Question

If a person shares 35.3% DNA (2501.2cM) with another, is the relationship sibling or half-sibling?

Answer 1

As others have said the cM (depending on how it was computed) points very strongly toward a full-sibling, however, full siblings and half-siblings are easily and instantly distinguishable not just based on the cM.

This is because roughly half one-third of matching segments between full siblings will be fully identical (shown in green on GEDmatch), which is the hallmark of a full sibling match, while roughly half two-thirds will be half-identical (yellow on GEDmatch). (thanks to Edison for correcting on the ratio of half-identical to fully-identical regions). Half-siblings will have only half-identical segments; any fullly-identical segments will either be tiny, isolated and coincidental, or a product of endogamy ("in-breeding", such as cousin marriages), but should still be dominated by half-identical regions.

The algorithms to report full siblings should take full matches into account and therefore be trustworthy barring outlying cases of incest or extreme in-breeding.

On the other hand, half-siblings matches are not, in isolation, distinguishable between aunt/uncle-niece/nephew or grandparent-grandchild matches. They would be expected to have the same cM range of half-identical regions. When classifying a match the algorithm will normally present all these possibilities, whereas in a sibling match it will definitively classify them as siblings.

Comments

If you can view the data at the service provider, Nathan has the best answer. I believe we're saying the same thing, but to restate: full siblings will be half-identical (HIR: half-identical region) on 50% of their DNA and also fully-identical (FIR) on an additional 25%. If we're looking at the "standard" total centiMorgan evaluation that most services use (GEDmatch, FTDNA, Ancestry, MyHeritage), which assumes (not quite accurately, but then cM calculation itself is only approximately accurate) the whole genome is 6,800cM, so that would equate to 1,700 HIR plus 850 FIR. As Nathan notes, half-siblings--assuming no immediate-generation pedigree collapse--should show no meaningful fully-identical regions at all. The only other relationship that will show significant FIR (well, other than identical twins, of course) would be double first cousins, and even then it would only amount to about 106cM, or around 1.6%.

GEDmatch provides this information, and you can at least view it as a graphical display (but you have to eyeball the values) at 23andMe in the Family Traits chromosome browser.

Back to the aggregate cM amounts, it's worth noting that 23andMe does things a little bit differently than everyone else. They compute the 22 pairs of autosomes as equalling 7,074.6cM instead of 6800cM. They also are the only company that adds the X chromosome into the total calculation. After including the single X, the male total cM at 23andMe is 7,256.8; with the two X chromosomes, the female total is 7,438.6cM. So unless you can view 23andMe results elsewhere, like GEDmatch, you can't get an apples-to-apples comparison with a test-taker from another company, and the 23andMe reported sharing will almost always be higher, at least for recent relationships.

Using the 6,800cM model, the theoretical average sharing for full-siblings would be 2,550cM; for half-siblings, 1,700cM.

Answer 2

2500 cM is right in the middle of the range for a full sibling.

Answer 3

Per 23AndMe's guidance, full siblings typically share from from 38%-61% with a full sibling and 17%-34% with a half-sibling. 35.3% is right in between, so could be either.

Answer 4

According to the Shared cM Tool there's a 99% chance it's a full sibling and .67% chance it's a half sibling.