Porcupine DNA Chart

Porcupine DNA Chart
Porcupine DNA Chart

Porcupine DNA Chart

There is a great post by Jim Bartlett Porcupine Chart to check out at This article explains and uses a chart called a porcupine chart to show how DNA is passed down from our ancestors.  It explains why we may have more than a normal amount of DNA from some relatives but maybe no DNA at all from some of our ancestors a few generations back.  The chart was originally from an article at  The Coop Lab – http://gcbias.org/2013/11/11/how-does-your-number-of-genetic-ancestors-grow-back-over-time/  The chart shows how the DNA that passes down gets smaller and smaller through the generations. When an ancestor drops out, his/her spouse/mate stays in the mix.

From the Porcupine DNA Chart article by Bartlett: “Although ancestors who contribute DNA to you continue to drop out with each succeeding generation going back, the number of contributing ancestors at each generation going back cannot get smaller. NB: some of the individuals may repeat as multiple ancestors, but the number of positions for ancestors in the Tree who contribute to your DNA never gets smaller. In the extreme, the number of genetic individuals gets very small at bottlenecks and deep ancestry; but the number of “slots” in the Tree is very great.”

Bartlett: ” it appears that the number of ancestors who contribute above-threshold DNA segments will usually include all of our 16 2G grandparents, maybe all of our 32 3G grandparents, and most of our 64 4G grandparents (5th cousin level). I think that most of our Matches are in the 6th to 8th cousin level (where some of our 7G grandparents have passed “sticky” segments down to us); and that it drops off after that, with some Matches out to 10-12th cousin level with a few of our more distant grandparents.”

The original article with porcupine chart is hard to read for the average person, but this is easier to read and important for those of you using atDNA to try to find birth families.  I see a lot of questions about why a know relative doesn’t show up as a DNA relative, why one son will match some relatives and other son will match some different relatives, etc.  This article will help you answer those questions.  Not all of your relatives will be your DNA relatives necessarily and some will not appear as DNA relatives unless you take the matching segments down to a very small number like 3 cM.

About the author

DD

I found my birth family after 40 years of looking for them. I used DNA tests, software to sort DNA match results, family trees, contacting DNA matches and several website tools. We want to provide you a "one stop shop" with all your resources to help YOU find YOUR family.

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