I am managing the DNA accounts of my adopted brother. He has been tested directly with Ancestry and 23andMe. In 2017 FTDNA was allowing users to upload their raw dna data for a fee, and they would be given their matches same as if they had submitted a direct sample. (Or so I've been told)

I have found both his birth parents (unmarried) and the birth father has been tested at 23andme to confirm paternity. He passed.

So. Apparently my adopted brother has a half sibling. A brother, who would share the same father, different mothers.

This half sibling was registered with FTDNA several years before my brother. He should have been his highest match, and yet he doesn't show up as a DNA match at all on ftdna. They should share 700 cm roughly, if I am not wrong. Half brother insists they don't match because he has a "mutation" of his y haplogroup.

My understanding is that should not affect them matching as half siblings? It is highly possible they share no DNA as the relationship between his parents was rocky at best.

Can 2 half siblings not show up as matches at all? And would the mutation in his brother's haplogroup cause this?


3 Answers 3


Half siblings, according to the Shared cM Project, should share 1317 – 2312 cM.

If they shared only 700 cM, they'd have about a 70% chance of being one of the following relationships (source):

  • Great-Grandparent
  • Great-Aunt/Uncle
  • Half Aunt/Uncle
  • 1C
  • Half Niece/Nephew
  • Great-Niece/Nephew
  • Great-Grandchild;

and about a 30% chance of one the following relationships:

  • Great-Great-Aunt/Uncle
  • Half Great-Aunt/Uncle
  • Half 1C
  • 1C1R
  • Half Great-Niece/Nephew
  • Great-Great-Niece/Nephew.

You've got three options, I think, for why your adopted brother and the half brother aren't showing as related:

  1. They're not actually half-siblings (or related at all) and one of them has the wrong man as their bio-father;
  2. The half-brother isn't allowing sharing so that he doesn't show up in Family Finder results;
  3. The half-brother only paid for a Y-DNA test and does not have Family Finder. I believe that these are completely separate tests with FTDNA and you can get a Y-DNA test without getting the autosomal (Family Finder) test done. HOWEVER, I do not know if this was always the case.

Currently, with FamilyTreeDNA, you can upload DNA test results from 23andme, AncestryDNA, and MyHeritage and get the matches for free - it costs an additional $19 to get the Chromosome Browser, myOrigins, and ancientOrigins features (source).

If your adopted brother and his bio-father tested at 23andme, they should get a ball-park Y-haplogroup. You should be able to compare that to what the half-brother got from FTDNA. HOWEVER, this will only show if they're related somewhere along the direct paternal line - not paternity.

So, in short, no: Half-brothers would 100% show up on an autosomal DNA test. There is absolutely no way they wouldn't.


The (presumed) half sibling can't match your adopted brother because he hasn't taken the same kind of test.

This has been mentioned in other responses but I thought it should be stated up front and clearly as the most likely possibility.

You need to know that there are multiple kinds of consumer DNA tests. The kind you are talking about in your question is now the most well known and widely used DNA test. This is called an autosomal SNP test (but which FTDNA brands as "Family Finder"). It tests all non-sex chromosomes (and sometimes also the sex chromosomes) at on the order of half a million places. In this kind of test one often talks about the "cM" size of a match.

Another kind of test is the Y-STR test (which FTNDA calls a Y-DNA test). This tests only the male Y sex chromosome, and tests it only at 12 to 111 locations. The concept of cM size of a match does not apply to this kind of test.

The Y-STR test was the first to be introduced and can only test for a males's male-only line of descent. FTDNA was a leader in this kind of test. Years later the autosomal kind of test was introduced. For several years after that, FTDNA did not offer that kind, until in 2010 it introduced its "Family Finder" test. Y-STR is still offered but is not as widely used now as autosomal tests.

It seems probable that the half-sibling's test was of the Y-STR ("Y-DNA") kind. This is suggested by:

  1. The half sibling talks about his "Y haplogroup". This is a central concern to those taking Y-STR tests, but to autosomal test users is a minor detail (and not even available in some versions of autosomal tests).
  2. The half sibling attributes the lack of a match to "a mutation" of his Y haplogroup. In Y-STR tests, a single mutation (difference) is a significant difference among the 12 to 111 test locations (although not a total mismatch, since it one will happen every few male generations). In autosomal tests, a single mutation is noise, and not of much concern at all since there are hundreds of thousands of locations tested and some mutations or misreads are expected.

Y-STR and autosomal tests are not comparable. If indeed the half-sibling's test was of the Y-STR kind, that would explain the lack of any match.


Your adopted brother has tested with both AncestryDNA and with 23andMe and has transferred his raw data to FTDNA. The ISOGG Wiki can tell us which tests each company provides and which tests which can be transferred.

In the AncestryDNA article, under Transfers, the Wiki says:

AncestryDNA results can be uploaded free-of-charge to the third party website GedMatch where many additional tools are available for advanced analysis. For more information on GedMatch and other third-party tools see the Wiki article on autosomal DNA tools.

AncestryDNA results can also be transferred to Family Tree DNA's Family Finder database where you can search for additional matches, join projects and use additional tools to compare results. The transfer is free but a small fee is required to unlock additional features. Details can be found at www.familytreedna.com/AutosomalTransfer. Note that AncestryDNA transfers after the introduction of the new chip in May 2016 will receive a reduced number of matches and will not receive matches in the speculative range. See the blog post from Louise Coakley Should you upgrade your Family Finder transfer?.

For 23andMe's test history, the wiki says:

Chip versions

A range of different chips have been used for the 23andMe test since the introduction of the service:

v1: November 2007
v2: September 2008, ~555K SNPs
v3: November 2010, >900K SNPs
v4: November 2013, ~570K SNPs
v5 August 2017, ~640K SNPs (Illumina Global Screening Array)

Tests done using the version 3 and version 4 chips can be transferred free of charge to the Family Tree DNA Family Finder database. For details see the FTDNA page on the autosomal DNA transfer program.

While you can get some Y-DNA data from an AncestryDNA transfer (see the section Extracting Y-DNA data in the AncestryDNA article), including a haplogroup assignment, you don't get the same amount of data as someone who takes a full-fledged Y-DNA test like the ones that FamilyTreeDNA offers. (See their article Y-DNA Basics.)

Could it be that your brother's half-brother took a Y-DNA test at FTDNA? If that's the case, they wouldn't match because the half-brother's results are not in the FamilyFinder Database (which has the results from autosomal DNA tests).

If my guess is right, and:

  1. They are in two different databases on FTDNA's site, and
  2. FTDNA doesn't match the results of Y-DNA tests to the autosomal database for you

-- then the only way they could match at FTDNA would be if they both participated in the same surname project. Joining projects is something you do voluntarily -- FTDNA doesn't submit your data to those projects without asking you.

So this is my long-winded way of saying that without knowing exactly which tests both of them took, we can't answer your question with any certainty. The previous answer assumes that both took autosomal DNA tests, but that might not be the case.

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