My great-grandmother belongs to Haplogroup H, and I always feel a little bad for her. Not that I have anything against Haplgroup Hâ€™ers, but they got the short end of the stick. You see, currently all mtDNA sequences are compared to the Revised Cambridge Reference Sequence (rCRS), an mtDNA sequenced derived in the early 1980â€™s and recently updated. Since the source of most of the mtDNA for that sequence belonged to Haplogroup H, people who belong to Haplogroup H often have no deviations at all and their sequencing results tend to be a little boring. Imagine if your mtDNA testing company sends your results and they say: â€œYou belong to Haplogroup H, and your deviations from the rCRS are as follows: 0.â€ You see, a little dull.
Comparing everyoneâ€™s mtDNA to a randomly chosen sequence has always seemed so artificial to me. It was out of necessity of course, and maybe it will only be temporary. A recent paper in Nucleic Acids Research proposes that with the ready availability of many full-length mtDNA sequences, researchers can begin to compose a â€˜consensus sequence.â€™ For the non-geneticists out there, a consensus sequence is sort of a master mtDNA sequence, the result of comparing many (or all) mtDNA sequences to create a single sequence that they can be used to describe them all.
The study used 827 recent high-quality full-length mtDNA sequences to create a consensus sequence and analyze the variability of human mtDNA. First and foremost, it is important that this is the very earliest stages of this type of study, and hopefully future research will use thousands of sequences from all over the globe (and from many different time frames using ancient mtDNA).
From the sequences studied, fully 84.1% of the mtDNA genome was invariant! Additionally, 43.8% of the variable sites were â€˜personal mutationsâ€™, mutations found only in a single sequence. One of the most interesting pieces of information was that the sequences differed from the consensus by 21.6 nucleotides. Thus (if this number holds true with further research), if you were to randomly pick a person off the street, your mtDNA sequence will only differ, on average, by 21 or 22 nucleotides. The full range, however, was from 5 to 89 nucleotides, a pretty big range:
Another great piece of information was the variance between the rCRS and the human consensus generated by the study – 73, 263, 325+C, 750, 1438, 2706, 4769, 7028, 8860, 11719, 14766, and 15326. Interestingly, â€œ[t]his list exactly parallels the changes necessary to go from the rCRS (Haplogroup H2a) through each of the intermediate haplogroups, to macrohaplogroup R.â€