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Some Basic Information On mtDNA, Tests, Results and Their Interpretation

The DNA (deoxyribonucleic acid) molecule can be thought of as a really long ladder that has been twisted into a spiral. The linking elements in the DNA molecule, comparable to the rungs of our ladder, are composed of four amino acids, guanine, adenine, thymine and cytosine, abbreviated G, A, T and C. These four amino acids occur together in pairs (G-C and T-A). The linking elements (or rungs) can be either of these pairs. These links are referred to as "Single Nucleotide Polymorphisms" or SNPs for short. The mtDNA molecule can be thought of as a circle with the two ends joined, as if our twisted ladder were bent into a circle with the two end appended. TheSNPs are numbered around the circle beginning on one side of the junction and ending on the other. The region around the junction is called the D-Loop (magenta "Origin" in the picture below, taken, with thanks, from Genebase Tutorials). The section on the low numbered side is called HVR2 and covers SNPs 00001 through 00574 and the other end is HVR1 and includes SNPs 16001-16569. The rest of the circle (yellow, green and white, SNPs 00575-16000) is referred to as the "Coding region".



The Mitochondrial Chromosome is passed only from the mother to all children but since only female children can be mothers, only female children can pass the maternal mtDNA along to their children. Thus, the mitochondrial chromosome, or mtDNA, follows the maternal line, being passed from some ancient mother to her daughter, then to her daughter . . . to her daughter for, literally, millennia. MtDNA is relatively stabile, showing mutations only over many hundreds of years. Many years ago, researchers at Cambridge University, England, needed some mtDNA material with which to define (sequence)the molecule. They obtained some of the placenta from a woman who happened to be giving birth in the Cambridge University Hospital at the time and used this material to sequence (define the makeup of the SNPs) the mtDNA molecule. This description of the molecule has become the standard, referred to as the Cambridge Reference Sequence or CRS against which all mtDNA is compared.

Your mtDNA test result lists those loci, or SNPs, at HVR1, HVR2 and the Coding Region where the amino acids (C,G,T,A) differ from those found at this SNP in the CRS standard, as well as indicating the specific changed amino acid. An example of a report is shown below. In the first entry, at SNP 16153, an adenine (A) was found in this sample where one of the other amino acids (T,C,G) was found at SNP 16153 in the reference sequence or CRS.

HVR1 differences from CRS 16153A
16288C
16362C


HVR2 differences From CRS 114T
146C
152C
195C
210G
263G
309.1C
315.1C

These data are useful in two ways. First, they define the ancient group of people from which you descend and, second, they can determine relatedness and non-relatedness among individuals. The first usage, determining ancient ancestral groups and migrations, makes the assumption that a mutation that first appeared at some location will tend to concentrate at that location. On the other hand, those carrying the mutation who do migrate will be spread progressively more thinly over the rest of the earth as they migrate more and more distal to the origin. Thus, in general, the greatest concentration of a particular mutation will occur at the location where it originated. Location, in this case, means a very general area such as a continent, subcontinent, country or group of countries and subregions in some cases. Therefore, localized (relatively) concentrations of people carrying a certain mutation suggests that the mutation probably originated at that locale. These large groups of people, carrying similar specific mutations, are called haplogroups. Basically, a haplogroup is defined as a group of people having, in common, mutations at specific SNPs. SNP mutations occur so infrequently that they can be considered a "one time only event". Those people carrying these SNP mutations are said to belong to the haplogroup defined by them. The first popular publication of a theory of human genetic history based on mtDNA genetics was Bryan Sykes' book, "The Seven Daughters of Eve". He gave the seven original mtDNA haplogroups' "clan mothers" names like Helena and Velda (haplogroups H and V). The ancient migrations of these peoples, as their haplogroups change when new mutations occur, have been heavily researched and suggest origins for our ancient ancestors. If we find that we belong to some haplogroup (say H or J) we can look at migration maps such as the one below (shamelessly lifted from the National Geographic Society Genographic Project) and see the migratory history of this haplogroup from its inception about 15,000 to 20,000 years ago to the present. The H and V haplogroups can be seen to have split off from the HV haplogroup and migrated to southern Europe and the British Isles. Maps of migratory routes for all known mtDNA haplogroups can be found on the web. There are links on our Links Page that will show some of these.

Soooooo, you now know the ancient "Earth Mother" from which you descend and the region of the world in which she originated. Great! This information, and a couple of dollars, will get you a cup of coffee most places. However, it does allow you to eliminate anyone who belongs to a different haplogroup from consideration as a possible relative in your genealogical line. The knowledge of your haplogroup can save you a lot of time, money, effort and frustration in your genealogical pursuits.

The second - and more relevant for genealogical applications - use of your results comes when you compare your test results, your mtDNA profile or haplotype (note, not haplogroup), to those of others. If your haplotype matches with another person, it means that the two of you share a common female ancestor. However, it does not tell you who this ancestor was, or when and where she lived. Therefore, one hopes for a match with some other individual and that the common ancestor is identifiable. Then each can use the other individual's information to, hopefully, expand their own knowledge of the shared ancestral maternal line. There are a couple of difficulties with this approach however. The mother's - the mtDNA source - surname changes with every generation so there is no constant surname line to follow. Also, for most of written history, including much of genealogical time, women's maiden names were neither as definitively, nor regularly, identified in written records as were men's surnames. These two factors make it much more difficult to trace the maternal line. Secondly, the mutation rate for mtDNA is much slower than that of Y-DNA so the number of generations back to the common maternal ancestor is likely to be well beyond the genealogical time range (since surnames came into common use around 1100), even with a high resolution (HVR1 and HVR2) match.

This treatment presents this subject as I currently understand it. I too am also relatively new at this and am very liable to misunderstand the data I have tried to digest as I attempt to become more competent in the area. If you find any errors in this treatise or find that you have questions that it doesn't answer, please contact me. Thanks!