My Answer to a DNA Question

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In a recent post within the Genetic Genealogy Group here on Ancestorian someone asked the following question:

“We are having our DNA tests now, and showing up against living family members we know … HOW is that DNA test proven against ancestors 200 years ago against their DNA which we don’t have”

Because the Genetic Genealogy Group is a private group, it was suggested that I repost the answer I gave as a blog post for the benefit of others who are not in that group but who may be wondering the same thing.

Most direct-to-consumer DNA testing e.g., Ancestry, 23andMe, LivingDNA, Family Finder at FTDNA, is autosomal testing (At-DNA). That is what the inquirer was inquiring about. That is what my response relates to.

Researching with DNA can be a bit of a learning curve. I was asked for a simple (that is, basic) answer to the question. So, that is what I am attempting here but with links to other sources of information for those who want to explore further.

Some Basic Background Information


You have 22 pairs of autosomes. That is, one copy of each of the autosomal chromosomes 1-22 from each parent.

You also have 1 pair of sex chromosomes. That is one copy from each parent. Males are xy. They get the y copy from their father and the x copy from their mother. Females are xx. They get one x from each parent. The x chromosome is sometimes referred to as chromosome 23.

While some of the companies include x chromosome data, others such as Ancestry focus only on the 22 pairs of autosomal chromosomes when determining DNA matching and cM values shared.

For the most part, the pattern of DNA inheritance for the sex chromosomes differs to that of the 22 autosomes. It requires separate explanation. My focus in this blog post is the 22 autosomal chromosomes.

DNA Recombines Randomly

DNA is mixed up at each generational level in a process called recombination. Recombination is the exchange of DNA between the two copies (one maternally inherited and one paternally inherited) of each chromosome (1-22).

During the creation of an egg or sperm for the next generation, DNA segments are swapped between the two copies of each autosome at one or two random spots on each of the autosomes (crossover points). The newly formed chromosome in the egg will be a mix of what the maternal grandparents have contributed and the newly formed chromosome in the sperm will be a mix of what the paternal grandparents have contributed.


  • The way in which DNA recombines at each generation is random; and
  • Which bits of your ancestors’ DNA you get is random

This also means that, because the creation of each egg and sperm is an independent event, siblings (identical twins aside) don’t inherit exactly the same portions of their grandparent’s DNA as one another. They share some in common but not all.

You inherit 50% of your autosomal DNA from each of your parents.

On average, you inherit 25% of your DNA from each of your grandparents (via each of your parents), 12.5% from your great grandparents, 6.25% from 2nd Great Grandparents, 3.125% from 3rd Great Grandparents … and so on.

In reality, you may get slightly differing amounts of DNA from each grandparent. This is because of the random way in which DNA passes from one generation to the next. As indicated above, 25% is the amount you inherit, on average, from each of your grandparents. If you are a little short for one grandparent within an ancestral couple, you will receive more from the other grandparent within the ancestral couple but the amount will average out across the two grandparents within the ancestral couple. Similarly, your parents may have received slightly differing amounts from their grandparents. So DNA washes out unevely over time (See below)

Useful links:

Your Genetic Tree is a Subset of Your Genealogical Tree

The number of ancestors you have doubles with each generation. When you consider how the number of generations increases moving back in time and the random way in which DNA recombines at each generation in order to move forward in time, it is not difficult to understand how the percentage of autosomal DNA coming from each ancestor is diluted with each new generation. Because segments from specific ancestors are passed down randomly, they can be lost quickly.

Because DNA washes out over time, you will have the DNA of some of your ancestors only. Somewhere between about 6-8 generations back, or so, ancestors start falling off your genetic tree.

The following table from ISOGG Cousin Statistics - - shows the probability of sharing no detectable DNA with your ancestors:

Also, because DNA is passed randomly, you will have relatives more distant than 2C who belong in your genealogical family tree but who do not share a detectable amount of DNA with you because they got different bits of DNA from your common ancestors than you did or, for more distant relatives, they inherited DNA from an ancestor when you didn’t. They are genealogically related to you but not genetically connected to you through shared DNA.

The following table from ISOGG Cousin Statistics - - shows the probability of sharing no detectable with your relatives:

You can only be matched to other people who share a detectable amount of the same bits of DNA with you. Because you don't inherit exactly the same DNA as your siblings, your DNA match list will look a little different to theirs for matches beyond second cousins (2C). It is quite possible, therefore that a sibling will match say a third cousin (3C), for example, when you don’t.

Useful links:

Making Connections

Where the testing company finds that you share a detectable amount of DNA of a reasonable size with another person in their database you are matched to that person. The company also tells you which other matches match you both

Where 3 or more matches match each other in the same place on the same chromosome, this indicates that they share a common ancestor. That is, that the same chromosomal segment has been passed down to you and each of the shared matches from the same ancestor. [Some people share more than one segment but I am trying to keep the answer simple].

Ancestry doesn’t provide detailed chromosomal information but, as with the other databases, you can make good use of shared matching to figure out groups of matches that all match one another. If you can’t connect them to your own tree straight away, you can investigate how they link together around a common ancestor. This may be a clue to one of your brick walls.

Reliability – Tree Completeness

At this point perhaps we should return to the question initially posed:

HOW is that DNA test proven against ancestors 200 years ago against their DNA which we don't have?"

The short answer is, it doesn't if you don't have their DNA ... not directly anyway.

If we assume that the average span between one generation and the next is about 25 to 30 years, then 200 years could take us back, 8 or more generations. As discussed above, it is quite possible that you won't carry the DNA of some of those ancestors. In that case, your autosomal DNA test won't directly link you with those ancestors. Indirectly, it may, if you have siblings who match others who link back to the ancestors who you don't share DNA with.

Even if you do share DNA with an ancestor, that in itself is not absolute proof of anything! You need to use documentary evidence to figure out how you and your matches connect together and to find who that common ancestor or ancestral couple is. Even when you have found that, caution is required! Sometimes people are too quick to conclude that they have proven something even though it is a few generations back and they have gaps in their tree.

Questions you need to ask yourself are ... Can I be sure that this is a genetic connection rather than just a genealogical one? How complete is my tree? Could I connect to these matches in some other way as well?

A great way to check how complete your tree is, and to remind yourself where your brick walls are, is to use the ‘TreeCompleteness’ function within the ‘AncestralTrees’ tool at DNAPainter - I have written about this before[1]

This is what it shows for me:

You can see that I have some significant gaps.

I treat DNA as another piece of evidence that contributes (alongside the documentary evidence etc.) to the conclusions I make about my Ancestry. It has helped enormously in places but I have varying degrees of confidence in the conclusions I have drawn and, because I have significant gaps in my tree, my conclusions stand only until other evidence comes along to make me rethink it. For this reason, you won't find the words DNA verified or DNA proved anywhere in my tree. I do, however, show where I have triangulated data around an ancestor as a contributing piece of evidence.

There may well be other people who have a greater degree of confidence in the extent to which their DNA information proves a relationship in their tree based on a more complete and well-evidenced tree.

To Conclude

I hope I have answered the question, albeit in a long winded way.





JaneChapman - avatar
About the author: JaneChapman Admin Icon
I am a hobby genealogist. I have been researching family history since the early 1990's. Since 2013, I have been using autosomal DNA-related information, in conjunction with traditional sources of evidence, to support, or challenge, the paper trail and to solve genealogical puzzles. I am always happy to collaborate with, and support others in, family history research. I am a member of the New Zealand Society of Genealogists (NZSG). I am also a member of the Guild of One Name Studies - You can also find me at:

Get involved!


Heather B. avatar
@peepso_user_2769(Heather B.)
Thanks Jane, I was actually wondering if it was possible to make a connection where there isn't one in the same way you can with paper records when I saw you had written; "Sometimes people are too quick to conclude that they have proven something even though it is a few generations back and they have gaps in their tree."
JaneChapman avatar
@peepso_user_2769(Heather B.) DNA evidence contributes valuable information to decisionmaking. As with any other evidence it doesn't stand alone. As you know, in genealogy it is all about how a range of evidence is weighed up - even paper trail records can be wrong. So, just think about DNA evidence as another bit of contributing evidence; another piece of the puzzle.

For example if you have a range of documentary evidence pointing to a particular person being the ancestor you are looking for, the DNA evidence may be the extra evidence you need to convince you. It isn't the DNA that has proven it, it is the range of evidence considered together (each bit weighed up against the other) that gives you confidence (to some degree or another) in the conclusion you have reached.

Sometimes people put too much weight on DNA evidence because 'DNA doesn't lie' (an often used comment on social media). No it doesn't lie but it can, and very… Read more