An intragenic tandem duplication of genomic DNA is responsible for the f3N mutation of Drosophila melanogaster.

Among the numerous X chromosome-linked forked bristle (f) mutations described in Drosophila melanogaster, one designated f3N exhibits the unusual property of reverting spontaneously to wild type at an inordinate frequency, a frequency that can be increased with x-ray irradiation. In contrast to the f mutants described thus far, all of which are associated with the insertion of mobile DNA elements, f3N is associated with an intragenic duplication of 2.8 kb of genomic DNA that resolves to the normal sequence when reversions occur. Consideration is given to intrachromosomal recombination as the mechanism of reversion and truncation of the forked protein as a cause for the mutant phenotype.