Extranuclear inheritance

Extranuclear inheritance or cytoplasmic inheritance is the transmission of genes that occur outside the

Mitochondria are organelles which function to transform energy as a result of

Mitochondrial diseases are inherited from the mother, not from the father. Mitochondria with their mitochondrial DNA are already present in the egg cell before it gets fertilized by a sperm. In many cases of fertilization, the head of the sperm enters the egg cell; leaving its middle part, with its mitochondria, behind. The mitochondrial DNA of the sperm often remains outside the zygote and gets excluded from inheritance.

Parasites

Extranuclear transmission of viral genomes and symbiotic bacteria is also possible. An example of viral genome transmission is

• Vegetative segregation results from random replication and partitioning of cytoplasmic organelles. It occurs with chloroplasts and mitochondria during mitotic cell divisions and results in daughter cells that contain a random sample of the parent cell's organelles. An example of vegetative segregation is with mitochondria of asexually replicating yeast cells.[5]
• Uniparental inheritance occurs in extranuclear genes when only one parent contributes organellar DNA to the offspring. A classic example of uniparental gene transmission is the
  fertilization via the egg. The father's mitochondrial genes are not transmitted to the offspring via the sperm. Very rare cases which require further investigation have been reported of paternal mitochondrial inheritance in humans, in which the father's mitochondrial genome is found in offspring.^[6] Chloroplast genes can also inherit uniparentally during sexual reproduction. They are historically thought to inherit maternally, but paternal inheritance in many species is increasingly being identified. The mechanisms of uniparental inheritance from species to species differ greatly and are quite complicated. For instance, chloroplasts have been found to exhibit maternal, paternal and biparental modes even within the same species.^[7][8] In tobacco (Nicotiana tabacum), the mode of chloroplast inheritance is affected by the temperature and the enzymatic activity of an exonuclease during male gametogenesis. ^[9]
• Biparental inheritance occurs in extranuclear genes when both parents contribute organellar DNA to the offspring. It may be less common than uniparental extranuclear inheritance, and usually occurs in a permissible species only a fraction of the time. An example of biparental mitochondrial inheritance is in the
  diploid offspring.^[1][5]

Poky is a mutant of the fungus Neurospora crassa that has extranuclear inheritance. Poky is characterized by slow growth, a defect in mitochondrial ribosome assembly and deficiencies in several cytochromes.^[10] The studies of poky mutants were among the first to establish an extranuclear mitochondrial basis for inheritance of a particular genotype. It was initially found, using genetic crosses, that poky is maternally inherited.^[11] Subsequently, the primary defect in the poky mutants was determined to be a deletion in the mitochondrial DNA sequence encoding the small subunit of mitochondrial ribosomal RNA.^[12]

See also

• Maternal effect
• Plasmid
• Xenia (plants)

References