within multicellular organisms is either endogenous or exogenous. Endogenous is generated by cell–cell signaling (e.g. during cell differentiation early in development), while exogenous is a cellular response to environmental cues. Removal vs. retention In sexually reproducing organisms, much of the epigenetic modification within cells is reset during meiosis (e.g. marks at the FLC locus controlling plant vernalization), though some epigenetic responses have been shown to be conserved (e.g. transposon methylation in plants). Differential inheritance of epigenetic marks