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When it comes to identification with DNA, forensic scientists have two basic approaches at their disposal. One approach analyzes nuclear DNA and is useful in identifying someone who is alive or has died only recently. This is the technique used to incriminate (or exonerate) suspects of crimes and in paternity testing. It's also the one used to identify the remains of victims of the September 11th attacks.

The other approach analyzes mitochondrial DNA, or mtDNA. Because mtDNA tends to survive long after nuclear DNA has disintegrated, this approach is useful in identifying the remains of persons who died long ago. It was used, for example, to identify the two crew members of Bomber 31 who have been identified and, more famously, to help identify Czar Nicholas II of Russia, who was killed along with his immediate family in 1918. This is also the technique scholars have turned to in order to assess how closely related extinct Neanderthals are to people today*.

What is mtDNA and why is it different? Unlike nuclear DNA, which is a mixture of genetic material from both parents, mtDNA is passed on, with no change, from mother to offspring. The father's mtDNA, on the other hand, is destined to die off; no trace of its genes will pass on to subsequent generations (unless a woman who inherited the same mtDNA—his sister, for example—has children of her own).

Outside the nucleus, but still within the cell, lie anywhere from 200 to 2,000 mitochondria. Mitochondria are tiny structures that help cells in a number of ways, including producing the energy that cells need. Every mitochondrion includes an identical loop of DNA about 16,000 base pairs long. By contrast, each cell contains only a single set of nuclear DNA (46 chromosomes), which is made up of 3,000,000,000 base pairs. Whenever an egg cell is fertilized, nuclear chromosomes from a sperm cell enter the egg and, together with the egg's nuclear DNA, form the DNA for the new individual. The mtDNA from the sperm cell, on the other hand, is left behind, outside of the egg cell. This is why only the mother's mtDNA carries on to subsequent generations.

Since mtDNA is passed on only through maternal lines, forensic scientists can use it to assist with the identification of someone who died long ago by matching that person's mtDNA to any ancestor or descendant on that line. This wouldn't be possible, however, if mtDNA wasn't as long lived as it is. Because there are so many more copies of mtDNA within the cells of our bodies, mtDNA tends to be around, even in bone fragments, long after nuclear DNA has fallen apart.

The following interactive feature shows who within a family tree shares the same mtDNA. It also shows how the mtDNA for many descendants can all be traced back to one common ancestor while their nuclear DNA can be a mixture of DNA from many ancestors.

• Tracing Ancestry (45K)

  Requires Flash plugin.

* This feature is adapted from an article that first appeared on the "Neanderthals on Trial" Web site. To find out more about mtDNA analysis of Neanderthals and its role in tracing human ancestry, go to Tracing Ancestry with mtDNA.

Rick Groleau is managing editor of NOVA online.