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I had a swab test performed to establish sibling-ship through DNA Diagnostic Center (DDC) wanting to confirm what I thought was certain paternity with myself, male sibling and and mother (father deceased). The results came back with a 10% probability that we shared the same father (at an allele level). Astonished by the results, I had a 2nd test (blood samples) done with same persons, different collection center, but that was analyzed by the same company (DDC). Conclusions were the same.

Assuming that I was not the child of the man that I was my father (because of the physical appearance of my brother to father), I set out the next few years looking for familial connections through Ancestry.com. Surprisingly, connections kept being made with the family of "the father"! So I assumed, okay the gentleman that I thought was my father is NOT, but someone in his family is.

Getting no closer to an answer and just out of curiosity, I submitted the DNA of my brother to Ancestry.com to see what connections would be shared. We definitely shared the same maternal family members, but we also shared all of the same what had to be paternal connections and at the same levels/relationships. Even greater than this, the results yielded we share 2,509 cM, longest segment is 187 cM, with 100% Sibling Relationship!

So confused, I reached out to both DDC and Ancestry to confirm their results when presented with the conflicting information. True to thought, they both independently stood by their results, without being able to speak to the processing of another company's testing method.

How is this possible?

Then the thought occurred to me, what would the cM range be for persons sharing the same mother and the fathers are brothers?

"The Father" has a brother (our Uncle?), and to my knowledge there are no other male sons from my grandparent's union (or outside of the union).

  • Note that DDC's result is not particularly strong: they are saying that 10% of the time, their test would indicate "not same father" when the two submitters actually did have the same father. – RobertShaw Sep 10 at 22:29
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The relationship that you are suspecting is one of three-quarter siblings and a useful blog to read about that is What is a Three Quarter Sibling?:

A 3/4 sibling is the child of one of your parents and the sibling of your other parent.

The same blog says:

the average amount of shared DNA between the three types of siblings are as follows (numbers are approximate):

  • Full siblings 2550 cMs
  • Three quarter siblings 2125 cMs
  • Half-siblings 1700 cMs

but does not specify the range that you are asking for. Instead it offers this warning:

Can you tell if you are a three quarter sibling just based on shared DNA?

The short answer is no. If you suspect that you and your sibling are three quarter siblings, meaning that one of you might possibly be the child of one parent and the sibling of the other parent, you cannot prove or disprove this theory simply based on your shared DNA.

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