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.

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.

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
920	371	549

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.

Summary

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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