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.