11 Responses

  1. Jane 30 May 2015 / 11:07 am

    In my family, when I go back more than one or two generations and check collateral lines as well as my direct lines, I see many inter and intra family marriages: cousins marrying and siblings from one family marrying siblings from another. This means that some of my cousins are not simply, say, second cousins, but double or triple second cousins. These types of gene pools need to be taken into consideration in order to make shared cM analysis meaningful. So, for example, one of my great-grandparents is probably either the first or second cousin of one of the others (I lack proof), but surnames, places, and circumstances strongly suggest this, and I am a strong match with a first cousin twice removed on this line. It is also important to separate X-chromosome matches from autosomal since males pass on a perfect replica, excepting mutations, of one of their mother’s X-chromosomes to daughters, whereas the autosomes are ‘polluted’ by recombination.

  2. Blaine Bettinger 30 May 2015 / 11:14 am

    Jane – thank you for your comment. Endogamy caused by recent cousin marriages is a very well known problem in genetic genealogy. The Shared cM Project took endogamy into account, requesting at the time of collection whether there was any known endogamy that could affect the results. See https://thegeneticgenealogist.com/2015/04/06/collecting-sharing-information-for-known-relationships-part-ii and other recent posts here that break down statistics in “ALL”, “NO ENDOGAMY REPORTED”, and “ENDOGAMY REPORTED” to account for these issues.

  3. Jane 30 May 2015 / 11:28 am

    One other issue: I just checked my match with the cousin mentioned above, and we match on 271.2 cM total at GEDMatch, given a threshold of >7cM. If I reduce the threshold, naturally we match at a higher level. Does your chart reflect data collected for different threshold levels? Also, he tested at 23andMe, whereas I tested at Ancestry. These companies use different chips.

  4. Susan Celotto 30 July 2015 / 12:01 pm

    I have a question. Since I am adopted and, from my non-ID information, it doesn’t appear that my birth parents married, I will only be a half sibling or a half cousin to anyone I match.

    I match two full brothers. Because of the pattern of DNA inheritance, I appear to be a closer cousin to one brother than the other (see chart below). The three of us are all within five years of age of each other, their mother was born about the same year as my birth mother’s older sibling, and our grandparents were born in the same year range. Because of these facts, I suspect that we are actually “regular” cousins and not “removed cousins.

    QUESTION #1: Except for half sibling and half niece/nephew, does the relationship chart only show the range of shared DNA percentages if people share both parents?

    QUESTION #2: If the relationship chart only shows shared DNA percentages when someone shares both parents, what happens to the relationship (and resulting percentage) if someone is a half-cousin? Obviously a half-cousin will share less DNA with someone than a full cousin.

    When I looked at the relationship chart that shows DNA percentage shared (see link) the three of us fall within the range of 3rd cousins. The numbers I gathered from FTDNA and GedMatch are below.

    QUESTION #3: Does that mean my numbers might actually show that I am a half-2nd cousin to these two brothers instead of a 3rd cousin?

    The chart shows that 2nd cousins would share an average of 246 cM (range of 47 – 760cM) and 3.12%. Does it make sense that a half 2nd cousin would share an average of 123cM (range of 23 – 380 cM) and 1.5% DNA PLEASE EXPLAIN.

    The three of us are trying to find how I fit into their family and I need to know if I should focus on all of the descendants of their great-great-grandparents or whether we could have a closer common ancestor

    *I couldn’t find a way to paste my numbers here. Instead of posting them all, here is my match with the numbers of both brothers average together.
    Total segments: 18
    Total segments with 5cM+: 4
    Total cM: 116.77
    Total cM on seg over 5cM: 85.39
    No x-match (divided by 68): 1.25% DNA shared:
    ** Note: one brother is much higher than these numbers and well in the range of the numbers of a 2nd cousin if all of the numbers in your chart were cut in half (if that actually works out correctly).

  5. Randy H 5 September 2015 / 11:35 am

    Why have you not posted information for siblings in the chart? We see as much variation there, if not more, as for grandparents. Appears to me as much as 40-60% (std dev window around an expected mean of 50%). Would really love to see those stats as well. Siblings are different than parents or children. It is not a fixed 50%.

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