I have been waiting and have asked Ancestry for this a few years ago. Apparently, I was not the only person of an endogamous background who had asked for it.
I went through my list looking for the first known Maori, just to see how large the longest segment would be. I have always advised Polynesians to look for anything at least 30cM for the largest segment (longest block at FTDNA) or as Ancestry is calling it, longest segment size in order to determine a true 2nd to 3rd cousin relationship. I know with other endogamous groups they tend to look for something around 20cM.
At Ancestry, you will have to click on that match’s name in order to see the longest segment size.
Notice how the longest segment size is below 20cM, but based on the total amount shared the predicted relationship is anywhere between a 2nd – 3rd Cousin. To show you what that looks like against known 2nd to 3rd cousin relationships, I am showing my match list about where the endogamous matches come in. I indicate the Hawaiian ones versus the Maori ones and my known cousins. I am inserting the longest segment size since you cannot initially see it on your list until you click on the match.
While I have a lot of 3rd cousin matches, my mother and one of my cousins have a lot of 2nd cousin matches. My cousin had over 500 of 1st – 2nd Cousin predicted relationships. Just looking at her top matches, I indicated the known relationships versus any Maoris and Hawaiian that she matches.
This definitely will help with determining the endogamous matches. But the longest segment size does get smaller the more distant of the relationship becomes. So by the 3rd to 4th cousin level, you may not really be able to tell, with the exception of the fact that we tend to get a lot more segments.
I have a 2nd cousin of whom we do not share a lot of DNA. While we still share in the range of what is expected for a second cousin, the longest segment size is just over 20cM.
At least sorting through these matches have become easier now that we have this additional feature. Again, this works well with the closer predicted relationships. This may not be as useful if you already have a lot of distant matches and your Polynesian matches fall within that range. A lot of my western Polynesian (Samoans and Tongans for example) matches are in that range.
Below are some of my Samoan matches and while their total shared is not a whole lot, their longest segment size is significantly smaller compared to what we normally see with eastern Polynesians. This is true with other DNA companies like FTDNA and 23andme.
My top FTDNA matches where the endogamous matches come in among my known relationships.
I am hoping that in the near future Ancestry will put the longest segment size immediately on the match list page so it will be easier to go through rather than click on each name to see if the match really is worth looking into. For now, what we have is definitely an asset to help us sort through these matches.