Dr. Steve Mount<\/a> is an Associate Professor of Cell Biology and Molecular Genetics at the University of Maryland.\u00a0 In February 2011, he wrote an article about his experience submitting his DNA to 23andMe and his experiences matching his cousins.\u00a0 More specifically, he became interested in one particular segment of DNA trackable to a specific ancestor.<\/p>\n He shares these insights.<\/p>\n In genetic genealogy, people who deal with autosomal DNA spend a lot of time trying to figure out which segments are IBD vs IBS \u2013 Identical by Descent versus Identical by State.\u00a0 In laymen\u2019s terms, identical by descent means that you do in fact share a common ancestor in a timeframe in which you might be able to identify them.\u00a0 Identical by state really implies, technically, that you just happen to have the same DNA due to spontaneous mutations, not because you share a common ancestor.\u00a0 In reality, it\u2019s taken to mean that you descend from a common population –\u00a0 in other words, you do share a common ancestor but the segment is so small that it implies that the ancestor is so far back in time that you can\u2019t possibly identify them. \u00a0Some people call these matches \u201cfalse positives\u201d which really isn\u2019t accurate.<\/p>\n Far from being useless, these small segments are very useful in identifying different ethnic populations<\/a> found in your ancestral tree and can, often in conjunction with larger segments also be useful in identifying ancestral lines.\u00a0 Discounting small segments, especially if you share a common ancestor, is akin to throwing away pennies because they aren\u2019t as useful and are more difficult to manage than quarters or dollars.\u00a0 Furthermore, small segments may be our only way of identifying ancestors that are many generations back in our tree.\u00a0 After all, we inherited all of our DNA from some ancestor, no matter how small the segments are today.<\/p>\n Because we have no better rule of thumb (or statistical model), we utilize the theory that one inherits about 50% of the DNA of each ancestor in each generation.\u00a0 We know this is absolutely true between Mom and Dad, but you don\u2019t receive exactly 25% of each of your grandparents\u2019 DNA.\u00a0 However, the mixture of what and how much of your grandparents\u2019 DNA you do inherit is approximately 25% and appears to be random, like a card shuffle.\u00a0 If it\u2019s not random, we don\u2019t know what the rules of inheritance are.<\/p>\n In the past few years, as we\u2019ve come to work more closely with autosomal results, we have learned that while the rules of thumb about how much DNA you inherit from specific ancestors are useful, they are not absolute.\u00a0 In other words, it\u2019s certainly possible to inherit a very large chunk of DNA from a very specific distant ancestor when the rules of probability and the rule of thumb of 50% would indicate that you should not.<\/p>\n This is shown clearly in the Vannoy project where 5 cousins who descend from Elijah Vannoy born in 1786 (5 generations removed) share a very\u00a0significant portion of chromosome 15.\u00a0 These people are all 5 generations or more distantly related from the common ancestor, (approximate 4th<\/sup> cousins) and should share less than 1% of their DNA in total, and certainly no large, unbroken\u00a0segments.\u00a0 \u00a0As you can see, below, that\u2019s not the case.\u00a0 We don\u2019t know why or how some DNA clumps together like this and is transmitted in complete (or nearly complete) segments, but they obviously are.\u00a0 We often call these \u201csticky segments\u201d for lack of a better term.<\/p>\n I downloaded this information into a spreadsheet where I can sort it by chromosome.\u00a0 Below you can see the segments on chromosome 15 where these cousins match me.\u00a0 Note that Buster is also a cousin from a second ancestor.<\/p>\n Given these incidental discoveries and the very large amount of DNA I share with these cousins on chromosome 15, I was quite interested in Dr. Mount\u2019s following commentary:<\/p>\n \u201cThe probability that fourth cousins share at least one IBD [identical by descent] segment is 77%, and the expected length of this segment is 10 cM.” Now consider the next step. There is a 50% chance that that one shared segment will not be transmitted at all, but a 90% chance that if it is transmitted it will be just as big as it was<\/strong> (the same 10 cM.). What this means for genealogy on 23andMe is that for two people sharing one segment identical by descent there is no way to reliably estimate how far back the common ancestor was. Furthermore, no improvement in software can possibly change that, because the limitation is imposed by the genetics itself.\u201d<\/p>\n Well, there goes the 50% rule – flying right out the window.\u00a0 The 50% rule of thumb says that in any given transmission, there is a 50% chance that it will be transmitted (so good so far) and that if it is transmitted, roughly half of it would be transmitted, or approximately 5 cM..\u00a0 That’s obviously not what is happening.<\/p>\n Dr. Mount goes on to say that, \u201cNo matter how far back you go, every nucleotide of one’s genome is derived from some ancestor, and even going back 20 generations, the chance that the bit which has been inherited is part of a block 5 cM. or greater is still appreciable. In fact, even for 19th cousins, there is a real chance (13%) that any segment of DNA they have inherited in common will be 5 cM. or greater. Of course, as mentioned above, there is very little chance that two 19th cousins will share any IBD segments at all, but this is offset if one has many 19th cousins, which is often the case.\u201d<\/p>\n 5cM is the line-in-the-sand cutoff number many genetic genealogists use to determine whether DNA segments are IBD or IBS.<\/p>\n What this really means is that the more distant, or 19th<\/sup>, cousins that you have, the greater the chance that one or more of them will test and\u00a0will indeed share a piece of DNA large enough to be identified by the testing companies as relevant.\u00a0 The software companies\u00a0will then apply their relationship estimating software to the size of the match and number of SNPs.\u00a0 The results are often inaccurate, as Dr. Mount says.\u00a0 Not inaccurate in that the match is incorrect, but the estimated relationship\u00a0is incorrect because the DNA did not divide in half as the mathematical model says it should.\u00a0 The “problem” is not in the software, but in the DNA itself.<\/p>\n \u201c23andMe reports a “predicted relationship” (e.g. “4th cousin”) and a “relationship range” (e.g. “3rd to 7th cousin”). However, these ranges are likely to be wildly inaccurate, because the likely distance to a common ancestor, given only the information that two people share a single IBD segment, can vary enormously, based largely on how many relatives one has.\u201d<\/p>\n And I will add, it will also vary by how and how much the DNA has or has not divided in every generation.<\/p>\n Dr. Mount goes on to provide the math and probability formulas for these various calculations, and explains what they mean, in English, then he summarizes by saying, \u201c<\/p>\n \u201cThus, if you have many more distant cousins, as would be expected if your ancestors had large families, then someone who shares a single IBD segment is more likely to be a distant cousin, because you have so many more distant cousins. The point where the increase in the number of cousins outweighs the loss of shared segments is five children per family. This is not extremely uncommon.\u201d<\/p>\n This actually makes a lot of sense when I look at my results.\u00a0 One of my ancestors, Abraham Estes (1647-1720) had at least 12 children of which 11 reproduced and had very large families.\u00a0 This line was extremely prolific.\u00a0 Many of my autosomal matches include Estes descendants.\u00a0 Some of my other lines where my ancestor was one of just a few children have far fewer matches, likely because there are far fewer people out there descended from them.<\/p>\n Dr. Mount confirms this by saying that, \u201cIf one family among [your] 32 [great-great-great-grandparents] had five children and their descendants did as well, while others in the family reproduced at replacement rates (two children per family), then your more prolific ancestors (the parents of just one of your 31 great-great-grandparents) would account for over 3\/4 of your fourth cousins.\u201d<\/p>\n So what is the take away message to us from all of this?<\/p>\n You can read Dr. Mount\u2019s article including his mathematical proofs, here.<\/a><\/p>\n ______________________________________________________________<\/p>\n Disclosure<\/strong><\/p>\n I receive a small contribution when you click on some of the links to vendors in my articles. This does NOT increase the price you pay but helps me to keep the lights on and this informational blog free for everyone. Please click on the links in the articles or to the vendors below if you are purchasing products or DNA testing.<\/p>\n Thank you so much.<\/p>\n DNA Purchases and Free Transfers<\/strong><\/p>\n Genealogy Services<\/strong><\/p>\n Genealogy Research<\/strong><\/p>\n The answer is, because inherited DNA segments do not always follow the 50% rule.\u00a0 I guess maybe no one told them??? Many times, when we receive our autosomal DNA results, we wonder why predicted relationships, particularly distant ones, aren\u2019t accurate.\u00a0 … Continue reading \n
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