Mitochondrial DNA

Mitochondrial DNA (mtDNA) is DNA that is located in mitochondria. This is in contrast to most DNA of eukaryotic organisms, which is found in the nucleus. It is often stated that 100% of the mtDNA contribution to a zygote is inherited from the mother, although this is controversial and may not be true for all organisms.

Unlike most of the cell, the function of which is defined by nuclear DNA, mitochondria have their own DNA and are assumed to have evolved separately. Human mitochondrial DNA consists of 5-10 rings of DNA and appears to carry 16,568 base pairs with 37 genes (13 proteins, 22 tRNAs and two rRNAs) which are concerned with the production of proteins involved in cellular respiration. However many proteins found in the mitochondria are encoded by nuclear DNA: some, if not most, are thought to have been originally part of the mitochondrial DNA but have since been transferred to the nucleus during evolution.

There is little change in the mtDNA from parent to offspring, unlike nuclear DNA which changes by 50% each generation. Since the mutation rate is easily measured, mtDNA is a powerful tool for tracking matrilineage, and has been used in this role for tracking many species back hundreds of generations.

However, mtDNA is very rarely used any longer in court cases, as it can be contaminated very easily. Simply touching or breathing on a sample may contaminate it.

Origin of mitochondrial DNA
The existence of mitochondrial DNA also supports the endosymbiotic theory, which suggests that eukaryotic cells first appeared when a prokaryotic cell was absorbed into another cell without being digested. These two cells are thought to have then entered into a symbiotic relationship, forming the first organelle. The existence of separate mitochondrial DNA suggests that, at one point, mitochondria were separate entities from their current host cells.

Genetic illness
Mutations of mitochondrial DNA can lead to a number of illnesses including exercise intolerance and Kearns-Sayre syndrome (KISS), which causes a person to lose full function of their heart, eye, and muscle movements. (See also Mitochondrial disease).

Mitochondrial 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 (Sutovsky et. al. 1999). Some in vitro fertilization techniques, particularly injecting a sperm into an oocyte, may interfere with this. Occasionally this process goes wrong, for example in inter-species hybrids. It has also been reported that mitochondria can occasionally be inherited from the father, e.g. in bananas. However, it has also been proven that about 1-2% of a person's mitochondria can be inherited from the father.

The fact that mitochondrial DNA is maternally inherited enables researchers to trace uterine lineage far back in time. (Y chromosomal DNA, paternally inherited, is used in an analogous way to trace the agnate lineage.) This is accomplished in humans by sequencing one or more of the hypervariable control regions (HVR1 or HVR2) of the mitochondrial DNA. HVR1 consists of about 440 base pairs. These 440 base pairs are then compared to the control regions of other individuals (either specific people or subjects in a database) to determine maternal lineage. Most often, the comparison is made to the revised. Vilà et al have published studies tracing the matrilineal descent of domestic dogs to wolves. The concept of the Mitochondrial Eve is based on the same type of analysis, attempting to discover the origin of humanity by tracking the lineage back in time.

Searchable Genetic Genealogy Databases
Mitosearch (FTDNA)