Paternal mtDNA transmission
In genetics, paternal mtDNA transmission and paternal mtDNA inheritance refer to the incidence of mitochondrial DNA (mtDNA) being passed from a father to his offspring. Paternal mtDNA inheritance is observed in a small proportion of species; in general, mtDNA is passed unchanged from a mother to her offspring, making it an example of non-Mendelian inheritance. In contrast, mtDNA transmission from both parents occurs regularly in certain bivalves.
Paternal mtDNA inheritance in animals varies. For example, in Mytilidae mussels, paternal mtDNA "is transmitted through the sperm and establishes itself only in the male gonad." In testing 172 sheep, "The Mitochondrial DNA from three lambs in two half-sib families were found to show paternal inheritance." An instance of paternal leakage resulted in a study on chickens. There has been evidences that paternal leakage is an integral part of the inheritance of Drosophila simulans.
In human mitochondrial genetics, there is debate over whether or not paternal mtDNA transmission is possible. Many studies hold that paternal mtDNA is never transmitted to offspring. This thought is central to mtDNA genealogical DNA testing and to the theory of mitochondrial Eve. The fact that mitochondrial DNA is maternally inherited enables researchers to trace maternal lineage far back in time. (Y chromosomal DNA, paternally inherited, is used in an analogous way to trace the agnate lineage.)
|“||Since the father's mtDNA is located in the sperm midpiece (the mitochondrial sheath), which is lost at fertilization, all children of the same mother are hemizygous for maternal mtDNA and are thus identical to each other and to their mother. Because of its cytoplasmic location in eukaryotes, mtDNA does not undergo meiosis and there is normally no crossing-over, hence there is no opportunity for introgression of the father's mtDNA. All mtDNA is thus inherited maternally; mtDNA has been used to infer the pedigree of the well-known "mitochondrial Eve."||”|
In sexual reproduction, paternal mitochondria found in the sperm are actively decomposed, thus preventing "paternal leakage". Mitochondria in mammalian sperm are usually destroyed by the egg cell after fertilization. In 1999 it was reported that paternal sperm mitochondria (containing mtDNA) are marked with ubiquitin to select them for later destruction inside the embryo. Some in vitro fertilization (IVF) techniques, particularly intracytoplasmic sperm injection (ICSI) of a sperm into an oocyte, may interfere with this.
|“||Over the last 5 years, there has been considerable debate as to whether there is recombination in human mitochondrial DNA (mtDNA) (for references, see Piganeau and Eyre-Walker, 2004). That debate appears to have finally come to an end with the publication of some direct evidence of recombination. Schwartz and Vissing (2002), 2 years ago, presented the case of a 28-year-old man who had both maternal and paternally derived mtDNA in his muscle tissue – in all his other tissues he had only maternally derived mtDNA. It was the first time that paternal leakage and, consequently, heteroplasmy was observed in human mtDNA. In a recent paper, Kraytsberg et al (2004) take this observation one step further, and claim to show that there has been recombination between the maternal and paternal mtDNA in this individual.||”|
Some sources state that so little paternal mtDNA is transmitted as to be negligible ("At most, one presumes it must be less than 1 in 1000, since there are 100 000 mitochondria in the human egg and only 100 in the sperm (Satoh and Kuroiwa, 1991).") or that paternal mtDNA is so rarely transmitted as to be negligible ("Nevertheless, studies have established that paternal mtDNA is so rarely transmitted to offspring that mtDNA analyses remain valid..."). One study stated that about 1–2% of a person's mitochondria can be inherited from the father."
The controversy about human paternal leakage was summed up in the 1996 study Misconceptions about mitochondria and mammalian fertilization: Implications for theories on human evolution. The following quotation comes from the abstract to that peer-reviewed study printed in the Proceedings of the National Academy of Sciences:
|“||In vertebrates, inheritance of mitochondria is thought to be predominantly maternal, and mitochondrial DNA analysis has become a standard taxonomic tool. In accordance with the prevailing view of strict maternal inheritance, many sources assert that during fertilization, the sperm tail, with its mitochondria, gets excluded from the embryo. This is incorrect. In the majority of mammals — including humans — the midpiece mitochondria can be identified in the embryo even though their ultimate fate is unknown. The "missing mitochondria" story seems to have survived — and proliferated — unchallenged in a time of contention between hypotheses of human origins, because it supports the "African Eve" model of recent radiation of Homo sapiens out of Africa.||”|
The mixing of maternal and paternal mtDNA was thought to have been found in humans and chimpanzees in 1999. However, there has been only a single documented case of human paternal mitochondrial DNA transmission.
According to the 2005 study More evidence for non-maternal inheritance of mitochondrial DNA?, heteroplasmy is a "newly discovered form of inheritance for mtDNA. Heteroplasmy introduces slight statistical uncertainty in normal inheritance patterns." Heteroplasmy may result from a mutation during development which is propagated to only a subset of the adult cells, or may occur when two slightly different mitochondrial sequences are inherited from the mother as a result of several hundred mitochondria being present in the ovum. However, the 2005 study states:
|“||Multiple types (or recombinant types) of quite dissimilar mitochondrial DNA from different parts of the known mtDNA phylogeny are often reported in single individuals. From re-analyses and corrigenda of forensic mtDNA data, it is apparent that the phenomenon of mosaic or mixed mtDNA can be ascribed solely to contamination and sample mix up.||”|
A 2002 study states
|“||Mitochondria may not be inherited solely through the maternal line, according to new research that promises to overturn accepted biological wisdom.||”|
In plants, it has also been reported that mitochondria can occasionally be inherited from the father, e.g. in bananas. Some Conifers also show paternal inheritance of mitochondria, such as the coast redwood, Sequoia sempervirens.
- Y-chromosomal Adam
- Human mitochondrial genetics
- Human migration
- List of genetic genealogy topics
- The mtDNA is passed from mother to offspring unchanged except for the occasional genetic mutation, the rate of which is unknown but is sometimes estimated as once every 600 generations (see Mitochondrial DNA Mutation Rates).
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