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Most of the genes in the mitochondria code for respiration-related proteins, and most of the genes in the chloroplasts code for photosynthesis-related proteins. The cytoplasmic genome, in contrast with the nucleic genome, exists in many copies in each cell: each cell contains numerous mitochondria and/or chloroplasts, and each such organelle contains multiple copies of its chromosome.
Mutations in the cytoplasmic genome occur spontanteously and at a much higher rate than in the nucleus, since the mitochondria and chloroplasts are exposed to high concentrations of reactive oxygen species (ROS, by-products of respiration and photosynthesis). Mitochondria and chloroplasts with mutant genes have the ability to cause wildtype alleles in other mitochondria and chloroplasts to become mutant as well; the way in which this is done is still not clear. A certain cell in which a mutant gene exists only in some of the organelles, whereas the wildtype allele exists in the rest, is a cytohet (or heteroplasmon).