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What is Haplogroup W?
The Bottom Line
A person is classified as belonging to Haplogroup W if they have a change to their mtdna at letters 1243, 3505, 8994, 11947, and 15884. In studies so far these five changes are unique to Haplogroup W. There are other mtdna changes that most or all W's share (see below). But these other changes are seen in other haplogroups and so do not unambiguously classify you as a W.
These changes do not involve the HVR1 and HVR2 regions (letters 16001 to 16569 and 1 to 574) that you probably had tested for genealogy purposes. However Family Tree DNA (FTDNA) conducts a separate test of 22 locations outside of HVR1 and HVR2 to assign customers to a haplogroup. FTDNA classifies you as a W if you are 1243C (the five other locations are not tested due to the expense).
Members of the W haplogroup, compared to the Cambridge Reference Sequence, do have other common changes in their HVR1 and HVR2 mtdna. While these are not used to define W, they are common to nearly all European-origin W's. The complete list of common W haplogroup changes are:
W Sub-Haplogroups
Tests of the complete mtdna for a handful of persons in Europe and Asia in the last few years have allowed some sub-haplogroups (W1, W2, W3 etc) to be identified. However so far these are identified differently from paper to paper. They have not yet been accepted as standards. Usually they involve changes at locations in the mtdna not included in the standard tests (see below for details).
The History of Haplogroup W
In the late 1970's, it began to become possible to read the human genetic code. At first this was very difficult, and only a single small group of letters here and there on the mtdna could be read. It quickly became apparent that some changes in letters to the code were shared by people in the same geographic region. These changes were in the stable 'coding region' of the mtdna, where there were very few changes (only about 250 out of 15,500 letters in this region were ever different in all the humans tested). Gradually it became clear that these were sections that accomplished work. Changes here would normally affect the functioning of the cell and could not last (because if that happened, the cell would not function correctly - the baby would either not be born or not have offspring). The changes in the coding region that stayed over time evidently did not negatively affect cell function.
A haplogroup was simply a group of people who share the same stable changes to their mtdna. These stable changes were found to correspond to human geographical areas. Soon, it became clear, they also corresponded to the ancestry of the human race. Letters were assigned to name the haplogroups as they were identified. The first haplogroups were identified among American Indians and were given the letters A to D. H to K were European groups. Haplogroup L was mainly found in Africa; haplogroup M in Asia. By comparing changes between the haplogroups, it became clear that L was the ancestor of them all.
Haplogroup W was one of five European mtdna haplogroups (T, U, V, W, and X) first defined in a scientific paper in 1996. In 1996 genetic testing was still difficult and expensive. The entire 16,568 letters of the mtdna could not be analyzed. The sample was limited to 67 Scandinavians. In the original 1996 paper, haplogroup W was identified as those with a changes in letter 8994 of the mtdna from G to A compared to the Cambridge Reference Sequence.
In the same paper the authors looked at the 'noncoding region' of the mtdna. This is what is now called HVR1 and HVR2 - areas of the mtdna which evidently are no longer used in the human cell. Changes here are not eliminated; they just stay. The rate of change here was 31 times greater than that in the coding region. The authors found that the two members of haplogroup W they tested showed a common pattern of changes: 16223T in HVR1 and 189G, 195C, 204C, 207A in HVR2.
As more areas of the mtdna were tested, the relationship between the different haplogroups became clearer. W was originally seen as descending from the African L; later it was thought to be derived from haplogroup I; then it became clear that both W and I were descended from haplogroup N; and later, depending on the tests conducted, W was seen as deriving from a subhaplogroup of N (N1 or N2).
In 2001, for the first time, it was possible to completely sequence the mtdna of a sample of 192 Scandinavians. All of the changes in the mtdna uniquely shared by members of Haplogroup W could be identified. This defined haplogroup W as containing coding-region changes at letters 709, 1243, 3505, 5046, 5460, 7864, 8994, 11674, 11947, and 15884. Furthermore two subhaplogroups were identified: W1 with additional changes at 5495 and 12669; and W2 with additional changes at 4928, 9612, and 10086.
By 2004 complete mtdna testing was accomplished for larger samples of individuals from south Asia, where haplogroup W was more prevalent. This led to further refinements and sub-haplogroups being defined. W was now seen as a subgroup of N2. N2 was defined by changes at 189, 709, 5046, 11674, and 12414. W was defined by additional changes at 195, 204, 207, 1243, 3505, 5460, 8251, 8994, 11947, 15884, 16292, and 16519. Several W subhaplogroups were defined:
Also in 2004, the first paper to study the deep history and detailed geographic distribution of haplogroup W's in southwest Asia appeared. This compared HVS1 relationships between a very large population sample. In this study W was defined by the change at 8994.
The pioneering papers (so far) that have defined and refined haplogroup W are:
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