TheSfm system of gene control inAntirrhinum majus.

InAntirrhinum majus,pal-rec, an allele of thepallida series, is considered to be involved in anthocyanin production. Thepal-rec gene normally mutates autonomously from the recessive condition to the dominant giving fully pigmentedPal spots on a colorless background. TheSfm element, when inserted at thepal locus, destabilizes the activity of thepal-rec gene in such a way that the repressor (Sf) component abolishes gene activity in almost all of the cell lineages, whereas the mutator (m) acts to release full gene expression in some of the cells. Such a flip-flop control of thepal-rec results in what is known as shifting.