- the grandson (referred to as A) of his mother’s sister
- the grandson (referred to as B) of his mother’s mother’s brother
The testee obviously will have shared DNA matches with both A and B. The testee, A, and B share a set of great-grandparents (Louis and Marie). The testee and A share a set of grandparents (William and Mary). Mary is the daughter of Louis and Marie.
The testee and A should have more shared matches that the testee does with B. A is more closely related. On paper:
- A and the testee should have 1/2 of the same genealogical ancestry–they share two out of four grandparents, four out of eight great-grandparents, etc.–because their mothers were siblings
- B and the testee should have 1/4 of the same genealogical ancestry. They share two out of eight great-grandparents, four out of sixteen great-grandparents, etc.
The paper percentages are a guideline of shared DNA and proportions of shared matches because:
- the amount of DNA you inherit from a great-grandparent may not be exactly one-eighth of your DNA–it’s been split and passed a few times by the time it gets to you.
- shared matches at AncestryDNA depend upon the size of your family, how many relatives have tested, and how much DNA the testee got from a specific ancestor
So just because A and the testee have 1/2 of the same paper pedigree and that the testee and B have 1/4 of the same paper pedigree does not mean that the number of shared matches B has with the testee will be half of what the testee has with A.
But when the shared matches were reviewed there was a problem.
The shared matches the testee had with A and the shared matches the testee had with B were all the same or could be traced to a relative of Louis and Marie. The testee had no shared matches with A that were not also a match with B or who could not be shown to be a relative of Louis and Marie.
Louis and Marie were ethnic French-Canadians born in upstate New York. William was born in Chicago.
But shouldn’t the testee and A have shared matches with each other that are not shared matches with B?