Please see 11/16/09 update about siblingship DNA tests here: "Even more about DNA half-siblingship tests"
In response to several questions over the last few weeks from some distraught donor conceived adults, I'm going to discuss DNA testing, specifically half-siblingship DNA tests, and how they can help donor conceived persons find paternal relatives.
In short, a siblingship DNA test is an autosomal (across the entire genome) test of typically 15 markers [see: Whatcha gonna do with all that junk, all that junk in your...DNA?? for more information about these markers] that two potential siblings do to determine HOW LIKELY it is that they are siblings. There is also half-siblingship tests that determine how likely it is that both individuals share one common parent. The latter is quite handy for donor conceived offspring to test whether or not they have found a sibling, especially if neither individual has a donor number.
Typically, at least one mother must be tested as well for this to be accurate. Since siblings only share 50% of their DNA, half-siblings only share about 25% of their DNA, which means that often many of the markers do not match. The mother can determine which allele at a certain marker the child inherited from her, and thus by process of elimination, which allele came from the father. If one mother is tested, this is usually enough information to determine if two individuals are related or not, but for a more conclusive result both mothers can be tested.
Siblingship DNA tests are inherently very different from paternity tests. Paternity tests are cut-and-dry. Either he's the father or he's not, with 99.999% accuracy (this has some exceptions, but not for today's discussion), because for each marker one of the child's MUST match one of his. Half-siblings are different, because for a given marker, child #1 could have alleles A and B and child #2 could have alleles C and D. While this looks like they couldn't be siblings, in reality they very well could be. The father, in fact, could have carried alleles A and C, and each child inherited a different allele.
Here's another example, which shows the importance of testing mom. Suppose child #1 carried alleles A and B, and child #2 carried alleles A and D. If child #1's mom was tested and they found that she carried alleles B and C, then these two children could be siblings. If both moms were tested and found that child #2's mom carried alleles D and E, then the possibility of the children being siblings is greatly increased (because it shows that allele A MUST have come from the father).
If mom carried alleles A and C though, then the A allele from child #1 must have come from mom, and therefore the two children do not share a match at that marker. It does not mean, though, that they're not siblings, since the father could have had alleles A and B, thus accounting for both children, even though it's not apparent on the DNA test.
This leads me to my next point. Siblingship DNA tests are not 100% for sure. For each marker, matches between the two children are analyzed with high-tech mathematical equations by professionals, using the frequency of the allele in the general population. That is, for each allele at a marker, millions of individuals in the world also carry that same allele, but some alleles are much more rare than others. If it is one of those rare alleles, the points assigned to that match are significantly higher than if the allele is a common on in the population. The more points that accumulate the higher the relationship index.
A relationship index that is below 1.0 means that these two individuals are not related at all - this is a very conclusive (but not 100%) answer that the two people are not siblings. A relationship index that is between 1.0 and 50 generally means that it is more likely that the two individuals are siblings, but that the test was inconclusive (this can especially be the case if neither or only 1 mother is tested). A relationship index between 50 and 100 means that it's not conclusive but that it's somewhat supported, and a relationship index above 100 is considered conclusive. Most half-siblingship tests (where at least 1 mom was tested) end up somewhere between 50 and 70, but I know of several donor conceived adults who found siblings that tested well into the 90s.
For older female offspring who may be unable to test one or both mothers, testing services at CaBRI are unable to determine siblingship status, because they look solely at the X chromosome, and in order to determine which X chromosome was inherited by the donor (and thus shared with a female sibling), mom needs to be tested to eliminate her X chromosome. However, all is not lost. Most DNA testing companies can do a half-siblingship DNA test with only one mom's DNA, and it's possible, but not recommended that they could do a test of only the two alleged siblings. However, the likelihood of getting anything above a somewhat supported relationship index is practically impossible, thus it would only give you an idea. It would tell you that there's "no way in hell" that you could be siblings, but it could not tell you with much accuracy that you are siblings.
For males who have found a potential male sibling, CaBRI is the easiest and cheapest solution (but it takes quite a few weeks to get results).
For offspring of opposite sexes a DNA testing company must be used to determine siblingship status.