Blog 2022 05 22 Why don’t your DNA matches also match your sibling

I was wondering what to write when I received an interesting call from a former client who is also a distant cousin.  When he was looking for a professional genealogist in Georgia he recognized my name as a DNA match, since his mystery ancestor married the sister of my maternal 2nd great-grandfather.  I know, some of you are wondering why I did not say, grandmother?  John Steven Akers was my mother’s father’s father’s father and Mahalia Holland was his wife.  Her sister married my client’s ancestor.

Anyway, the purpose of his call concerns a common misconception.  Just because you and a distant cousin know how you are related and may match many of the same people, why do you two not match?  This is why we should try and test several of our siblings.

Several years back I used a combination of mitochondrial DNA (mtDNA) and autosomal DNA (atDNA) to solve a brick wall.  My father’s mother’s mother was an unknown.  We knew her name and date of birth in what was then Telfair County, Georgia but is now Dodge County.  However, there was nothing that linked this Mary Martin to a specific family.

Through the use of standard genealogical research, I deduced that the most likely candidate was the granddaughter of a Keziah Davis Martin.  While there were other candidates, Keziah’s granddaughter was the most likely candidate based on the 1870 Census entry [i] and Keziah’s estate six years later. [ii]

Then, I researched other descendants of Keziah through her daughters and their daughters until I found some who are living and were willing to test.  I also had two direct female descendants of Mary Martin Livingston.  Since Mary is on my father’s side, I and my siblings were not candidates for taking the mtDNA test BUT…  We are for taking an atDNA test which my sister and younger brother have taken.  All of my other test subjects were mtDNA candidates except one whose father would have been a prime candidate if he was still alive but she was an atDNA candidate.

Just to remind you, all of you, men and women, carry ONLY your mother’s mtDNA as we do not inherit any from our fathers.

Let me show you

We are right at that five to six generations mark where atDNA falls off.  See the chart below.

The results of the mtDNA showed they all had exactly the same mtDNA which was what I expected but that does not answer the question.  Mitochondrial rarely changes for up to 52 generations which means sometime in the last 1,000 years we shared a common mother.  That is where the atDNA comes into play.

While we did not all match everyone, we all matched someone or several others.  As you can see below, the results showed that my Mary Martin was indeed the granddaughter of Keziah and by way of deduction, the daughter of Keziah’s youngest, unmarried daughter, Mary Martin who already had another son with no known father.

While my Mary’s father is still a mystery, the first half of a brick wall is solved.  This is a long way of explaining the question that my distant cousin and a former client asked.  You can see below that the one person who was a known descendant of Keziah but was not an mtDNA candidate only barely matched me and neither of my siblings or other cousins.

[i] 1870 U.S. Census, Telfair County, Georgia, population schedule, PO Temperance, p. 60 (penned), dwelling and family 385, Kessiah Mirtin (sic); ( : accessed 14 March 2021), citing FHL microfilm: 545675.

[ii] Dodge County, Georgia, Will Book 1, p. 7-8, Kiziah Martin (1876), Office of the Probate Court Clerk, Eastman, Dodge County, Georgia.