Double Cousins and DNA

UPDATE:
The kindness of the genealogy community continues to overwhelm … literally! We have enough data for now and are slowly making our way though the contributions of dozens of volunteers. We cannot thank you all enough!


What Are the Odds? is the most powerful tool available today for figuring out where someone fits into a tree of their DNA matches.  That person could be an adoptee, someone with an unknown parent or grandparent, or even an unidentified body.

It has limits, though, especially when endogamy, pedigree collapse, or incest are involved.

The good news, though, is that The DNA Geek has teamed up with Margaret Press of the DNA Doe Project to develop a WATO-on-steroids that can handle complex cases.  It’s still in development, but the basic programming is in place.  Right now, we’re in need of simple, real-world data to put it through its paces.

 

Double Cousin Data Wanted

Do you have double first or second cousins?  A double cousin occurs when two siblings from one family marry two siblings from another family and both couples have children.  Those children are first cousins to one another twice over, through both of their parents.  Double second cousins are the children of double first cousins.

If you do, perhaps you can help us test the new tool.  The diagrams below show what we need.

In words, we’re looking for cases where someone with both a single first cousin and a double first cousin (dark gold above) from the same family has done an atDNA test, as have the two cousins (medium gold).  We need to know the centimorgan amounts shared by the main person to each of their two cousins (indicated by the red arrows).

If your matches don’t quite line up with this diagram or the double second cousin one below, we can’t use your information just yet.  But don’t lose hope!  We’ll be testing different scenarios in the future.

The best way to submit the data would be to download or print out the diagram, scribble names (or aliases) in the gold boxes, and write in the centimorgan values in the arrows.  If your matches are at 23andMe, please omit any X segments from the total, and if they are at FamilyTreeDNA, please subtract out segments less than 7 cM.

Doing it this way will streamline the data tracking on our end.  You can email a copy of your finished diagram to margaret.press (at) gmail (dot) com.

Here’s the diagram for double second cousins:

Thank you in advance to this wonderful community that works together toward our common goals!

67 thoughts on “Double Cousins and DNA”

    1. My Father’s younger brother married my mother’s sister, so I have three double cousins, one deceased. Is this the data you want?

  1. Hi Leah and Margaret, thanks for this!

    I’m guesing you want people with ‘just’ double first and double second cousins but no endogamy?

    1. For now, yes. The goal is to test increasingly complex scenarios to prove that the new tool works.

      1. I was told after my dad’s death, my mother already deceased it’s my uncles child, my daddy’s brother married my moms sister???? I have no clue what to do

        1. It’s not uncommon for two siblings to marry siblings from another family. What question about your family are you trying to answer?

  2. I have some double cousins, but not that close. But maybe this tool will help me figure out whether my great-grandparents were first cousins? Still hoping to find the answer someday!

    1. Yes! Your case is one of the first that I want to try once we’ve vetted this tool.

  3. How am I suppose to tell how much that segment is on the X chromosome? I tried hoovering over with my mouse but nothing. Am I missing something in 23andMe?

    1. If you scroll down the page in the chromosome browser, there should be a table of matching segments.

      1. Hello , I’m currently pregnant with my second child. I ended up getting pregnant by my sister baby father’s 1st cousin so my niece will have a double cousin from both sides. So my question is how high is the rest risk of having a disability child

        1. Based on what you describe, your baby’s father isn’t related to you, so there is no increased risk. If you are concerned about other factors that might affect your baby, you should discuss it with your obstetrician. Congratulations on the baby!

  4. RE- Double Cousins- so do you mean when 2 brothers marry 2 sisters, and the children of each of those couple’s children are double cousins? Then that is I and my 3 female cousins, all of whom had 23 and me tests. We’d like to help if we could but I don’t know what you mean about centimorgan value or how we would find it.
    Best,
    Susan

    1. Yes, that’s exactly what a double cousin is! Do you also have single cousins (through other siblings of your parents)? If so, we’d love your help!

      At 23andMe, use the “Family and Friends” pulldown at the top of the page and select “Advanced DNA Comparison”. That will take you to the chromosome browser. Make sure your name is in the “Compare” field then add your cousins in the “With” fields. You can do up to five people at a time. After you click “Compare”, you’ll see which segments you share with each cousin and the total centimorgans will be at the top. If you share any segments on the X chromosome, scroll down to the table to see how much, and subtract that amount from the total. (Sorry it’s so complicated!)

  5. Please disregard last. I figured it out. I had to use the Advanced DNA Comparison.

    1. Wonderful! Please email your version of the diagram to margaret.press (at) gmail (dot) com.

    1. The long-term goal is to help everyone struggling with pedigree collapse and endogamy. Baby steps!

  6. I have what I have thought are double second cousins, but can’t figure out how to place them into your diagram. My grandfather William had a brother Benjamin. William married Lena and Benjamin married Lena’s sister Josie. Aren’t the first generation children of these two couples first double cousins? And aren’t their descendants second double cousins? If so then I as couple #1’s descendant and the children of couple #2 would be double second cousins and we have DNA matches. Pleas tell me if I have misunderstood double cousinship.

    1. Sounds like you’ve got it right! In the 2C diagram, William and Benjamin’s parents would be Grandpa A and Grandma A. Lena and Josie’s parents would be Grandpa B and Grandma B. William and Benjamin would be Sibling A2 and Sibling A3, respectively. Lena and Josie are Sibling B1 and Sibling B2. You can then put yourself in the diagram and the grandchild(ren) of Ben and Josie. The one additional thing we need is a 2C who is not a double cousin, so the grandchild of a sibling of either William and Ben or of a sibling of Lena and Josie. Thank you!

  7. My father has double 1st cousins and then I have double 2nd cousins.

        1. If you also have match data to a single 2nd cousin, that’s what we’re looking for.

  8. I have a (1st and 2nd), a (1c1r and 2c1r), and a couple of (2c and 3c). These are probably also clouded by endogamy though. I guess you’re not ready for these yet?

  9. I share 1517 cM with my double first cousin my sister shares 1584 cM with that same cousin.

    I share 775cM with this double first cousins daughter and my sister shares 860cM with her.

    His mother and mine were sisters
    His father and mine were brothers.

  10. Hi!

    My family definitely has the qualifications! However, you do not mention data from Ancestry. If we have shared our data with GEDmatch, can you use that?

  11. My 2x Great Grandmother married my 2x Great Grandfather, and her sister married my 2x Great Grandfather’s brother.

  12. I’ll be very interested to see these results since I have a serious nest of double cousins one generation back in my paternal tree! They played havoc with my Ancestry ICW lists until I found the source.

  13. I have two first cousins that are double first cousins. They are showing as brother and sister on ancestry. Their fathers are brothers and moms are sisters. Both moms did ancestry as well.

  14. Good afternoon
    My husband has double second cousins x 2 that have tested, I ll try to find a single from the other siblings of his grandparents to go with them
    Just checking if that’s what is required
    Thanks
    Belinda

    1. Thanks for checking. Right now, we want to compare double 2nd cousins with single 2nd cousins (and similarly for double and single 1st cousins).

  15. Would you be needing results from a pair of half-siblings who are also half-first cousins?

    Their mother’s were half-sisters and father is shared. Their father also had an identical twin, but unfortunately the only child of father’s twin is already deceased.

    1. Oh, that’s interesting. It might take me a couple of weeks to get to it, but if you wouldn’t mind sharing the information, I’d love to see it. You can email it to me at theDNAgeek (at) gmail (dot) com. I’d like to know how much DNA total they share with one another (and how much is FIR if the testing site shows that) and how much they share with known first cousins if any have tested.

  16. Ok, so wouldn’t you still be double cousins or at least share double the DNA if you have this scenario or are you just looking for double cousins between siblings marrying siblings? I have one who’s father is my father’s paternal 1st cousin and who’s mother is my father’s maternal 1st 1x removed; opposite sides of the tree, but related to both parents for me.

    1. We’re trying to keep it “simple” for now. That definitely an interesting case, though!

    1. So they’re her double and single 1C1Rs? Cool! We’ll expand to scenarios like hers hopefully soon.

  17. My dad has 2 sisters who married 2 brothers. I think some have tested. Not sure. Are you willing to provide fee test kits if they are willing to participate in your study

  18. Are cousins whose paternal grandparents were siblings, double first cousins? If so, then I can send you some info.

    1. Your father was the product of incest? That’s a bit of a different situation, but we’d love to take a look. Please email me at theDNAgeek (at) gmail (dot) com.

  19. I have several double first cousins. Would love for them to take the dna test and see what the results show.

  20. I am desended from three generations of such marriages. My grandparents, my great grandparents and my 2nd great grandparents marriages involved sets of brothers and sisters marrying, I have double first cousins, once, twice and three times removed. Will send info for the generation you’re now researching.

  21. I am a DNA Search Angel and have a search I am working on right now to determine maternal grandparents. At first glance, it appears that the searcher’s BM would descend from BT and AH (I have created a WATO for T line and for the H line). BT and AH have a daughter who tested and shares 546cM with searcher. For both WATOs the daughter can only be a half sib. But, BT has a sister, ET, whose grandson tested and shares 517cM with searcher. It is my believe that ET has slept with one of AH brothers and searcher’s BM is the result. When will your tool be available? If I understand your project, this may help in cases like this?

    1. I’m not the programmer of the tool and I’m not sure when it will be available. We’re still testing it. I have hope for it, though!

  22. I have 5 double cousin’s.my mother and her brother ,married a brother and sister of another family.we share the same set of grandparent’s.

  23. Interested in trying to find out more about my linear tree. Found out I have double cousins on ancestry dna. Trying to find my biological father.

    1. Double cousins can make the search more complicated. How close are your closest paternal matches?

  24. My parents married within different families. 3 brothers married 3 sisters from difffernt families. Are the children considered double first cousins? Would our DNA be similar to my siblings– sisters/brothers??

    1. Yes, the children of those three marriages would be double first cousins to one another. Their shared DNA will be higher, on average, than regular first cousins and lower, on average, than half siblings, but could overlap either of those possibilities. Some double 1C could share in the range of full siblings but they will typically share less than full sibs.

  25. What happens to the DNA of their descendants when 2 “double second cousins” have kids together. This happened in a family tree I am researching. Trying to locate the birth mother of an adoptee. The birth mother would be either a kid of the “double second cousins” or a grandkid. The cm numbers between all the family members are confusing… I do have an extensive tree, that is well researched.

    1. If two double 2nd cousins had a child together, that child would match a little more closely to relatives through both sets of “doubled up” 2-great grandparents. Exactly how much closer is hard to say because the range for 3rd cousins is so broad even before doubling up is factored in.

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