Very little is known about the physical and molecular organization of the genomes of plant organelles. Unlike the genes of a nuclear genome, which can be recombined by selective matings and mapped by recombinational events, the genes on an organelle genome are usually inherited as a unit, solely through a single parent. Therefore, when a uniparentally inherited mutation is observed, it is usually not possible to determine the location of the gene relative to other genes on the organelle genome, nor is it usually possible to determine genetically the organelle genome responsible for the mutant phenotype.