Molecular Genetics

Facts

• In the 1930's, heritable epigenetic effects due to the interaction of homologous alleles were described and designated paramutation, since Mendel's laws were violated.
  • Paramutagenic allele = an allele that will convert a paramutable allele to a paramutant allele.
  • Paramutable allele = an allele that can convert to a paramutant state.
  • Paramutant allele = an allele of lower function caused by paramutation, inherited somatically and germinally.
• Paramutant alleles do not require the continued presence of the paramutagenic allele to maintain their state. They do, however, revert to the paramutable state with some frequency.
• For the Rlocus of maize, the paramutant state is correlated with increased methylation of Rlocus DNA.
• Transgenes will sometimes become silenced. Often, the silencing correlates with increased methylation of the transgenes.
• Silencing by methylation resembles RIP'ing.
• Not all gene silencing occurs by methylation coupled mechanisms. Some may be triggered by heterochromatization of the repeated gene. Others (Co-suppression) appear due to post-transcriptional events.

Interpretations

• Induced changes in methylation status are metastably heritable and can affect gene expression.
• Paramutation may be a manifestation of an important as yet undiscovered regulatory mechanism.

Further information

• In some cases, there is a relation between methylation and paramutation.
• In others, such as at the B locus of maize, there is not.

This is page 1383 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 2003