To understand what happened next, you have to know a little about mitochondrial DNA (mtDNA). Mitochondrial DNA is contained in small, football-shaped inclusions outside the nucleus of a cell. It’s widely believed that mitochondria were once independent bacteria that invaded primitive cells millions of years ago. Instead of being digested, these bacteria took up residence in the cell, forming a symbiotic relationship with it. The cell provided them with food and water, and the mitochondria provided the cell with energy for metabolism and heat. The arrangement worked out so well that millennia later, a human cell has up to 1,000 mitochondria, each carrying five to ten copies of its own genome.
Mitochondrial DNA is passed along the exclusively female line of the family. A mother passes it to all of her children, but only her daughters pass it on to the next generation. My brothers and my sister and I have my mother’s mtDNA, but only my sister has passed it on to her children. My brothers’ children inherited their mtDNA from their mothers, who are my sisters-in-law.
Not many people understand why this happens. An egg cell is large, containing hundreds of mitochondria. The sperm is small, with only a few in its tail. In the act of fertilization, the sperm injects its genetic material into the egg, but is otherwise destroyed along with its mitochondria. The fertilized egg divides, becoming an embryo that develops into a fetus that eventually is born as a child carrying the mitochondrial DNA of only his or her mother.
Back to the Unknown Child…
The mitochondrial genome is shaped as a ring, 16, 563 or so bitpairs (bp) long. The positions are numbered starting at the “top” with 1 clockwise to 16,563. Not everyone has exactly 16,543 bit pairs in their mtDNA genome. It is common for the mtDNA genome to experience insertions and deletions, making it slightly longer or slightly shorter than this.


When the original mtDNA tests had been performed on the Unknown Child, only the HVR1 region had been tested. That is, only low resolution tests had been done. This had been sufficient to rule out four of the children. However, because the DNA profile (called the haplotype) of the Unknown Child is very common – the HV haplotype is shared by about 15% of Western Europeans – it is not surprising that the HVR1 results of two out of the six candidates matched those of the Unknown Child. Further DNA testing should have been done before an identification was announced, but because the age estimate given by the teeth was thought to be reliable, it had not been considered necessary.

So in forensic terms, the second round of testing was still tied.
There were only two remaining DNA tests possible. One was a last attempt to obtain Y-DNA from the remains, but there was only a remote chance that any Y-DNA had survived, and there had never been any effort made to locate paternal references. The focus had been on finding mtDNA references for the candidate children. The second hope was to test the coding region of mtDNA, the region outside the HVR1 and HVR2 control region. However, the first two attempts to distinguish the children using standard testing of the coding region failed. We were quickly running out of options.
To be concluded…
Part I, Part II, Part III, Part IV (Conclusion)