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My sister and I have a perplexing situation. She believed a family friend to be a half brother, so they got a test at Biogene DNA. It states:

Based on the results of the analysis obtained from the DNA loci listed, the probability of Half-Siblingship is 94.3110% (in other words, 16.6 times more likely to be Half Siblings than Not Related). This probability is calculated by comparing to an untested, unrelated, random individual of the Other population (assumes prior probability equals 0.50).

My understanding is that this considered conclusive.

However, my mother, my sister, the friend and myself all did MyHeritage DNA tests and in the DNA comparison tools my sister and mother come up as matches. In fact my sister comes up as a full-sibling not a half-sibling. So this means either my sister and the friend are not half-siblings or my biological father is the same as the friend.

I have also downloaded the raw data from MyHeritage and uploaded it to GEDmatch. I'm able to get the expected matches for my mother and my sister. If I use the default settings for the one-to-one match it finds no matching dna for the friend and me or the friend and my sister. If I reduce the Minimum segment cM size to 1, I do get some matching with the friend.

Can anybody help me figure out if A) the Biogene test is wrong, or if B) I'm not using GEDmatch or MyHeritage correctly?

  • 1
    dnapainter.com/tools/sharedcmv4 will let you understand the range of probabilities for each match you've found. Matches under 7cM are essentially meaningless. Biogene doesn't do the same sort of tests as MyHeritage et al – ColeValleyGirl Jun 13 at 5:27
  • Yes. I uploaded MyHeritage download into GEDmatch. For my sister and me it shows: Largest segment = 109.4 cM Total Half-Match segments (HIR) = 2568.3 cM (71.613 Pct) Estimated number of generations to MRCA = 1.2 56 shared segments found for this comparison. 624621 SNPs used for this comparison. 75.387 Pct SNPs are full identical For my sister and Friend it shows: No shared DNA segments found 627098 SNPs used for this comparison. The Biogene report states: (as shown above in the post) – Eric Gauvin Jun 13 at 11:04
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The Biogene result I would not characterize as "conclusive". By their own analysis, it is wrong about 6% of the time. If they give the same result for fifty test submittals, they will be wrong about there being a relationship for three of those test takers. That's definitely more than a shadow of a doubt.

(Note also that the statistic Biogene gives is really for whether there is any relationship at all, within several generations, not that the relationship is specifically a half-sibling relationship.)

If you didn't make an error in using the One-to-One Autosomal DNA Comparison on GEDmatch the results should be reliable. Typical places for errors are misidentifying a kit when uploading it, using X-DNA Comparison instead of Autosomal, entering a wrong kit number (you can verify the kit name on the results page), or using non-default settings on the comparison. (It's fine to use the "Position Only" option, though, for a more compact results page.)

So, if you correctly compared your sister against the friend and the One-to-One Autosomal showed "No shared DNA segments found" then it means they are not half-siblings nor any other close relation, and thus the Biogene test is misleading since it is contradicted and presumably their result is due to the 6% inaccuracy of that test.

As ColeValleyGirl pointed out, you can use the DNApainter utility to get an estimate of the probabilities for possible sets of relationships. For the 2684 total cM of your match with your sister, it indicates that it is 99.7% probable that you and your sister are full siblings.

  • Thanks. We three submitted new tests with ancestryDNA. Still waiting for the results. As long as the user account preference settings are correct, I guess it should show the matches. Would there be a benefit in downloading from ancestry and uploading to GEDmatch? – Eric Gauvin Jul 20 at 15:10
  • @EricGauvin, I'm not familiar with the tools on AncestryDNA, but I'm sure they will list for you close relatives, and if the friend does not appear in that list for your sister, then they are not half-siblings. GEDmatch does have more extensive tools than AncestryDNA, but that may not be very relevant to you if all you are interested in is whether friend and sister are related. ...[continued] – RobertShaw Jul 21 at 18:58
  • However even then, if you would like to see an explicit result if Ancestry does not list the friend on the matches list, then you might still want to upload to GEDmatch so that you can run an explicit comparison and see it say explicitly that there are no autosomal shared segments (and thus likely not even a relation as close as third or forth cousins). This should be completely redundant to Ancestry's (implicit) report, but maybe you'd feel better to have a second software program do a compare, and to see it explicitly reported. – RobertShaw Jul 21 at 19:06
  • Thanks for the info. In combing through all my info, I revisited the ethnicity estimate from myHeritage and I am struck by how sister/friend clearly resemble each other, while me/mother clearly resemble each other. I am aware that the ethnicity estimate is not very accurate. However, it is very strange how similar sister/friend are. It will be interesting to see how this all turns out. – Eric Gauvin Jul 22 at 23:25
  • Still waiting for the ancestry results, but to rule out errors uploading the myHeritage kits to GEDmatch, I did it again with the kit for the friend/sister. I double checked all my settings and I still get "No shared DNA segments found". A question I have for you is why GEDmatch would say No shared DNA segments found, while the Biogene test says it's 94% correct. Seems like the Biogene test was completely wrong somehow, so I'm wondering how that could happen. At this point I wouldn't expect that Ancestry will be different. That seems unlikely. – Eric Gauvin Jul 23 at 10:49

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