My sister and I share 2,277 centimorgans. It seems low for full sibling but high for half sibling.

Any thoughts?


The Shared CM project tool says that 2277 shared CM is 59.76% likely to be a sibling (range 2209 to 3384) and 40.24% likely to be something else including half-sibling (range 1317-2312). So, more likely a full sibling than not, but the luck of the draw means you're at the low end of the sibling range and the high end of the half-sibling range.


2277 cM is, as ColeValleyGirl points out, definitely in the range for full sibling. The next question you want to ask is if it is in the range for anything else.

Full sibling: 2209-3384
Half sibling: 1317-2312
Aunt: 1349-2175

You can rule out aunt (this can happen in families where a teen gives birth to or fathers a child who is then passed off as a younger sibling). The cM range isn't close enough.

You and your sister, however, are in that small overlap between full and half sibling.

Where do you go next? If there are no family stories or hints that something might be off, then you can just assume you're full siblings and leave it there.

If you want to explore it further, you will need to test other family members.

Obviously, testing your parents is the best way to go, but it's not always possible.

If you have another sibling and you each test as full siblings to her/him, then you are good. This would show you and your sister are full siblings.

Also look for cousins or other close relatives, some on your mom's side and some on your dad's side. If both you and your sister match cousins on both sides as expected, this will also confirm that you are full siblings. Look also for close matches to people you don't recognize.

DNA can do seemingly random things. Sometimes segments split up in unusual ways (or don't split at all). We look to averages a lot, but not DNA split to lead to the creation of a child is average. There are hard and fast rules (for example, 1500 cM would rule out full sibling) but there is a lot of wriggle room here and there. It's a good tool but, in the end, it's only a tool.


On DNA painter, 2277 cM could be:

  • 59.76%: Sibling
  • 40.24%: Aunt / Uncle †, Niece / Nephew †, Grandparent, Half Sibling, Grandchild

† this relationship has a positive probability for 2277cM in thednageek's table of probabilities, but falls outside the bounds of the recorded cM range (99th percentile)

We can rule out the following: Grandparent (unlikely), grandchild (unlikely) and Niece (2277 cM is outside the range) and Aunt (2277 cM is outside the range).

This leaves us with the possibilities "Sibling" and "Half Sibling".

Both are within the range. The cM alone can't tell whether you are full siblings or half siblings. But even if the amount of shared cM overlaps with both ranges, there's still a way to find out whether your sister is a full sibling or not.

This is a comparison between me and my full sister:

And this is a comparison between my father and his half-sister:

For full siblings, about 25% of your segments are completely identical (purple on 23andMe, green on GEDmatch), 50% are half-identical (pink on 23andMe, yellow on GEDmatch) and the rest 25% are non-identical (gray on 23andMe, red on GEDmatch).

For half-siblings, about 50% of your segments are half-identical and the rest 50% are non-identical.

So here's the thing. Upload your test results to GEDmatch and see if you share any completely identical segments (green). If you do, you are definitely full siblings. If you don't, I'm sorry to say you are half-siblings.

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