Please Help Collect Sibling DNA Statistics

I need more data from full and three quarter siblings that are either tested at 23andme or have uploaded to GEDmatch. Ever since I published my article on why full siblings don’t share more DNA, I have been getting inquiries from people who are concerned that they are only ¾ siblings, so I would like more cases in order to help them figure that out.

My article (click here for it) included the insight that if you sum the FIRs (fully identical regions) and HIRs (half identical regions) of full siblings you get a result of about 3600 cM. This is roughly the same amount shared with a parent, thus a true half of the inherited DNA. My article also discussed the expected numbers for ¾ siblings.

Here is the form to help me collect this information, Please do not include the X if you are using 23andme. Below are some more instructions if you need them.

If you are using GEDmatch, the instructions for collecting this data are in my previous article, click here. For the 23andme instructions click the read more below.

At 23andme when you use the chromosome browser to compare two people you get a display like the following. The number for Half identical is what I call HIRs and the number for Completely identical is what I call FIRs in the form.

My brother and I at 23andme. The darker segments are the FIRs. Click or the full image.

Underneath the image of all those chromosomes there are many lines of numbers with the title Detailed segment data. At the bottom of those lines the segments shared on the X are listed. Subtract the X cM amounts that have half in the final column from the HIRs. Subtract the X cM amounts that have complete in the final column from the FIRs (only two sisters will have these). These new numbers are what are needed for the form. Below is an example of what those lines segment data look like. In this case I would subtract both 74.20 and 65.61 from the 2811 HIRs.

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32 thoughts on “Please Help Collect Sibling DNA Statistics

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  1. I have 4 brothers on Gedmatch. The above statistics are from a One to One comparison with me and my eldest brother. I can run more comparisons if you like, e.g. me and each of the other three; each brother with another, etc. I’m happy to help you in any way I can!

  2. I submitted the data for myself and my sister. One thing I didn’t see a way to indicate on the form was where the data was originally collected. Although the data I submitted was from Gedmatch mine came from Ancestry while hers came from FTDNA. Does that matter? We seem to be on the low end of half matching DNA but I’m sure we are full siblings.

  3. Hi, and thanks for doing this study! I have 2 sisters, and sent you data from compares among the 3 of us. I only realized I’d made an error in my first compare after it was too late to correct it. Inadvertently, I used the value of the longest segment for the FIR total instead of the correct number, which is
    809.5 cM. I did include my email address, so it should be easy to locate and either toss or correct as you prefer. Sorry for the error. I should have suspected an error when the number I got was less than 100 cMs!

  4. Are you interested in half sibling data?
    In case you are: my paternal half brother and I share 1585.6 HIR and NO FIR…

  5. Submitted for myself & my full sister. Through Gedmatch, both kits FTDNA. Our cM numbers are not as high as your examples. Attempting to narrow down matching chromosomes between ourselves & (Paternal surname) known cousins has been difficult.

  6. My grandmother had 5 kids by at least 4 different men. As a result her grandkids are half cousins since we each have a different grandfather. 3 of us have tested and downloaded to GEDmatch if that will help your research.

  7. Submitted for myself & my brother, along with my 4 kids. Their paternal grandparents are related to each other, somewhere between pedigree collapse and endogamy, which might impact their shared DNA – feel free to contact me for more info.

  8. “Subtract the X cM amounts that have complete in the final column from the FIRs (only two sisters will have these).”

    Two brothers could also have these?
    As in this real example:

    JB / Stu X 95686854 116986871 19.09 1778 Complete
    JB / Stu X 1470937 22056132 30.69 2312 Complete

  9. I don’t know if the form was meant to be completable, but I couldn’t, so here goes my statistics:
    Sister (myself) and full brother, both from 23andme:
    Half identical (HIRS) minus X is 2503 cM
    Fully identical (FIRS) is 698.

    Sister (myself) and a different full brother via GEDmatch, mine from 23andme, his from Ancestry:
    Half identical (HIRS) 2666 cM
    Fully identical (FIRS) is 874.

    • Were you trying to complete it on a smartphone? IF not what device? So far Several nundred people have completed it successfully. It is just a google form.

  10. My brother and I are supposed to be full brother/sister. We both tested at Ancestry about a year apart. We match there at 2344 cM. At GEDmatch, our HIRs are 2413.9 cM, while our FIRs are 675.3 for a total of 3089.2 (which I already posted in the survey). We both have matches on all sides of our family. We did have a paternal grandfather not expected (father was adopted; adoptive father was supposed to be father. Long story and numerous Sicilian matches in a very non-Sicilian family later…). My mother is the only daughter of her parents; father was raised as an only child four hours from his birth mom. So, I dunno.

    • Cheri, your data looks like full siblings to me. 3/4 is fairly rare.

      Did your father have a full brother who could be the father of one of you? from what you said that sounds unlikely so yes you are full sibs. 3/4 is pretty unusual.

  11. Nope! He has a biological half sister four states away. That’s it. The low numbers have always interested me, but a range is there for a reason, I guess. Back to seeing if a grandchild of the likeliest candidate for my biological paternal grandfather will test. Four brothers (all of whom are long gone, as is my dad), along with most or all of the children of the brothers. I have wryly thanked my brother for testing for fun three years ago. Pandora’s box opened, genetic genealogy chops still being acquired.

  12. OK, I realized I messed up in some of my submitted calculations. (Two sisters from one family, and four sisters/one brother from another family. I forgot to subtract the complete X CM from the total Full CM for the some of the sisters.)

    Can you just delete everything you’ve received from my email address and let me start over? Sorry for the hassle!

    Actually, I can send you the link to a spreadsheet with this information, let me know if that would be better.

  13. I submitted numbers for the comparison with each of my two sisters and the comparison between the two of them. My middle sister and I are on the low end (although still within statistical norms) and I always tease my mother that it is a good thing she had a third child or she might have some explaining to do.

  14. I wrote you a few weeks ago about possible 3/4 siblings….I may have not completed properly, or,…cannot find it on here lol

    Okay I am trying to help this friends family. At their mother’s death bed, she revealed the youngest was not their father’s bio dad. She refused to reveal the true father.
    These ladies all took ancestry DNA tests. a few have passed away now. However, here is the data I see (I wrote them, and was told unfortunately they do not provide a full analysis of their matches)
    first half sister
    1904 total 47 segments largest seg 124cm
    second half sister
    1764 total 52 segments largest seg 105cm
    third half sister
    1762 total 45 segments largest 123cm

    I realize this can be difficult to analysis from this, but, I am concerned on the number of segments…….Is that something that should concern me?

    I appreciate any advise or help, Cheryl

  15. I just submitted an error. My aunt had a child with her husband and with her husband’s nephew (they were about the same age). I submitted the numbers for these 2 children. I don’t know what their relationship to each other would be called because they would be more than half brothers but less than 3/4 brothers. Write me at the email address below and I will give you their numbers.

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