Paramutation

Para mutation is an epigenetic interaction between two alleles of a gene, which leads to an inheritance that is not compatible with the Mendelian rules. The phenomenon was discovered in 1956 by Canadian geneticist RA Brink in maize ( Zea mays).

Para mutation at the b1 gene in maize

The B1- gene of maize encodes a protein that activates the anthocyanidin signal are synthesized by the purple pigments. Plants that are homozygous for the BI allele have high expression and b1 show a purple color; Plants that are homozygous for the B ' allele, however, are bright because b1 is only weakly transcribed. Crosses between these two plants produce heterozygous plants, which also have a bright color. This means apparently that BI ' is recessive to B. However, if the heterozygous plants are crossed together, all plants of the following and future generations are bright This violates Mendelian splitting rule according to which some of the plants homozygous for BI and thus would have to be purple.

Mechanism

The nucleotide sequence of BI and B 'is identical, but the alleles have different epigenetic state, ie chromatin structure is different. By an interaction between the two alleles of the epigenetic state of BI is reprogrammed by B 'such that occur in subsequent generations no more plants, the BI are homozygous with respect. The exact molecular mechanism of interaction is not yet understood, but appears to play a major role in the communication between the alleles of non-coding RNA such as siRNA.

Other cases of Para mutation

In other genes in maize as well as in other organisms cases were found by mutation parameters, including Arabidopsis thaliana, and mouse. There are less clear-cut cases of para mutation in animals known as in plants. The example of the b1 locus in maize is particularly easy to analyze because the B ' allele is very stable and has a wholly owned penetrance. This is not so often you can in most other cases of para mutation, the expression of a gene change gradually, and the effect can wear off in subsequent generations.