Mitochondrial DNA: Not As Pure As Once Thought

The analysis of mitochondrial DNA (mtDNA) has been a useful forensic technique since it was first admitted as evidence in a US court in Tennessee v. Wade in 1996. Unlike nuclear DNA, the DNA used for DNA fingerprinting, mtDNA does not match a crime scene sample to a particular individual. That is, it can’t point the finger directly at one person. What it reveals is the maternal lineage of the individual or crime scene sample in question. Often this is all that is necessary to prove that a particular individual was at the crime scene, but it is not the “smoking gun” that nuclear DNA is.

But now some new evidence casts a different light on this test. Before addressing that, let’s look at an excerpt from my book Howdunnit: Forensics so we can understand exactly what mitochondrial DNA is.

From Howdunnit: Forensics:

The DNA used for standard DNA testing is nuclear DNA and it can be extracted from any nucleated cell. But cells also contain non-nuclear DNA. This DNA is found within the mitochondria, which are small organelles that reside within the cytoplasm of the cell and serve as the cell’s energy production center. A small amount of DNA is found within the mitochondria, but each cell has many mitochondria.

Mitochondrial DNA has several characteristics that make it unique. These include the fact that it is passed from generation to generation by the maternal linage, mutates rarely, is found in places where nuclear DNA doesn’t exist, and is exceptionally hardy.

Your mtDNA is inherited unchanged from your mother and only from your mother. And she received hers from her mother, and her mother from her mother, and so on. Why is this? At fertilization, the egg supplies the cell and half the DNA while the sperm supplies only half the DNA. The sperm cell itself breaks down and disappears after passing its genetic material into the nucleus of the egg cell. This means that the actual cell and all the cell components (including the mitochondria) of the developing zygote come from the mother. As the cell divides and multiplies, these mitochondria are copied and passed on, generation after generation so that all the cells of the body contain identical mtDNA.

Since, mtDNA undergoes a significant mutation approximately once every 6500 years, it is unchanged over many generations. This means that your mtDNA is virtually identical to your mother’s, your great-great grandmother, and your maternal ancestors from 1000 years ago. Thus, anyone’s maternal linage can be accurately traced over many generations and this fact can be used to prove if two people share the same maternal linage.

That is the current state of mtDNA. Now to the new study:

Research reported in the March 4 issue of Nature suggests that mitochondrial DNA is not as pure as we once thought. I should point out that nothing in nature is absolute and there are variabilities in virtually any natural system you want to discuss. This holds true for mtDNA. But the entire basis for using mitochondrial DNA is that it varies little in a given individual so that the mitochondrial DNA found throughout the body remains consistent. This new study suggests that this might not be so. They found approximately 7% variability in skeletal muscle mitochondria and a disturbing 90% in kidney tissue. What does all this mean?

It does not mean that the individual will not share mitochondrial DNA with all of his maternal ancestors, nor that a non-relative would have mtDNA that matched the maternal linage in question, but rather that all of his mitochondrial DNA will not be identical and that this variability itself varies from organ to organ and tissue to tissue within a person’s body. So a sample taken from a skeletal muscle will be less variable than would one taken from a kidney.

This means that if possible the same tissue should be used for comparison. That is, if the suspects mother’s or grandmother’s mtDNA is obtained from a blood sample then the suspect’s mtDNA should be obtained from the same type of sample. This should lessen the variability, or at least make the degree of variability more predictable, and this should in turn make interpreting the test much easier. But this is still supposition and the final word will come only when further research is done.

One word of caution: never draw firm conclusions from a single study. Scientists do not do that, or at least should not, and therefore, though this study is intriguing and does raise some interesting questions, the truth will require further study by other scientists and only then can we know what all this actually means. But it is intriguing.