People often talk about characters — hair colour, a hooked nose, a talent for the trombone — missing a generation. This is simply the result of recessive genes acting on complex traits. However, Susan Lolle, Robert Pruitt and colleagues now report in Nature that, for the plant Arabidopsis thaliana at least, this can be quite literally true.

The A. thaliana HOTHEAD (HTH) gene encodes an enzyme related to the glucose-methanol-choline oxidoreductases. Recessive mutant alleles produce flowers with fused sepals, petals, stamens and carpels. However, around 10% of the progeny from crosses between homozygous mutant hth parents revert to a normal phenotype.

Many trivial explanations, such as contamination, could produce this result, but Lolle and colleagues carefully excluded each of these. Instead they found that the sequence of one, and occasionally both, of the hth alleles had reverted to the wild type in the phenotypically normal progeny. Such precise correction could not result from random mutation, however a search for related sequences in the genome that could function as a source of corrective recombination drew a complete blank.

This unusual process is not limited to changes in the HTH gene. In third-generation plants that were derived from crosses between two different Arabidopsis ecotypes, as many as 1 in 25 of the ecotype-specific marker genes had sequences that were not present in the parents' genome, but only in their grandparents'. Such deviations from conventional inheritance were only seen in homozygous hth mutant backgrounds, irrespective of the precise mutant allele that was present.

To explain this curious phenomenon, the authors propose that the sequence of some, or perhaps even all, plant genes is passed down through the generations independently of the conventional genome, but in a form that can be used to correct changes when they arise. Such a corrective mechanism could be triggered by metabolic stresses, for example. No such system is currently known, indeed it would defy the tenets of Mendelian genetics, but one possibility is that it involves RNA. Double-stranded RNA molecules that are responsible for gene silencing can persist through several generations in Caenorhabditis elegans, and the ability of RNA to function as a template for modifying DNA is well established.

Many plants have been responsible for fundamental discoveries in genetics: Gregor Mendel's wrinkled peas or Barbara McClintock's variegated ears of maize, to name but two. The hothead mutants of A. thaliana could be destined for similar notoriety for uncovering an emergency back-up to the genome.