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. 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. 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. It is now understood that the tail of the sperm, which contains additional mtDNA, may also enter the egg. This had led to increased controversy about the fate of paternal mtDNA. 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...'). A few studies indicate that very rarely a small portion 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: The mixing of maternal and paternal mtDNA was thought to have been found in chimpanzees in 1999 and in humans in 1999 and 2018, resulting this last finding quite impressive, as biparental mtDNA was observed in subsequent generations in three different familias, leading to the conclusion that, although the maternal transmission dogma remains strong, there is evidence that paternal transmission do exist and there is a probably a mechanism which, if elucidated, can be a new tool in reproductive field, for example, avoiding the mitochondrial replacement therapy, and just using this mechanism so that the offspring inherit the paternal mitochondria. However, there has been only a single documented case of 90% 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: