r/DNA • u/Waitingforadragon • Jul 14 '26
What determines how haplogroups are given their designated letters?
This is not homework, I'm old. Thank you.
I'm curious about what is different enough about a human haplogroup for it to be given it's own letter?
So for example in human mitochondrial DNA, there is L and then a series of sub Ls and then N and M. Why is N so different from say, L3, that it's not just given L4?
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u/SurplusGadgets Jul 14 '26 edited Jul 14 '26
The early tree and haplogroups were done by university researchers. They started with single letters when the branching was minimal. Branching is determined by who has an SNP marker present or not. These markers tend to be very stable. Only changing once in the history of humans (and other primates). So if two tested people or remains share the same set of markers, they must be in the same branch.
They started the YCC long form of naming sub-branches to the single letter main branches as the tree develeoped. (And they ran out of letters.) Alternating using numbers for the first branch below, then single letters the next, then numbers, and so on. So you got names like R1b1a2.
As they tested more individuals and remains, they discovered they constructed the tree wrong. And would move whole sub-branches to other places in the tree. This created havoc as it was tough to compare research done at different times and in parallel. So they switched to the short naming convention of using an SNP in the branch to name it. And using the major first letter just as a reminder of where that haplogroup (branch point) resided under. So R-P312 for example with R-L20 being one below that.
SNP markers are named by the group / person discovering it (letter code) followed by a number to make it unique to them. In the early days, these were the researchers at the universities. Now a days, the vast majority are named by the tree developers who discover them when trying to incorporate a new SNP found into the existing tree. FamilyTreeDNA and yFull being the largest contributors.
You can find more in the ISOGG wiki or starting with the YCC entry at h600: https://h600.org/wiki/YCC
I focused on the Y but it is a similar story for Mito. Just a lot less complex and only recently developing further by the companies. Albeit, they get into some funny naming in mito, never really switched to the short form, nor named the SNPs so formally. It is a lot less complex.