Thursday, January 1, 2015

Centimorgans or Percentages?

Average Estimated cMs for Autosomal Testing Comparisons. click to enlarge
Kristina Gow Dunnaway, ISOGG Facebook Page, 2014, used with permission


I will take a moment away from the narrative to answer the most commonly presented question put to me by readers regarding autosomal DNA matching.

And yes, I just heard that audible group sigh from all of you chomping at the bit to hear about my mother's reunion with her father. All in due time. All in due time. 

But let's start out the new year right. It is important for all genealogists - novice and experienced alike - to start 2015 with a cheek swab or a vial of saliva. And if your response is that you have already done so, then you need to start 2015 getting your older relatives, who are regrettably finite resources, to spit or scrape. Remember that DNA testing benefits both you, the researcher, and those out there desperately looking for a match. Take a lesson from what you have read in my blog. My path to Harold James "Brighton" Daugherty, my grandfather, began with my mother's relatively tiny 0.52% match to Brighton's first cousin, thrice removed: Brian Joseph Ryder. 

Brian's great-great-grandmother, Bertha Daugherty, was a woman who died at the age of thirty-six years: sixty-three years before Brian Ryder was born. He never knew the woman. He never knew her name. When he started to poke around into his ancestry at the time of his 23andMe test, she became a passing fill-in-the-blank on his family tree. But Bertha's brother, Ira Daugherty, was my great-grandfather, and Ira's son was the man I spent 2014 searching for.

If Brian Ryder had not tested out of sheer curiosity, I would not have had the starting point for my search.

The larger the various DNA databases become, the more helpful they will become to the genealogist, the adoptee searching for his or her birthparents, the foundling with no history at all, the millions of children born of sperm and egg donations that have made modern-day genealogy so technologically baffling. DNA testing helps everyone.

Do it. Do it now.

So that takes me back to the question I am asked most frequently.

When I discuss relationships and DNA matching, I often do so in terms of percentages. It is one way that 23andMe lists their genetic matches, and the mathematics makes more sense to me and my analytical brain. I have posted a graphic with my blogs indicating how known relationships should theoretically match each other by percentages. Siblings match each other by 50%. Half-siblings match each other by 25%. First cousins match each other by 12.5%. And the biggest revelation for my search came when my mother matched Ken Ryder by over 4%; and I knew second cousins match on the average of 3.125%.

But not all DNA sites list percentages. And the total amount of DNA tested by each company varies slightly, as well as how they report it. Additionally, the percentages by which different sexes match is skewed a bit by counting the matches on the X-chromosome, as women have two of these to the man's one. Roughly, the centimorgans of DNA you match with another person divided by 6800-7100 should give you a ballpark percentage.

What the hell is a centimorgan anyway?

Wikipedia defines it this way. "In genetics, a centimorgan (abbreviated cM) ... is a unit for measuring genetic linkage. It is defined as the distance between chromosome positions (also termed, loci or markers) for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01. It is often used to infer distance along a chromosome. It is not a true physical distance however."

Confused? Don't be. What I wanted to point out by this definition is that a centimorgan is not a tangible distance, such as an inch or a centimeter. It does infer a length of segmentation along a chromosome, and as genealogists we can think of it as a "sort of distance." All DNA testing companies report the amount of DNA you share with a match in centimorgans.  When you upload your results to GEDmatch.com, the many user tools also show matches in centimorgans and not percentages.

Oh, and when I say you should all get autosomal DNA tested in 2015, that automatically means you have to follow up with an upload of your results to GEDmatch.com. If you are only going to test through one company (cheapskate), you can still compare your results to others on GEDmatch.com who have tested through other platforms, and who have also uploaded their results to this site. It's free. Free is good.

Do it. Do it now.

So I am presenting you here with a chart similar to the one I have posted before in which the percentages of DNA are shown that you have in common with known stated relationships. This chart I give you today shows you the theoretical average of shared DNA you have with known stated relationships in centimorgans. This handy chart was made by Kristina Gow Dunnaway, and she gives permission for its reproduction and personal use. If you publish a book with this chart included and make a ton of money, that's another story, but I will leave copyright law to Judy Russell at Home - The Legal Genealogist.

You will see that the chart uses 6800 cM of autosomal DNA (atDNA) as its base figure for total DNA measured per person. This is the amount tested by FamilyTreeDNA. A more detailed discussion regarding the numbers game, the testing companies, and counting the pesky X-chromosome can be found at the International Society of Genetic Genealogy (ISOGG)'s wiki page at Autosomal DNA statistics - ISOGG Wiki. I have visited this page so often my browser recognizes it as soon as I type "au" only.

And remember, Mother Nature does not follow the rules set out on either one of the charts that I have given you. These are averages. The numbers are based on a purely theoretical assumption that DNA is passed perpetually in a tidy 50:50 split every generation. It is not. The only true 50:50 split you will ever get is a child compared to his or her parents.

The key to remember is that the larger the number, the more reliable the relationship assessment should be. I knew at the beginning of my search that my mother's father was not the man she thought he was, because she matched her sister by only 26% (1935 cM). There is no way you can make an argument for that being a full-sibling relationship. But as the numbers become smaller and smaller, the known relationship gets fuzzier and fuzzier.

Additionally, remember that if you have cousin marriages in your ancestry or come from a highly admixed population that may have had limited choices for marital partners, due to say religion or perhaps geographical isolation, the numbers become wonkier and less defining. The more families intermarry and their common ancestors' DNA is "reinserted" into their offspring, the more of it will be passed to the present generation. The numbers will be larger than expected.

Remember when I said life was messy?

Get busy setting up the 2015 budget, and make sure there are ample resources set aside for DNA!

65 comments:

  1. Thanks for the explanation for us non-medical types... Eventually will sink in.. LOL..I did not see the chart by Kristina Gow Dunnaway that you referenced Did some Google searches but failed to find it.. Help.. anxious to see it
    Larry

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    1. It is the chart that heads this blog post.

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  2. Duhhh Right under my nose.. Thanks

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  3. Just for amusement, I received an email yesterday from a researcher in Tennessee, trying to find matches. Here is what he wrote: "There are 18 samples with the surname [XXX] that show up. Many of these folks with samples don't even post their lineage or ancestrial surnames. Makes one wonder about their IQ instead of their DNA." This was through FTNDA. I encouraged him to upload his and his hope-for match cousin to gedmatch.com.

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    1. Sadly, as much as I work in all the DNA databases, I find the response rate to be abysmally low. And this person is correct - few people list surnames or geographical areas to even help identify a common ancestral link. Matches on GEDmatch are different.... people upload to this site because they WANT to further their DNA experience, so responses are better, but even so, I have gotten many unanswered emails on that site too.

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  4. Could you please explain how the "numbers become wonkier" if we have cousin marriages in our genealogy? My grandparents were 1st cousins 3 times removed, my great grandparents were double 1st cousins 1 times removed, my great great grandparents were 1st cousins 1 time removed. They lived in an isolated area and everyone was related.

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    1. If first cousins marry, they share a common set of grandparents. Their children will have four grandparents, but only six great-grandparents instead of eight. Those "duplicated" ancestors have the potential of sending more of their DNA downstream to their descendants. So cousinhood marriages amplify the DNA numbers, and the theoretical values lose their predictive value. The matches with other distant cousins are still valid, but because more of the duplicated couple's DNA ended up in their descendants, the relationship will falsely seem closer. In populations where there was a lot of intermarrying, DNA from common ancestors gets intermingled to the point that it is often carried in larger amounts in distant descendants, but also requires a careful eye to make sure your DNA matches aren't from multiple common ancestors.

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    2. Thank you for your explanation. I have learned a lot from your blog and it was a fascinating story of your grandfather search.

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  5. Important info but yes, I was definitely hoping for the "reunion" post! Or even how your mom took the news that her father was still alive. Fabulous reading!

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  6. Why would aunts and uncles have just as many cMs as grandparents? Same for great aunts & uncles, etc.

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    1. It's all about the mathematics. It is easy to understand that you get half of your DNA from your father, and the other half from your mother. In turn, since they got half of theirs from each of their parents, you roughly carry 25% of each grandparent's DNA in you. You share 50% of your DNA in common with your siblings because even though you have the same parents, the random shuffling you get is different from the combination your brothers and sisters got. So if you share 50% of your DNA with your siblings, their children would get half of it as well, making an aunt/uncle/niece/nephew relationship also 25%.

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  7. I am hoping somewhere DNA testing will help me find my Fathers maternal German ancestors that I have looking for years for!
    Can't wait to hear all about the reunion!
    For those that don't follow Michael on Facebook- you might want to start... A few great pictures there!

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  8. Thank you for give very good information.
    Medigene DNA

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  9. The main focus concerns the diagnosis of hereditary diseases
    Every paternity establishes a special relationship. This not only means a lifelong emotional bond, but also includes extensive and ongoing legal obligations. As large as the scope of that relationship is, the questions can be just as excruciating if there are any doubts about the Paternity Test, DNA Relationship Test and Identity Proofs. Unlike the mother, as a general rule, the father is naturally often left with his suspicions unless he decides to take a paternity test.

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  10. My sister and I share 1788 centimorgans and 81 segments. From your chart I am assuming we do not have the same father. Would that be a fairly clear assumption?

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    1. Yep...it is more clearly in line with a half-sibling, rather than a full one.

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    2. Yep...it is more clearly in line with a half-sibling, rather than a full one.

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  11. Thank you for your blog and research. We took the Ancestry DNA test, and an unknown person matched my husband at 1804 cM's. We felt this person was a child of one of my husbands brother's. One brother passed away and we had his son tested. The son's came back at 1928 cM's to the unknown person. Ancestry list him as "possible 1st cousin". My question is, do we need another DNA test to confirm half sibling? or could he in fact be a first cousin, and the other brother (still alive) actually be the father?

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    1. Georgia, I am always a fan of "more is better" when it comes to DNA, so additional testing to make sure all arrows point in the right direction is never a bad thing. With that being said, both test results are in the 25% range, which would indicate only grandparent/grandchild, uncle/nephew, or half-sibling. The scenario you paint is the most likely, as I would find 1928 cM ridiculously high for a first cousin, and most likely reflects a half-sibling. If the living brother is willing to test, it would confirm all of this, and additionally give you more DNA of your husband's ancestry to play with! But I think you're definitely on the right track.

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    2. Thank you! I have another question. What would be your theory as to Ancestry not saying "possible half sibling" rather than possible 1st cousin? The numbers certainly agree with half sibling.

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    3. I am not a fan of AncestryDNA's functionality for comparison purposes, and I just read somewhere (although I cannot recall where), that they just throw out half-relationships when creating their match algorithms. Although it certainly messes things up, it is an all-too-real phenomenon that cannot be ignored. AncestryDNA did not list "possible half sibling" because they do not have that category. To them, it wasn't an option.

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  12. Thank you Michael, your the best! I'm your newest fan!

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  13. What relationship would you consider 1580cm to be?

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    1. You can see by the chart that this would be closest to a grandparent/grandchild, aunt/uncle/niece/nephew, or half-sibling. A double first cousin could also be in this range.

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  14. Hi Michael,
    Female A (deceased) and Male B share the same mother. A's father is unknown. A's daughter, female C and B share 868 Cm. across 34 DNA segments. Shouldn't this be higher for uncle/niece? or are we missing something?

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    1. A and B are HALF siblings, thus C is a HALF niece of B. The theoretical expected shared DNA should be 850 cM. You are right on the money!

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  15. Female/female. Ancestry DNA test matches at 1175 cMs with 41 DNA segments. X-match on Gedmatch reports 196.1.
    What relationship might this be?

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    1. The most likely scenario here is two female first cousins whose fathers are siblings. The total shared DNA falls within the reported range of first cousins (850cM average, with 533-1379cM reported via Blaine Bettinger's shared DNA project). The key here is that there is an entirely intact X-chromosome shared. Fathers pass on their entire X-chromosome to their children; mothers pass a recombined X. These two females having an entire X-chromosome in common would have to get it unchanged from their fathers, who in turn got their common X from their mother, making the two females in this example having shared paternal grandparents.

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    2. Michael, thank you for your reply. The conundrum is that there is no brother. He only had one sister. Non-Identifying adoption paperwork is perfectly matched to this particular man. Could this be an outlier case?

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    3. I am not sure what you man by "outlier" or whose adoption paperwork you are referring to, so I don't know how to reply. I HAVE seen GEDmatch glitches where someone shared an entire X-chromosome, and when it just didn't fit, the folks there found a problem in the upload. I'd either notify GEDmatch, or reload the test to make sure this X-chromosome match is real. Until then, you need to tell me more. Feel free to email me directly at INPowerMac@gmail.com if you want to bypass the public forum here.

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  16. 1175 cMs nibling or 1/2 sibling; http://gliesians.com/autosomal-prediction-grid.faces?cms=1175.0

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    1. 1175cM is a bit on the low side for an aunt/niece relationship or for a half-sibling. Neither is impossible, but at the very extreme end of the bell curve. Note Blaine Bettinger's Shared cM Project, where the lowest reported value for these relationships, respectively, are 1301cM and 1320cM; and theoretic values should be around 1700cM. And that's still ignoring in this situation the passage of an entire X-chromosome, unless they were half-sisters of the same father.

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    2. Cool tool, Robert James Liguori! Thanks for posting it. ^_^

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  17. Thank you, Michael. I will follow your directions with Gedmatch first & then contact you directly.

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  18. Tom is my 2nd cousin once removed. His grandfather is brother to my grandfather. DNA match is indicated to be 1.01% .
    Kate is my 2nd cousin once removed. Her gr grandmother is sister of my
    gr mother. DNA match is indicated to be 2.22 % in ten segments.
    The Ancestors included in a pedigree back to 1601 contain four marriages of cousins. A collapsed pedigree. At last, I have an example comparing a collapsed pedigree cousin with a regular cousin. 1.01% to 2.22 % .How can I convert the cM value to centimorgan?

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    1. I assume you meant to ask how do you convert percentage to centimorgan. Different companies use slightly different figures, and the presence of two X-chromosomes in women over one in men changes overall total numbers. Even so, generally using 6800 cM as your figure for total DNA per person is pretty accurate. Therefore, you share 3400 cM of DNA with each parent, or 50%. Theoretically, second cousins once removed share 1.563% of DNA with each other. That percentage of 6800 is about 106 cM. Your example of 1.01% and 2.22% therefore indicates shared cM values or approximately 69 and 151 cM of shared DNA. This is perfectly compatible with the expected relationship. I am not sure what you might be asking regarding the cousinship marriages, but if you are related along the lines where this has happened, then the likelihood of having larger numbers of shared DNA rises because of the concentration of certain ancestral DNA along these lines. I hope that helps!

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  19. My question is if someone has less than the 7cM default limit are they possibly a false match or just harder to predict how distant they are? My theory is even a 1cM match is still a match.

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    1. The lower you set your threshhold, the more garbage you are going to get. I don't really get too excited over a match unless it is 10 cM or larger. Depending on who you read, 8 cM is approaching the 50:50 level of real versus false matches. And no, a 1 cM is rarely a match. Remember that your chromosomes are paired. There is yet no way to discern which of the pair you got from which parent. So for the sake of a simple example, say you got a C-C-C-C-C string of alleles from your father, and at the same location, a G-G-G-G-G from your mother. The resulting strand on your chromosome is CG-CG-CG-CG-CG. Anyone who matches EITHER allele is considered a match. But if your match had parents that contributed a C-G-C-G-C and a G-C-G-C-G, the result looks the same, but there is no way they are related. Follow? The longer the matching segment, the less likely this random erroneous matching can occur. So no, a match is not necessarily a match.

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  20. This comment has been removed by a blog administrator.

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  21. Thanks for making DNA relationships a little easier to understand. I am a woman who was adopted at birth and the only thing I knew for certain was the first and last name I was given by my birth parents before I was adopted. I have two questions on 2 of my Ancestry DNA matches. My birth surname is a rather obscure last name and a few months ago I found an Ancestry DNA match with my birth surname. According to Ancestry we have a 4th cousin type of relationship. Since she was born in the same year as I was (1954) and we share 73 cM’s, wouldn’t a 3rd cousin relationship be a more accurate possible relationship?
    Second question; I share 168cM’s with my “closest” DNA relation other than my children and grandchild. When first posted to Ancestry he was listed as 2nd cousins and when they “refined” the process, he became a 3rd cousin relationship. He was born about 1963-1967. Based on the cM’s and the approximate birth year, what would you say our relationship is?

    Debbie

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  22. Yes, I agree 73 cM would be more in line with a third cousin instead of a fourth cousin. 168 cM would be closer to second cousin, once removed. But here's the thing.... yes, these are both significantly high numbers, and you should be able to discern a relationship to them within a reasonable time period, but due to the unpredictability of DNA inheritance, you could be anywhere in the spectrum of second to fourth cousins. Averages are great aids, but they just aren't reality. See Blaine Bettinger's revised chart similar to the one on this blog, but taken from actual test results, and you will see that the averages ARE close to the theoretical values, but the ranges are broad. http://thegeneticgenealogist.com/wp-content/uploads/2016/06/SharedcMProject.png

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  23. I'm an adoptee and recently received my results from 23andMe. I found that I have two first cousins! Hooray! 1 cousin matched at 917 cm and the second at 703. Pretty typical results right? My closest blood relatives that I know! But of course there has been a complication. The cousins' brother (my fauxdaddy ;-D) is convinced I'm his daughter. Now my fauxdaddy is starting to get attached and I know he will be incredibly disappointed to find out I'm not his daughter. So my question is could I possibly be my cousins' niece? Everything I've read says no, but the family doesn't have/know of an aunt or uncle that could be my mother/father. Also, fauxdaddy was living in the town in which I was born and some other info lines up, however; there are GLARING problems between fauxdaddy's account (eg: supposedly he didn't know about me, but my birth records say that he should). Could my cousin be my uncles at only 700-917 cM? Any insight would be great! Thanks for all you do! PS: I do know that they could be my half-uncles, but my fauxdaddy claims that all his siblings have the same parents.

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    1. Based on Blaine Bettinger's ongoing Shared cM Project, nobody has yet reported an aunt/uncle/niece/nephew relationship under 1301 cM, so no, I would doubt that these people are your uncles. You did not state how the two first-cousin matches are related to each other (or if they are). Your first cousin's niece (depending how you define the relationship) would, at best, be your first cousin, once removed. The Shared cM Project has reported shared DNA up to 753 cM with an average of 512 cM, so there is feasibility there. Bottom line: Fauxdaddy can test to clarify things, or like in my blog, once you identify the possible candidates, more testing can be done.

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    2. Wow, thank you so much for your reply. The 1st cousins that I have are brothers. Ultimately it looks like a paternity or avuncular test is in order. Thanks again for your confirmation!

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    3. If you found a cM values of 917 and 713, you are quite lucky!! Congratulations. I have been comparing DNA for years and have never found a match over 430 !
      Please email me at terencepau@aol.com in order that we may discuss further in a confidential environment.

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    4. If the two matches are brothers, than yes, I think first cousins are the most likely, but half-uncles are always possible too. It's a great start, but yes, more testing well help you narrow down the identity. Testing someone on their paternal and maternal side will help narrow down identities as well.

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  24. If I had DNA matches of 700 and 900, I would investigate further to discover the pedigree going back a few more generations. The fuller the pedigree the greater the chances of a DNA match of a female grandparent or great grandparent.

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  25. My brother(?) and I had our autosomal DNA tested by Ancestry.com. The match(>7cM) was 1918cM and the largest segment was 139 cM. Virtually all of the matches were half matches, not full matches.
    Are we almost certainly half-brothers? Do we need to do a Y-DNA test?

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    1. Yes, 1918 cM of shared DNA is more in the line of half-siblings instead of full siblings. Know that the listed matched cM on AncestryDNA are different than on other sites because their algorithm "deletes" matching segment they deem "pile up" areas. You could upload both tests to GEDmatch for another comparison, but frankly it is not going to change a lot the reality of the situation. In real life data analysis, full siblings have shown as low as 2150 cM matching, and half-siblings as much as 2134 cM. So the next question is whose daddy is whose? Y-DNA can be helpful only if other men of the same paternal lineage have tested. For example, I have tested my Y-DNA out of academic curiosity, but I have no matches. That's because there just aren't a lot of Lacopo men being tested. I am not sure what you mean by "half matches" and "full matches", but if you know your presumed paternal lineage, obviously the brother who has autosomal DNA matches to known members of dad's side of the family has the known father. The one who does not has the unknown father. That brother needs to reread my blog from the beginning to see how one tracks that man down! Best of luck!

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  26. Thank you for the very quick and complete reply. I left out an important detail. The information I gave was from GedMatch. I submitted it to GedMatch after I downloaded the data from Ancestry.com. The "half matches" and "full matches" I referred to are from the GedMatch graphic. I don't think that changes your suggestions. Btw, we do have matches showing that one brother does have matches with the paternal lineage.
    I do have another question. If the identical twin of the mother in this case also had a son with her own husband, what what would you expect the match to be? Close to the same as the half brother?
    We will refer to your full blog for further information. Thanks again.

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    1. Oh good God, you're trying to throw bizarre scenarios at me now! Firstly, I kinda thought that's what you meant by "half" and "full," so yes, that delineates half-siblings. Theoretically, if two identical twin women had children by their respective husbands, those resulting first cousins would, yes, actually have values closer to half-siblings.

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    2. Thanks again, Michael. The twin question wasn't hypothetical. :)

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  27. Hello, I was wondering if there is any way humanely possible that full siblings can share 2076.6 cM. They have 50 matching segments. I am in complete shock that my mom and my uncle could possibly be half siblings. I am sure you hear this all of the time but honestly my grandparents would seriously be the last people on earth to have something like this happen. They were married almost 60 years when my grandmother passed away. I have read on a chart that half siblings on average share 1700 cm but the range can be 800-2100 Cm. Since the shared cM is so close to that 2100 range could this be an outlier. Another genetic oghef suggested that his could be an outlier on the low end of full siblings. What are your thoughts?

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    1. I have mentioned Blaine Bettinger's Shared cM Project before. He is taking KNOWN relationships and recording the amount of cM DNA they share. This is gathering REAL data in place of theoretical amounts. Interestingly, they are coming very close to the theoretical values. As of June 2016, his data showed the lowest full sibling match at 2150 cM, and the highest half sibling match at 2134 cM, so your value is pretty darn close to this breaking point. The true test would be to compare both tests on GEDmatch on a one-to-one basis. Full siblings will have numerous large chunks of their chromosomes as full base-pair matches. Remember that we have PAIRED chromosomes - one from mom, and one from dad. It would make sense that full siblings would have matches on both. These are represented on GEDmatch by large green bars on the graphic of their one-to-one comparison. Half-siblings would share little to no full base-pair matches, and share only half-pair matches. These would be graphically represented as large yellow bars. There will be almost no green. The "oh my God, this could never happen in my family" stories are becoming so commonplace now, I believe them now to be the norm rather than the exception! Best of luck!

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  28. Hello, maybe you can help me. On Gedmatch. I received a page of DNA matches. The cMs are all around the 40.1 - 69 range except for 3 members who's cMs are in the 850 range. According to your chart that would place them in the Half Aunt/ Half Uncle range or the 1st cousins range. In order for this one match to be my cousin who mean our parents are siblings which is crazy since her parents are in their late 80s or dead and my mother is her early 40s. SO I'm leaning toward Aunt because I do not know who my father is. I'm confused by the 1st cousins having the same amount of cMs as Half Aunt or Uncle. Can you email me at evaesmith@live.com. I have a millions questions. I believe I may have found my sperm donors family but I am not sure.

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    1. You mention "3 members" matching in the 850cM range, but then discuss "this one match." As my experience relayed in the blog proves, you have to interconnect all the matches not only with yourself but with each other. It is like on large logic problem. You don't have enough information to make a conclusion yet.

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  29. Sorry about not explaining better. The lady who I'm referring to uploaded her dna twice and the third dna match is her brother. I match the first match with the same 893.1 cMs and her brother my third match with 843.7.

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    1. As you mentioned, that range is most compatible with first cousin, half-aunt/uncle, but also full great-aunt/uncle. Again, you don't know enough to make a conclusion. You already mentioned that this match has a parent in their 80s. That person needs to be tested. You can only start wtih what you know. Read the blog in its entirety and you will see that. One match leads to more testing. You have to lay out a family tree of these matches and identify candidates and test accordingly.

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  30. Oh wow I didn't even see the full great aunt range until now. I'm testing a possible half sibling soon. Ancestry dna kits take 7 -10 days ughhh.. I don't live very far for her so I'm hoping to find out in the next 2 months. I know this is probably just a guess but I have to start somewhere. DNA doesn't lie so I'm related to this family I just need to figure out how. My mother would have been 46 this year and the lady who's match I mentioned is in her 50s. Her parents are pretty up there in age. I mean I guess I was thinking the only option that would make sense is the half aunt range. I'm so hoping I found my father's side. If not I'll just keep praying and hoping more people test and submit the results to gedmatch.

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  31. Hi there. My mom and I recently got our results back from 23andme and my mother has a strong match to a man and woman (they are brother and sister to each other) and they are listed as predicted 1st cousins to my mom. My mother matches the brother at 1126cm with no match on the X and sharing 32 segments. She matches to the sister at 1181cm and shares 33 segments, 3 of the segments match on the X. We have reached out to them and are having a hard time trying to figure where my mom fits in to the picture. Do you have any suggestions that could point us in the right direction?

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  32. Forgot to mention that my mom was adopted and have no family history past herself

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    1. Marcus,

      1126 cM and 1181 cM certainly falls into the first cousin range. You didn't state the ages of all involved. There's also those tricky half-relationships too. That level of matching could be half-aunts and half-uncles to half-nieces and half-nephews. The same goes for great-aunts and great-uncles. Even so, the values are on the high side for halfs and greats. First cousin is a sound assumption.

      If your mother shares no X-chromosome with brother, but she does with the sister, it is most likely (but not definitely) that the shared ancestor is from the father of the brother-sister. This father would have supplied his X-chromosome to the daughter, but not to his son. He would have only gotten the Y-chromosome.

      There are many good discussions on X-chromosome inheritance. One of them is Roberta Estes's "X Marks the Spot" at https://dna-explained.com/2012/09/27/x-marks-the-spot/. Truthfully, I have to wrap my head around it every time I discuss it, so that I don't misspeak. It is a topic I like to explain visually. Roberta has the inheritance diagrams on this blog post.

      So.... if your mother and these siblings are first cousins, and we suspect that the shared DNA comes through their father, the logical conclusion would be that your mother's grandparents are the same as the siblings' paternal grandparents.

      Since your mother is - of course - a woman, she got both her X chromosomes from each parent. This does not differentiate whether the match with the siblings is through your mother's unknown father or mother, but it seems likely that one of her parents is the brother or sister of the matching siblings' father.

      Clear as mud?

      This made my brain hurt. Let me know what you find out!

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  33. Had my brother's DNA tested with Ancestry. A match came in with with a unknown person at 190 cms. and 8 segments. Question is, could my grandfather be this person's great grandfather?

    Thank you, Carol S.

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    1. If your grandfather was this person's great-grandfather, it would make you first cousins, once removed....assuming that you also have the same grandmother/great-grandmother. You see from the chart above that the theoretical amount of shared DNA would be 425 cM, so your shared DNA does not support this. BUT, if this relationship is as you stated, but by two different women, then you would be HALF-first cousins, once removed, and thus sharing a theoretical value of 212.5 cM of DNA. This would be very much in line with what you describe. I will take the time here to also share another chart made by Blaine Bettinger. The above posted chart contains theoretical values. Blaine's chart is a result of his research into real-life scenarios. You can see that the actual values reported do not fall far from the theoretical values, but it is another important tool to keep at hand. It can be found at http://thegeneticgenealogist.com/wp-content/uploads/2016/06/SharedcMProject.png.

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