Just came across this... "In population genetics, effective population size is not a direct measure of the total number of people that lived at a given time. It is rather a measure of genetic diversity. Experts trace an individual’s DNA back through history, looking for differences in the DNA sequences between the two copies of his or her genome. Essentially, they estimate how many generations of relatedness separate the maternal copy of a gene from the paternal copy. If a population is small, they can expect to reach the common ancestor relatively quickly; if it is larger, it takes longer. 'It’s amazing that you can get this much information out of a single individual,' Rogers said." https://www.quantamagazine.org/genetics-spills-secrets-from-neanderthals-lost-history-20170918/ Best regards, Stephanie On Thu, Sep 28, 2017 at 5:19 PM, Belinda Dettmann <[email protected]> wrote: > Thanks for that, Tim. The degree of endogamy becomes important in the > context of predicting relationships. Examples are in the statistics and > guidelines given in references such as ISOGG where values only apply to > populations with a degree of endogamy similar to that found in the USA. As a > practical guide, the commonly used predictions (at FTDNA, 23andMe and > ancestry.com) for known relationships from third cousin outward are not > correct for ancestry which comes from mixtures of populations with less > inbreeding than in the USA overall. > As an Australian with mixed ancestry from 8 different regions of the British > Isles, plus Germany and Poland, I have thousands of DNA matches, for which, > for third cousins and worse, the predictions are all wrong. The > relationships are much more distant than predicted and most links are > impossible to find. Ranges usually cover the right value, but the actual > predictions are rubbish. Similar situations apply for testers who live in > the British isles, or South Africa, or New Zealand, or Poland, to my certain > knowledge. > We all find it disconcerting when told how close our middle-distance matches > should be, when we know they are nothing of the kind. > I agree that chromosome mapping is essential in doubtful situations, and I > routinely download DNA for my known relatives to Gedmatch for that purpose, > if they agree. There again, the guidelines for length of segments need some > adjustments, as I find that fewer long and more short segments are the order > of the day for known cousins of third degree or more in testers from mixed > populations. > > -----Original Message----- > From: GENEALOGY-DNA [mailto:[email protected]] On Behalf Of > Tim Janzen > Subject: Re: [DNA] Endogamy does exist in USA data > > Dear Belinda, > You make some valid points. A point I would like to make is that endogamy > is population specific. There is a lot of admixture in some populations and > little in others. Even in the U. S. you have some areas and populations > where there is a lot of endogamy and others where there is little. As a > general rule, the Americans who live on the West Coast (outside Utah) have > less endogamy than those who live on the East Coast who have early Colonial > American roots... > The importance of chromosome mapping can't be overemphasized since you can't > use segments linked to one ancestral branch as part of the quantitative > analysis of a genealogical research question for a different ancestral > branch. > Sincerely, > Tim > > -----Original Message----- > From: GENEALOGY-DNA [mailto:[email protected]] On Behalf Of > Belinda Dettmann > Subject: [DNA] Endogamy does exist in USA data > > It is important to realize that DNA testers with predominantly US ancestry > come from an endogamous background, compared with the rest of the world. > That is, there is a degree of inbreeding in the USA, compared with the rest > of the world. This may not be apparent to local researchers, as most of the > statistics are calculated from USA data, where endogamy has been ASSUMED to > be zero. It is small indeed when compared to Ashkenazi or French-Canadian > populations, but it is larger than in mixed populations generally. > > > ------------------------------- > To unsubscribe from the list, please send an email to [email protected] with the word 'unsubscribe' without the quotes in the subject and the body of the message