Concepts – Why DNA Testing the Oldest Family Members is Critically Important

Recently, someone asked me to explain why testing the older, in fact, the oldest family members is so important. What they really wanted were talking points in order to explain to others, in just a few words, so that they could understand the reasoning without having to understand the details or the science.

Before I address that question, I want to talk briefly about how Y and mitochondrial DNA are different from autosomal DNA, because the answer to the “oldest ancestor” question is a bit different for those two types of tests versus autosomal DNA.

In the article, 4 Kinds of DNA for Genetic Genealogy, I explain the differences between Y and mitochondrial DNA testing, who can take each, and how they differ from autosomal DNA testing.

Y and Mitochondrial DNA

In the graphic below, you can see that the Y chromosome, represented by blue squares, is inherited only by males from direct patrilineal males in the male’s tree – meaning inherited from his father who inherited the Y chromosome from his father who inherited it from his father, on up the tree. Of course, along with the Y chromosome, generally, the males also inherited their surname.

Y and mito

Mitochondrial DNA, depicted as red circles, is inherited by both genders of children, but ONLY the females only pass it on. Mitochondrial DNA is inherited from your mother, who inherited it from her mother, who inherited it from her mother, on up the tree in the direct matrilineal path.

  • Neither Y or mitochondrial DNA is ever mixed with the DNA of the other parent, so it is never “lost” during inheritance. It is inherited completely and intact. This allows us to look back more reliably much further in time and obtain a direct, unobstructed, view of the history of the direct patrilineal or matrilineal line.
  • Changes between generations are caused by mutations, not by the DNA of the two parents being mixed together and by half being lost during inheritance.
  • This means that we test the oldest relevant ancestor in that line to be sure we have the “original” DNA and not results that have incurred a mutation, although generally, mutations are relatively easy to deal with for both Y and mitochondrial DNA since the balance of this type of DNA is still ancestral.

Testing the oldest generation is not quite as important in Y and mitochondrial DNA as it is for autosomal DNA, because most, if not all, of the Y and mitochondrial DNA will remain exactly the same between generations.  That is assuming, of course, that no unknown adoptions, known as Nonparental Events (NPEs) occurred between generations.

However, autosomal DNA is quite different. When utilizing autosomal DNA, every person inherits only half of their parents’ DNA, so half of their autosomal ancestral history is lost with the half of their parents’ DNA that they don’t inherit. For autosomal DNA, testing the oldest people in the family, and their siblings, is critically important.

Autosomal DNA

In the graphic below, you can see that the Y and mitochondrial DNA, still represented by a small blue chromosome and a red circle, respectively, is inherited from only one line.  The son received an entirely intact blue Y chromosome and both the son and daughter receive an entirely intact mitochondrial DNA circle.

Autosomal DNA, on the other hand, represented by the variously colored chromosomes assigned to the 8 great-grandparents on the top row, is inherited by the son and daughter, at the bottom, in an entirely different way.  The autosomal chromosomes inherited by the son and daughter have pieces of blue, yellow, green, pink, grey, tan, teal and red mixed in various proportions.

Autosomal path

In fact, you can see that in the grandfather’s generation, the paternal grandfather inherited a pink and green chromosome from his mother and a blue and yellow chromosome from his father, not to be confused with the smaller blue Y chromosome which is shown separately. The maternal grandmother inherited a grey and tan chromosome from her father and a teal and red chromosome from her mother, again not to be confused with the red mitochondrial circle.

In the next generation, the father inherited parts of the pink, green, blue and yellow DNA. The mother inherited parts of the grey, tan, teal and red DNA.

The answer to part of the question of why it’s so important to test older generations is answered with this graphic.

  • The children inherit even smaller portions of their ancestor’s autosomal DNA than their parents inherited. In fact, in every generation, the child inherits half of the DNA of each parent. That means that the other half of the parents’ autosomal DNA is not inherited by the child, so in each generation, you lose half of the autosomal DNA from the previous generation, meaning half of your ancestors’ DNA.
  • Each child inherits half of their parents’ DNA, but not the same half. So different children from the same parents will carry a different part of their parents’ autosomal DNA, meaning a different part of their ancestors’ DNA.

The best way to understand the actual real-life ramifications of inheriting only half of your parent’s DNA is by way of example.

I have tested at Family Tree DNA and so has my mother. All of my mother’s DNA and matches are directly relevant to my genealogy and ancestry, because I share all of my mother’s ancestors. However, since I only inherited half of her DNA, she will have many matches to cousins that I don’t have, because she carries twice as much of our ancestor’s DNA than I do.

Mother’s Matches My Matches in Common With Mother Matches Lost Due to Inheritance




As you can see, I only share 371 of the matches that mother has, which means that I lost 549 matches because I didn’t inherit those segments of ancestral DNA from mother. Therefore, mother matches many people that I don’t.

That’s exactly why it’s so critically important to test the oldest generation.

It’s also important to test siblings. For example, your grandparent’s siblings, your parent’s siblings and your own siblings if your parents aren’t living. These people all share all of your ancestors.

I test my cousin’s siblings as well, if they are willing, because each child inherits a different half of their parent’s DNA, which is your ancestor’s DNA, so they will have matches to different people.

How important is it to test siblings, really?

Let’s take a look at this 4 generation example of matching and see just how many matches we lose in four generations. We begin with my mother’s 920 matches, as shown above, but let’s add two more generations beyond me.


As you can see in the above example, the two grandchildren inherited a different combination of their parent’s DNA, given that Grandchild 1 has 895 matches in common with one of their parents and Grandchild 2 has 1046 matches in common the same parent. Those matches aren’t to entirely the same set of people either – because the two siblings inherited different DNA segments from their parent. The difference in the number of matches and the difference in the people that the siblings match in common with their parent illustrates the difference that inheriting different parental DNA segments makes relative to genealogy and DNA matching.

However, if you look at the matching number in common with their grandparent and great-grandparent, the differences become even greater and the losses between generations become cumulative. Just think how many matches are really lost, given that in our illustration we are only comparing to one of two parents, one of four grandparents and one of 8 great-grandparents.

The really important numbers are the Lost Matches, shown in red. These are the matches that WOULD BE LOST FOREVER IF THE OLDER GENERATION(S) HAD NOT TESTED.

Note that the lost matches are much higher numbers than the matches.


In summary, here are the talking points about why it’s critically important to test the oldest members of each generation, and every generation between you and them.

Autosomal DNA:

  1. Every person inherits only half of their parents’ DNA, meaning that half of your ancestors’ DNA is lost in each generation – the half you don’t receive.
  2. Siblings each inherit half of their parents’ DNA, but not the same half, so each child has some of their ancestor’s DNA that another child won’t have.
  3. The older generations of direct line relatives and their siblings will match people that you don’t, and their matches are as relevant to your genealogy as your own matches, because you share all of the same ancestors.
  4. Being able to see that you match someone who also matches a known ancestor or cousin shows you immediately which ancestral line the match shares with you.
  5. Your cousins, even though they will have ancestral lines that aren’t yours, still carry parts of your ancestors’ DNA that you don’t, so it’s important to test cousins and their siblings too.

Y and mitochondrial DNA:

  1. Testing older generations allows you to be sure that you’re dealing with DNA results that are closer to, or the same as, your ancestor, without the possibility of mutations introduced in subsequent generations.
  2. In many cases, your cousins, father, grandfather, etc. will carry Y or mitochondrial DNA that you don’t, but that descends directly from one of your ancestors. Your only opportunity to obtain that information is to test lineally appropriate cousins or family members. This is particularly relevant for males such as fathers, grandfathers, paternal aunts and uncles who don’t pass on their mitochondrial DNA.

I wrote about creating your DNA pedigree chart for Y and mitochondrial DNA here.

Be sure to test the oldest generations autosomally, but also remember to review your cousins’ paths of descent from your common ancestors closely to determine if their Y or mitochondrial DNA is relevant to your genealogy! Y, mitochondrial and autosomal DNA are all different parts of unraveling the ancestor puzzle for each of your family lines.

You can order the Y, mitochondrial DNA and Family Finder tests from Family Tree DNA.

Happy ancestor hunting!



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32 thoughts on “Concepts – Why DNA Testing the Oldest Family Members is Critically Important

  1. Thanks for the article. I’d like to add, when testing the oldest family members, make sure contact info is kept up to date. My husband is a 54-year old adoptee who suddenly became curious about his bio family. His closest match (3 segments between 10-20 cM each) passed away in 2011 and then his wife in 2013. There was no tree associated with his account and his email is no longer monitored.

  2. And, I am 70-something and sometimes when I share a 6 cMs match with a 30-year old, I tell them had their parent tested, I probably would have shared close to 12 cMs with the parent which would have been well over the threshold for a good match. And so sometimes 6 cMs is more important when there is a variance of age between the two testers.

  3. Families will soon be gathering to celebrate the holidays. If you’re the designated genealogist for your clan, Thanksgiving and Christmas offer an opportune time to solicit family members to take a DNA test.

    Imagine this; Uncle Jim, I want to talk to you about estate planning and my inheritance. Now that you have Uncle Jim’s full attention, I presume your leaving everything to your children, but there is one thing I’d like you to leave me. Would you take a genetic DNA test to help me with our family research?

    Create interest in DNA testing using the ethnicity sales pitch. Are you curious about our origins in the old country? I wonder if we have any Native American ancestors?

    Saliva in a vial, or a cheek swab will become the greatest family heirloom your relative could pass down. It won’t get lost, broken, stolen, destroyed in fire, or sold on eBay.

    For those relatives that are nervous about potential NPE’s (non-parental events) scandals, remind them that that horse already left the barn when you originally got tested. To quote Irish playwright George Bernard Shaw, “If you can’t get rid of the skeleton in your closet, you’d best teach it to dance.”

    Life is unpredictable, talk to those older relatives about DNA testing ASAP.

  4. Hi Roberta,
    I have a question about the X chromosome, which I have not been able to find the answer to. I have some people who match my father on the x chromosome but don’t match my paternal grandmother, which I know is impossible due to x inheritance.
    1) Keith – 40 shared cm with my father; longest block 9cm, x match – 17cm
    2) Virginia – 50 shared cm with my father; longest block 35cm, x match – 7cm
    My thinking is that Keith is a genuine maternal match due to the 17cm x match and that the longest segment has some IBS at the end for my father which allows Keith to show up as a match with him but not my grandmother.
    My thinking is that Virginia is a paternal match and the x segment match is IBS.
    Do you agree?
    Thanks very much
    Gareth Nelson

  5. For the autosomal DNA, is there a reason the DNA tends to pass in segments? That is what is tested, so I am just curious of the logic behind that passing. Most of the sites that talk about the splitting, do not describe the reasons behind the segment grouping.

  6. By the time I jumped on the DNA testing bandwagon, all the older members of my family were dead. I’m an only child and the closest relatives I have left are 1st cousins, once removed. Unfortunately I don’t have the funding to pay to have them tested. It is very disappointing to realize that my autosomal DNA and mtDNA tests were for all practical purposes a waste of money.

  7. I’m looking into getting testing done of my parents, but am overwhelmed by all the info and choices. Obviously I want to get my fathers Y dna done, but how many markers? Is it possible to add markers on at a later date if you went with the 37 or 67?

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  14. I want to test my grandmother on my father’s side since she is the oldest living member of the family. I was going to get her the autosomal test through Ancestry since that is the one that I just did. Would it be worth getting a test for my father and grandmonther?

    • Yes, I would because your father’s matches can in essence be divided in maternal and paternal by virtue of his mother’s DNA. Very worthwhile. You’ll have three generations then to work with. I’d also suggest testing at Family Tree DNA. You can transfer, but since the two tests, Ancestry and FTDNA only share half the locations, you only get the closest matches. But more importantly perhaps is that FTDNA, if you swab with them, archives the DNA for 25 years. Once they are gone, if new tests emerge in the future, their DNA is still available to test.

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  16. FTDNA offers Y-DNA. MtDNA full sequence, and Family Finder. Under what category does one find or order autosomal DNA test kit?

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