Yes, DNA can usually tell the difference

It’s one of the most common questions people have about DNA testing. Just this past week two different people wrote in to The Legal Genealogist with some variation on the theme.

Can a basic genealogical DNA test tell the difference between half-siblings and full siblings?

Most of the time — as in one of the questions this week — the questioner’s parents have passed on but brothers and/or sisters remains, and there are still niggling doubts about whether they all share the same set of parents.

Could they be half-siblings — sharing only one parent — instead of full siblings, sharing both mother and father? And can a less-than-$100-buy-the-kit-and-wait-for-the-results DNA test really tell the difference?

Half or full

The answer, of course, is usually yes, it can tell the difference, as long as the test results are combined with all the facts and properly understood.

Now… let’s be clear here: we’re talking about the autosomal DNA test here, and not YDNA testing or mitochondrial DNA (mtDNA) testing. Autosomal DNA is the kind we all inherit from both of our parents and that’s used, in genealogy, primarily to find cousins to work with collaboratively.1 YDNA is the kind only men have that’s inherited only from their fathers and so is used to trace the direct paternal line.2 And mtDNA is the kind we all have but that’s inherited only from our mothers and so is used to trace the direct maternal line.3

Here’s the deal: with autosomal DNA, we all inherit 50% of our DNA from our fathers and 50% from our mothers. Because of a random jumbling of the DNA that occurs every single time a child is produced — a process called recombination4 — brothers and sisters won’t share the same exact DNA. One sister might have gotten a bit more from the maternal grandmother’s line than the others; one brother might have gotten a bit more from the paternal grandmother’s line than the others.

But on average full siblings will share about 50% of their autosomal DNA.

Now… since half-siblings are only inheriting autosomal DNA from one shared parent and not two, the amount of shared DNA they’ll receive is cut in half: to 25%.

And that difference usually shows up fairly dramatically in the standard DNA tests we take for genealogy when we look at the number of centiMorgans (cM for short) that we share with a sibling. That’s a unit of measurement in DNA that helps determine genetic distance — in other words, just how closely related two people are.5

The higher the number of cM shared with a sibling, the closer the genetic distance — or, in plain language, the more likely it is that you’re full siblings and not half.

Now the charts we used to use in genealogy tell us we should expect 2550 cM in common with a full sibling and 1700 cM in common with a half sibling.6 But more recent research has shown us there’s a range we can expect to see.

The Shared CentiMorgan Project, pioneered by Blaine Bettinger, tells us that the average for full siblings is 2629 cM, and the range is reported from a low of 2209 to a high of 3384. It also tells us that anything above about 2900 is a real outlier and not likely to be a correct interpretation of the data.

For half siblings, the average is 1783 cM, the range from 1317 to 2312 but anything above about 2150 is a real outlier and not likely to be a correct interpretation of the data.

That’s shown in the histograms on the Shared CentiMorgan Project: the charts you can see there that show you where most of the results show up. The results out at the ends are the outliers.

You can see this represented in terms of the statistical probabilities — the odds that two people are full or half siblings — in the DNAPainter tool called The Shared cM Project 3.0 tool v4. Plug in the total cM in common in the box at the top, and the tool will give you the odds: at 1700 cM, the odds are 100% that the two people are half siblings; at 2620 cM, the odds are 100% in favor of full siblings.

My own half sibling, for example, has a range of 1622-1975 cM with his half-brothers and half-sisters. The percentages tool gives us 100% odds for half sibling on that entire range. And my range with my full siblings is from 2566-2858 cM, with the odds for full sibling from 99.71% to 100%.

Where it gets dicey is starting around the 2000 cM level — that’s where there is some small amount of evidence that a match could be either full or half sibling. At 2250 cM, the odds are pretty close to 50-50 either way. And at that point, of course, you may very well need more than an over-the-counter DNA test to answer the question.

The results can also be skewed by issues like one parent being an identical twin (say two identical twin brothers each had a child by the same woman) or by incest.

Bottom line: yes, usually, the test data tells you whether you’re full or half siblings, as long as you look at all the data, and understand the test results fully.

Oh… and remember… there are real ethical issues involved in DNA testing, and we need to be prepared for all of them before we ask anybody — especially a maybe-full-maybe-half sibling — to test.


SOURCES

  1. ISOGG Wiki (http://www.isogg.org/wiki), “Autosomal DNA,” rev. 15 Sep 2018.
  2. Ibid., “Y chromosome DNA tests,” rev. 4 Dec 2016.
  3. Ibid., “Mitochondrial DNA tests,” rev. 1 Oct 2018.
  4. Ibid., “Recombination,” rev. 2 Dec 2017.
  5. Ibid., “centiMorgan,” rev. 21 May 2017.
  6. See ibid., “Autosomal DNA statistics,” rev. 3 Sep 2018.
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