Paramutations

"Paramutation" is an epigenetic phenomena coined by Alexander Brink in the 1950s. Paramutation describes the situation when one or more alleles induces a heritable change in the phenotype of another allele. Because the inheritance pattern is not based on the base-pair information of alleles, paramutation is an example of non-mendelian inheritance. In the 1940s, Barbara McClintock first observed paramutations, without giving them a name, in her studies of Maize.

Paramutations can occur between two genes in the same DNA locus both (cis), or between two distant genes (trans). The allele that induces the epigenetic change is termed the paramutagenic allele and the allele that is epigenetically altered is referred to as the paramutant allele. Paramutable alleles are "sensitive" to paramutation. The paramutagenic allele is epigenetically silenced as a result of seemingly random chance or most likely environmental etiologies. The silence, paramutagenic allele then silences the another allele, called the paramutant allele. In the future generations, the paramutant allele can become paramutagenic and silence another paramutable alleles or can revert back to its pre-paramutant state, non-silenced state. There is no universal pattern whereby one allele is always paramutagenic and another is paramutable. These roles can switch.

Interestingly, in contrast to nonsense gene mutations, which result in complete loss of the associated phenotype, paramutations affect phenotype along a spectrum of gene expression patterns. Several known mechanisms of paramutations include DNA methylation, changes in chromatin structure and RNA directed silencing of transcription. Every epigenetic modification involved with paramutations and the biochemical mechanisms remain to be elucidated. One potential explanation for the evolutionary basis of paramutations is to have a fast mechanism to create advantageous traits that are heritable. This enables organisms to quickly adapt to a rapidly changing environment.

Example of Paramutation 1:

http://www.mun.ca/biology/scarr/bio4241_paramutation.htm

Note how each generation is colorless. Mendelian Genetics would predict 25% of plants to be colored in the F2 generation.

Example of Paramutation 2:

In some cases there is also a relationship between paramutation and the sex of the progeny's parent. This is sometimes observed in Maize. For instance, if there is a parent that is heterozygous for a paramutable allele (R-r) and a mutagenic allele (R-mb) the result of crossing this heterozygote with a homozygous recessive is dependent on the sex of the crossing parent. If the crossing parent is male than the mutagenic trait is more prominently displayed and if the crossing parent is female than the mutagenic trait is less severe and the phenotype is more similar to the recessive phenotype. See Image below.

http://www.genetics.org/content/148/4/1973/F1.large.jpg

R-mb is the paramutagenic allele and confers a marbled (darked spots) phenotype. R-r is the paramutable allele and confers a solid dark color phenotype and r is a recessive allele that confers a yellow phenotype.

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