Cytoplasmic Inheritance
Cytoplasmic inheritance is a property of genes present outside the nucleus and located in the mitochondria of animals, plants, and fungi, and in the chloroplasts of plant cells. Cytoplasmic inheritance is a non-Mendelian type of inheritance and is typically associated with the phenomenon of somatic segregation, where a cytoplasm containing mutant and wild type mitochondria (or chloroplasts) will hold only one a homologous population of these organelles after several cell divisions.
The first evidence for the presence of extranuclear genes was provided by the observation of variants of plants whose leaves are striped green and yellow because mutations in the chloroplasts of some cells result in the absence of chlorophyll and the green color. Mitochondria were later shown to exhibit cytoplasmic inheritance mainly through studies of mutants in yeast and other single-celled organisms. The most important evidence that mitochondria must contain their own genetic system was provided through experiments performed by Boris Ephrussi who, about 1950, studied the genetics of respiration in yeast. Certain yeast mutants exhibit abnormal mitochondria, are incapable of oxidative phosphorylation, and reliant on the availability of high glucose in their growth media. When grown on media with low glucose they, therefore, form unusually small colonies and were called cytoplasmic petite mutants. By performing genetic crosses between different haploid yeast strains, Ephrussi and other yeast geneticist later concluded that the petite mutation does not segregate with any known nuclear gene or chromosome. In further experiments reported in the 1960s, the existence of DNA in mitochondria was demonstrated. Mitochondrial DNA of most of the petite mutants was shown to contain large deletions with subsequent failure to synthesize mitochondrial proteins that contribute to energy production through oxidative phosphorylation.
During yeast fusion, both haploid yeast cells that fuse to produce a diploid cell contribute equally to the cytoplasm of that cell. This diploid yeast will in turn show random segregation of both chromosomes and mitochondria during meiosis. Mitochondrial inheritance in yeast is, therefore, called biparental. This is not the case with higher eukaryotes where the sperm contributes little if any to the cytoplasm of the zygote and most mitochondria in the embryo are derived from those of the egg. More than 99 % of mitochondrial DNA is believed to be maternally inherited in most mammals. Uniparental maternal inheritance is found in many higher plants where mitochondrial DNA is inherited exclusively from the egg (female parent) and not from the pollen (male). A successful procedure used to test for extranuclear inheritance in plants is the performance of reciprocal crosses in two different strains. Because extranuclear genes from both chloroplast and mitochondria are contributed by the female gamete only, the progeny of a cross male X (green) x female Y (yellow) should be different from the cross female Y (green) x male X (yellow).
The maternal inheritance of extranuclear genes has been valuable in forensic analyzes in human genetics as well as in tracing population migrations. Mitochondrial DNA disorders in animals are believed to arise by direct inheritance or through spontaneous mutations in the embryo or accumulation of somatic mutations during the life of the individual. In human, these disorders have only resulted in affection of a limited number of tissues, primarily those that require high amounts of ATP produced by oxidative phosphorylation. Most defects affect the muscles, eye and the optic nerves. Kearns-Sayere syndrome, for example, is characterized by both eye defect, central nervous system degeneration and abnormal heart beat. Myoclonic epilepsy and ragged red fibers (MERRF) is one of the first syndromes associated with cytoplasmic inheritance described in human and is caused by a mutation in mitochondrial lysine tRNA. It is characterized by improperly assembled mitochondria in the muscles and jerky movements.
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