Mitochondrial Dna Analysis

DNA is the messenger, which illuminates (our connection to the past), handed down from generation to generation, carried, literally, in the bodies of (our) ancestors. Each message traces a journey through time and space, a journey made by the long lines that spring from the ancestral mothers.

(Bryan Sykes, The Seven Daughters of Eve)

Conventional short tandem repeat (STR) typing systems do not work in every instance. Ancient DNA specimens or samples that have been highly degraded often fail to produce results with nuclear DNA typing systems. However, recovery of DNA information from environmentally damaged DNA is sometimes possible with mitochondrial DNA (mtDNA). While a nuclear DNA test is usually more valuable, a mtDNA result is better than no result at all. The probability of obtaining a DNA typing result from mtDNA is higher than that of polymorphic markers found in nuclear DNA particularly in cases where the amount of extracted DNA is very small, as in tissues such as bone, teeth, and hair. When remains are quite old or badly degraded, often bone, teeth, and hair are the only biological sources left from which to draw a sample.

The primary characteristic that permits its recovery from degraded samples is the fact that mtDNA is present in cells at a much higher copy number than the nuclear DNA from which STRs are amplified. In short, though nuclear DNA contains much more information, there are only two copies of it in each cell (one maternal and one paternal) while mtDNA has a bit of useful genetic information times hundreds of copies per cell. With a higher copy number, some mtDNA molecules are more likely to survive than nuclear DNA. Table 10.1 contains a comparison of some basic characteristics for nuclear DNA and mitochondrial DNA.

This chapter will review the characteristics of mitochondrial DNA, the steps involved in obtaining results in forensic casework, and issues important to interpreting mtDNA results.


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