DNA tests can’t necessarily prove racial ancestry

What is the chance that you will inherit any DNA from a specific ancestor? What are the odds that specific segments of DNA that determine a specific race are still present 5 generations later?

People do not understand DNA tests and ancestry. A DNA test will not necessarily prove Elizabeth Warren is or is not descended from Native Americans That is, even with legal documentation proving descent from a specific Native American princess, the DNA may not show that. In fact, none of the DNA from the that ancestor may be present in the tested genome.

How much of your genome do you inherit from a particular ancestor?
[Via  | gcbias]

How much of your genetic material do you inherit from a particular ancestor? You inherit your mitochondria through your matrilineal lineage (your mum, your mum’s mum, your mum’s mum’s mum and so one) and your Y chromosome from your patrilineal lineage, but how is the rest of your genome spread across your ancestors in any given generation?


We will not even examine the fact that the supposed Native American was not full blooded or even Native American (ie married into the tribe).

Or that the accuracy and precision of the DNA tests are high enough to be definitive.

We will just look at the math.

Many races display specific regions of DNA that are characteristic for that race. So, if they are found in your genome, there is good evidence that you are descended from that population.

But if those sequences are NOT found in your DNA? That does not prove you are not descended from that race.

First, there is the logic. If “If the sequences are present, then you are descended from that race” is true, then only the statement “If you are not descended from that race, you will not have the sequences” is true.

“If the sequences are not present, you are not descended” may not be true. And in fact, as this referenced blog post discusses, there is a good chance that you have little DNA from your ancestors back 5 or more generations.

Second, while on average people get 50% of their DNA from each parent, actual biological processes mean that for any 1 individual, the amount can range from 45% to 55%.

DNA is often swapped between the DNA you get from your mother and that you get from you father. This is recombination. Women also have greater recombination rates than men. That means that blocks of DNA are exchanged between similar chromosomes, mixing up father and mother segments of DNA.

Third, see that as you go back farther you have more ancestors. 2,4,8, 16, etc. Five generations back and you have 32 different ancestors. So at best, only 1/32 of your DNA can come from a specific ancestor 5 generations back. That is. only about 3% of your total genome would possibly come from that specific ancestor

But what we are looking for here is what are the odds that specific segments of DNA (the ones that specify say Native American DNA) make it out five generations?

The linked post details this, using real data and simulations. Here is what was found for the specific probability that you inherit DNA  from a specific ancestor, looking at each chromosome.


Since some chromosomes are smaller and mix up DNA more than others, there are different chances for each chromosome. But one can see that after 5 generations you have about a 50% chance NO DNA from a specific ancestor will be found on the entire Chromosome 1.

And an over 80% chance that you will have no DNA from that ancestor Chromosome 22. 

I do not know where the markers are to determine Native American ancestry. But depending on the chromosomes, there is a reasonable chance that someone like Elizabeth Warren may not have any DNA from any possible member of the Cherokee tribe.

And that is assuming that relative was full blooded with no other racial ancestors themselves (for example Latino). If so, the odds are even greater for Warren not showing any Native American DNA.

The only way to really prove ancestry with DNA tests alone is to have the DNA sequences of every one of Warren’s ancestors. Not very easy.

Image: Scott A. Wright