Paternal mtDNA transmission


 * Paternal leakage redirects here.

In genetics, paternal mtDNA transmission and paternal mtDNA inheritance refer to the incidence of paternal mitochondrial DNA (mtDNA) being passed on to offspring. Paternal mtDNA inheritance is observed in a small number of species, but in the vast majority, mtDNA is believed to be passed unchanged from a mother to her offspring.

In animals
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."

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 techniques, particularly injecting a sperm into an oocyte, may interfere with this.

In humans
Many sources, notably companies that sell genealogical DNA tests, state that paternal mtDNA is never transmitted to offspring. This belief is central to mtDNA genealogical DNA testing and to the theory of mitochondrial Eve. However, there have been some studies that claimed to have found some cases of paternal mitochondrial inheritance in humans (e.g. Schwarz & Vissing 2002), and it was proposed that in sexual reproduction, the tail of the sperm does enter the egg, and thus "paternal leakage" may occur.

The ideas about human paternal leakage were summed up in the 1996 study Misconceptions about mitochondria and mammalian fertilization: Implications for theories on human evolution:

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 has been found in humans and chimpanzees. However, there has been only a single conclusively proven case of human paternal mitochondrial DNA transmission. All other reported cases can be ascribed solely to contamination and sample mix up.

Responses to this evidence involves saying that so little paternal mtDNA is transmitted as to be negligible ("Some researchers argue that a fragment of the father's mtDNA is in fact passed on, though it represents much less than 1 percent of the total." ) or saying 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..." ). It has also been shown that about 1-2% of a person's mitochondria can be inherited from the father.

According to the 2005 study More evidence for non-maternal inheritance of mitochondrial DNA?: In addition, heteroplasmy is a "newly discovered form of inheritance for mtDNA. Heteroplasmy introduces slight statistical uncertainty in normal inheritance patterns."

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 mixed or mosaic mtDNA can be ascribed solely to contamination and sample mix up."

On the other side of the issue are people who want to dismiss maternal mtDNA inheritance completely. A surname project tutorial states, "A male will rarely get his father's mtDNA. So far, this has been observed one time in the past 20 years of research." This single instance is cited as evidence that the whole field of Human mitochondrial genetics is invalid.

In either case, these phenomena throw doubt on the theory that all mtDNA can be traced back to a single female. This theory of mitochondrial Eve, which aims to trace all humans to "a woman living in sub-Saharan Africa about 200,000 years ago" allows no room for the inheritance of paternal mtDNA.

In protozoa
Some organisms, such as Cryptosporidium, have mitochondria with no DNA whatsoever.

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.