Why don’t we share more DNA with a Sibling?

When I first started doing DNA testing and I saw that full siblings shared only about 2800 cM of their DNA, I was surprised. I expected more. Then I thought about it. We have 23 pairs of chromosomes but the testing process cannot separate the two parts of each pair so our matches are seen as if we had only 23 unpaired chromosomes.

The matching segments are listed and totaled as if there was just one side of each pair even though it is using the data from both sides for the comparison; that is why they are called half identical regions or HIRs. Click here for my blog post with a deeper discussion of why doing comparisons that way can create false matching segments.

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

Siblings are usually listed as sharing about 50% of their DNA but that is only the half identical part that most companies measure. How many places did they get the same DNA from both parents on both pairs of a chromosome? In other words how many fully identical regions (FIRs) do siblings share? In my experience, they share about 800 cM or so that are fully identical. Thus the total of the FIRs and HIRs is about 3500-3700 cM; the same amount expected of a parent match. That intuitively makes much more sense to me.

So what would be the logical basis for this? Each parent gives each child half of their DNA but not all of it will be the same half. Roughly half of what each parent gives me will be the same as what my brother got so about 2 times 1800 for 3600; yet most full siblings are shown as sharing only about 2600-2800 cM of half identical segments. The rest of that 3600 is found in the fully identical segments.

A recent query I got was “I only share 2553 cMs with my sibling are we full or half? By the online calculator at DNApainter they are full siblings sharing a low amount, but another possibility is the father of one sibling was the brother of the other. That is called ¾ sibings. To tell the difference total up the FIRs and the HIRs. If they total less than 3000 rather than the 3600 for full siblings then they are most likely three quarters siblings or some similar relationship.

23andme totals both the FIRs and the HIRs unlike any other testing company, as shown above, however those totals include the X which throws it off a bit for my calculations.

Wherever you tested, you can get the FIRs and the HIRs by comparing the two kits at GEDmatch.com.

Here’s how. Presuming you have both uploaded to GEDmatch (and hopefully both opted in to helping law enforcement identify violent criminals and victims – click here for my post on that), start with a One-to-One comparison of the two kits.

The default comparison after you click submit shows the HIRs (half identical regions) and totals them at the bottom as shown below. The total for my brother and I is 2705.2. Since this does not include the X chromsome it is slightly lower than the total at 23andme.

Now click the back button on your browser to return to the form with your two kit numbers and check the box that says “Show only Full-Match (FIR) segments. “ then submit again.

For FIRs my brother and I have 816.1 cM. Adding 816.1 to 2701.2 gets us 3507.3. Now that’s more like it!

Even though this logic seems clear to me, I went ahead and looked at a few more examples of full siblings in my data.

So the answer is to look at the FIRs if you suspect you are ¾ siblings or some other relationship. Click here for an example of how an unusual relationship was determined using the FIRs.

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25 thoughts on “Why don’t we share more DNA with a Sibling?

Click here to add your thoughts at the end of the comments
  1. I have a case where two men supposedly are brothers and following your steps above they match HIR 2518.5 and FIR 537.1.

    Logic, location, etc. points to the fathers being a father/son combo. One brother shares DNA matches of the son’s mother side of the family and other doesn’t.

    Half brothers, 3/4 siblings, ??

  2. I meant to say I have an example with 2 siblings sharing 3170.1cm (2521.5 and 648.6 fir). Do you think this is an example of what you spoke about in your last post?
    Thanks
    Bill Hickey

  3. This is one of your best posts ever. I’ve puzzled over the rather wildly differing DNA matches for half-siblings, from the different testing sites that they used. These were direct tests, not uploads. Your clear and concise explanation was extremely useful. Now I’ll go back and run the GEDmatch utilities as you suggest. Thank you so much!

    • Thanks distant cousin,
      I rewrote it many times trying to make the concepts clear so this is lovely to hear. But did you mean full siblings in your comment? That is where the FIRs come in.

  4. What about when siblings seemingly share too much DNA? Sib 1 shares 2659 HIR with 894 FIR but Sib 2 shares 3002 HIR and a whopping 1145 FIR. According to 23andme that’s a 10% difference between sibs.
    I’m sure that endogamy must be to blame somewhere but wouldn’t that mean that both sibs should share excess amounts with me, instead of one less and one more?

    • Oceania-
      Endogamy does not usually play much of a role in close family matches unless the parents are related. While there can be a wide variation due to the randomness of DNAthis seems extreme.
      23andme includes the X and segments down to 5cM. Try totalling just the segments 7cM or larger excluding X and see what you get.
      Better yet if you all upload to Gedmatch and do the comparison there plus check if your parents are related. And please be a good citizen and opt in to helping law enforcement identify victims and violent criminals

      • Thanks Kitty. I’ll give that a try. According to Gedmatch my parents aren’t related. I am opted in for law enforcement too.

        • Thanks for opting in Oceania – now do one to one comparisons there with each sibling as per this article and report back with the new numbers!

  5. Thank you for your interesting post. I tried the GEDmatch technique and came up with 3221.5cM total (2454.6cM HIR + 766.9cM FIR). Could this be full siblings or does it clearly indicate 3/4?

  6. Interesting post. My GEDmatch results are 2442.6 and 550.4, total 2993.0 cM, with basically no possibility of anything but being a full sibling.

    • Interesting. There is always a variation in DNA results. Your numbers would tend to be a 3/4 but obviously can also be a full sibling.

      Do both of you have matches on your father’s mother’s side? If only one of you does, then there is the possibility of the other way a 3/4 sibling occurs: a father and a son having children with the same woman.

    • And the answer is that there were far fewer SNPs used for Fred’s sibling comparison because they were tested on different chips!

      At the bottom you can see something like
      405732 SNPs used for this comparison.  

      You want to have some 800K plus to compare, here is mine with my brother
      876580 SNPs used for this comparison.  

      • For my results above, I am using 600266 SNPs for this comparison. Does that change my situation? I don’t think so as they still have different matches-one matches son’s mother relations and other doesn’t. Just trying to be sure….

  7. Hi Kitty,
    I was under the impression that the HIR amounts identified in the one-to-one comparison on GEDMatch are, in fact, the SUM of the HIRs and the FIRs. I recall learning this in the Genealogy Tips and Tricks Facebook group. What am I missing? Thanks!

    • Chris –
      Either you misunderstood or whoever told you that mispoke. That is just not the case.
      Look at the screen shots for me and my brother above and see that the it says in the normal one to one “Total Half-Match segments (HIR) = “. Also notice that the number just a tiny bit smaller than 23andme totals …

  8. Hello again,
    I’m curious about something else that ties into this (please bear with me, this is still sinking in! 🙂 ). I’m reading/inferring that you mean the sum of the HIR and FIR should be in the 3500-3700 cM range. Per the ISOGG Wiki, full siblings are expected to share roughly 25% full identical regions, 50% half identical regions, and 25% no shared DNA. If you are adding HIR and FIR and getting the total amount, what happened to the 25% that isn’t shared?

    • The problem comes in the way percentages are used. The norm is to use precentages in reference to just half your DNA since the HIRs are all most DNA testing companies show.
      In other words, the statement you are quoting is confusing. I will see if I can get it corrected.
      The numbers you are quoting are based on half your DNA, if you look at a diagram. About 25% will be FIRs and and another 25% will be HIRs thus 50%. See this post for the 25% explained: http://www.andreasancestors.com/2011/12/questions-from-my-inbox-siblings.html

      • In other words this statement later in the article explains it better, “Thus, for example, the shared cM for full-siblings will on average be 75% of the total length of the genome, of which on average 50 percentage points are half-identical and 25 percentage points are fully identical.”

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