Shortcut to finding top DNA matches for endogamous populations in 23andMe and Ancestry

For background, see Genetic Genealogy for Jewish Ancestry and Overcoming Endogamy in DNA (YouTube) by GeneaVlogger.

If you take a DNA test to find relatives, and you’re from an endogamous population (such as being Ashkenazi), you need to look at your matches with the longest shared stretch of DNA. That’s easy enough to do on MyHeritage or FamilyTreeDNA, but not possible in Ancestry or 23andMe as they do not display or let you sort by longest segment/block.

The next best thing is to approximate the longest segment by dividing the total amount of shared DNA by the number of shared segments. This can be a bit of a pain to do repeatedly, so I’ve created a bookmarklet to do this. I tried to include the bookmarklet here directly but WordPress is filtering the Javascript so instead you’ll have to add any link at all to your link/bookmarks bar, then replace the URL with the code in this gist.

It doesn’t actually sort your matches. What it does do is calculate the ratio (total/segments) for each match displayed on the current page, hide matches below a certain threshold, and add the ratio number to each match. On Ancestry, which uses infinite scroll, you’ll want to scroll the page to reveal lots of matches before you click the bookmarklet. On 23andMe, you’ll want to paginate and rerun the bookmarklet on each page.

The threshold it uses is a minimum ratio of 12 and minimum total centimorgans of 47, but that can be changed by editing the beginning part of the javascript

The calculation uses the total amount of shared DNA in centimorgans, however since 23andMe displays the total as a percentage, it firsts multiplies the percentage by 75 to get a fairly accurate centimorgan number. Some websites claim that the multiplier should be 68, but I believe this number was calculated for FamilyTreeDNA, not 23andMe.