Mitochondria are the energy packets of the cell. They are the only organelle, other than the nucleus to contain DNA. However, there are many differences between nuclear and mitochondrial DNA. For example, unlike the nucleus where half the genetic material is inherited from the father and half from the mother, mitochondrial DNA is solely inherited from the maternal line. The organelles are present in egg cells, and though present in sperm cells, they are not passed on.
Mitochondrial DNA Forensic Science
Mitochondrial DNA (mtDNA) has many attractive features that make it an invaluable tool in forensic science. First of all is its abundance in the cell – there are hundreds of mitochondria in each cell and therefore many copies of mitochondrial genes – as opposed to two versions of each gene inside the nucleus. In fact the copy number in the mitochondria ranges from hundreds to thousands.
Mitochondria are relatively sheltered inside the cell and the genetic material appears to survive where nuclear DNA becomes degraded easily. Mitochondrial DNA has been extracted from aged and degraded samples of hair, bones and teeth, where there just hasn’t been enough decent nuclear DNA for analysis. There is a great deal of variation between the mtDNA of unrelated individuals but not so between relatives, unless there is a random mutation. Scientists and investigators can compare two hypervariable sections of an mtDNA sample with the hypervariable sections of another.
The fact that the genetic material inside the mitochondria is inherited down the maternal line is both an advantage and a disadvantage. It is an advantage in so far as every maternally related individual shares the same mitochondrial DNA sequences, which means that any living maternal relative can provide a reference sample. It is a disadvantage in that it is not an unique identifier because every person in a mitochondrial DNA lineage shares the same genetic sequence.
However, the technology is a boon to forensic science and is helping investigators to solve some extremely difficult cases. It is now an essential complement to nuclear DNA fingerprinting.