The second classical maize epigenetic system I’ll discuss is R locus paramutation. A tenet of classical genetics is that alleles of a gene segregate unaltered from a heterozygote. Paramutation is the name given by Alexander Brink to a phenomenon that he discoverd in maize in the 1950s that violates this most fundamental of genetic laws.
In R paramutation, one allele imposes a directed heritable change on the expression of another allele. The maize R locus, which we now know encodes transcription factors required for expression of anthocyanin biosynthetic genes, is subject to paramutation.
Brink reported that the expression of a wild-type R-r allele decreased in the next generation after exposure to either an allele called r-st (or another such allele, called r-mb, which was later identified). The alleles that do the job are termed PARAMUTAGENIC, and those which change are called PARAMUTABLE.
The alteration is heritable and can be maintained, but can also be gradually lost in certain genetic configurations.
More than that, after exposure to the paramutagenic allele, the paramutable allele can itself become paramutagenic.