The Science of Sustainability

DNA Ancestry Tests: Simultaneously Powerful and Limited

  • share this article
  • Facebook
  • Email
Using a common DNA ancestry test, President Obama would be 100% Caucasian.


Using a common DNA ancestry test, President Obama would be 100% Caucasian.

Sometimes genetic tests aren’t as useful as you think they will be. For example, if President Obama were to take a common ancestry DNA test, it would almost certainly come back as 100% Caucasian. Useful, huh?

This sort of test, a mitochondrial DNA (mtDNA) test, can look into the deep past but it can only see mom’s side of the family. And it isn’t even really that powerful. It not only ignores dad’s side of the family, but in reality it can only see a sliver of mom’s as well.

So with this test you can see your mom and her mom and her mom’s mom and so on. You can also see your mom’s sister and her kids and your grandma’s sister and her kids. What you won’t see, though, is your mom’s brother’s kids or your grandma’s brother’s kids or any male relative’s on your mom’s side of the family. It really only follows back a direct, specific maternal line.

This is why President Obama would look Caucasian in this test…because his mom is Caucasian. Now of course with a little knowledge about these sorts of tests, President Obama wouldn’t be surprised. But not everyone knows they have a relative on their direct maternal line who is of a different ethnic group.

I know of a case where a self-identified African-American woman had a mtDNA test done to try to determine what part of Africa her ancestors came from. Apparently they came from Europe; her test came back 100% Caucasian. This is despite the fact that all her living relatives were African-American and no one can remember any relatives who were Caucasian.

As this result shows, we don’t have to come up with an explanation like President Obama’s—it doesn’t have to be that her mom was Caucasian. Because of how these tests work, the DNA from this test could have come from a woman five, ten or even fifty generations back. Mitochondrial DNA passes from mother to child virtually unchanged, generation after generation.

What this means is that there is no dilution of mtDNA as the generations pass. Mom’s mtDNA is pretty much the same as grandma’s which is pretty much the same as great grandma’s and so on. This DNA doesn’t dilute over time.

Here is one way to show what might have happened to this woman’s DNA over the generations:

mtDNAft3

In the diagram, circles are women and squares are men. Also, white means Caucasian and black means African.

On the left, we are following the mtDNA of the tested woman and on the right we are following the rest of her DNA. She is the circle at the bottom of the family tree on each side.

What you can see on the right is that over the generations, the original Caucasian DNA is being diluted out. It goes from 50% to 25% to 12.5% and so on as you’d expect. The same isn’t true for the mtDNA on the left. It stays Caucasian generation after generation.

So if we look at most of her DNA, she looks African-American. We may not even see that sliver of Caucasian! But her mtDNA is 100% Caucasian. No wonder she was confused.

Obviously she can’t find out what part of Africa her ancestors came from with this test. But let’s say she got a more typical result and her mtDNA could be traced back to some part of Africa. What does that result really mean?

As the result here shows, it may not mean as much as she hopes. Imagine the previous scenario but instead of being Caucasian, the woman from five or ten generations ago came from a different part of Africa than the rest of her relatives. She would come away thinking her relatives were from one part of Africa when most of them were really from another.

She would not get the answer she was looking for but there is no way for her to know she got the “wrong” answer. She would simply go on believing she was from a part of Africa she really wasn’t.

Unfortunately, there really isn’t a better DNA test for looking back in time. The most useful tests, the autosomal ones that look at all our DNA, can only go back four or five generations before becoming hopelessly jumbled. For most African-Americans, this isn’t far enough back to find their African ancestors.

The other kind of test, the Y chromosome test, can go as far back along the paternal line as the mtDNA test can along the maternal line but it suffers from the same problems. In fact, a surprising number (35%?) of African-American men actually have Caucasian Y chromosomes (well, given plantation life, maybe not so surprising). None of these men will learn anything about their African heritage with this test.

So the bottom line is don’t put too much faith into DNA testing alone. It is kind of fun to trace back your history this way but you are really only following one strand of your ancestral web back in time. The rest of the web is invisible to DNA testing.

Of course mtDNA tests have been incredibly useful scientifically. They’ve let us trace human migration as we spread out of Africa and even trace all of our lines back to a single woman, Mitochondrial Eve. Recently these tests were even used to confirm that a skeleton found under a parking garage in England belonged to King Richard III.

Just don’t count on mtDNA tests giving you a broad understanding of your own family history. They won’t.

Related

Explore: , , ,

Category: Biology, Blog

  • share this article
  • Facebook
  • Email
Dr. Barry Starr

About the Author ()

Dr. Barry Starr is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA and runs their Stanford at The Tech program. The program is part of an ongoing collaboration between the Stanford Department of Genetics and The Tech Museum of Innovation. Together these two partners created the Genetics: Technology with a Twist exhibition. Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.
  • http://www.facebook.com/PaulGustavYates Paul G Yates

    I wouldn't write off the autosomal DNA tests so quickly by saying they only give relevant information going back 3-4 generation. The amount of populations, quality of those populations, and expertise in which the comparative algorithm used is written and weighted can make the autosomal results much more useful. Of course this depends on the company or person you use to complete the autosomal DNA test.

    • Barry

      You are right in that I shouldn't give autosomal tests short shrift…I think we can definitely say they do not go back as far as mtDNA or Y-DNA though. Also the autosomal tests I have taken suggest to me that they are more useful for finding relatives than divining ethnicity but I could be wrong on that one. Do you think that is true?

      • runjackson

        Autosomal test I took has helped be on my family tree back 7 generations, and I would say a good helpful tool ,coupled with hard research. As far as y dna test the fun part is that one small line of your y chromosome can be traced back a long time in past. Certainly a very tiny part of you. But interesting.

  • http://twitter.com/eshowoman Friday Foster-ABWW

    This is not news. African Americans have known about their white ancestry for years. Now we just know like Wanda Sykes and Vanessa Williams it can come from the maternal side of the family too. The presidents Caucasian side may have had slave ancestry according to some genealogists.

    • Barry

      All definitely true! I just wanted people to think long and hard about these tests before jumping to any conclusions about their ancestry.

      • James

        What is your evidence that great masses of people don't know what a mitochondrial DNA test is? It's really not that complicated. You seem to be very condescending, explaining things which really don't need explaining. Without evidence to the contrary, I would assume that adults who are willing to spend typically anywhere from $100 to $200 to have one of these tests done probably comprehend the fairly simple description of the nature of the test they are paying for.

        • DCKeene

          (Someone needs to chill out, or put the vodka bottle down)

  • Hmmmmm

    "You can also see your mom’s sister and her kids…" I am struggling to imagine how one can see their mother's sister's kids (their cousins)? What if they are not born? Hmmmmmm! Editors, please help!

  • http://www.facebook.com/edea.krammer.3 Edea Krammer

    Yep! DNA is the 21st century finger print that’s one thing for sure and can trace down where you can from but the thing is it doesn’t change anything for who you are today for what your ancestor had done in the past. The point is it only proves!
    BostonPaternity.com

  • Ian Brett Cooper

    Somehow I think that before he wrote this, the writer must have had a completely wrong idea of how DNA works and been disappointed when he found out the truth. Of course DNA testing only covers one direct line of ancestors – mtDNA covers the direct female line, while yDNA covers the direct male line. It is able to do this purely because certain types of DNA do not change from generation to generation. This is not a bad thing – it's the very thing that allows us to get anywhere at all in genealogical terms. If mtDNA nad yDNA acted like all the other DNA areas, DNA testing would tell us nothing at all about previous generations.

    • Barry

      Thanks for the summary of the blog, I think I covered all of your points in the article. I am aware of how DNA is passed down but many people who take these tests are not. I wrote this for them so they would know the limitations of the tests they are taking. That is all.

      • James

        Who is this "we" you presume to be too dumb to know what the test they shelled out in all likelihood $200 bucks to have done is actually testing? You're a real piece of work.

        • Mr Olorin

          And you have obviously never taught Statistics 101 to the average American.

  • Ian Brett Cooper

    "Sometimes genetic tests aren’t as useful as you think they will be. For
    example, if President Obama were to take a common ancestry DNA test, it
    would almost certainly come back as 100% Caucasian. Useful, huh?"

    Yes, it is useful. You say this as if it's all the story. Obama's father was black. Obama's yDNA would show 100% black. You can't just take a yDNA test and assume it tells you 100% of the story. It doesn't, nor is it meant to.

    This article is like saying that yeast is useless because it's only a small part of making bread.

    • Barry

      It is useful only if you realize the limitations of the test. I think the better analogy might be that you think you are figuring out everything about the ingredients of bread and you find out only about yeast. Yes learning about the yeast is important but you can't make bread with just that.

  • James

    These tests must be something of an embarrassment to those who have been piously parroting the feel good line that "race is a social construct!" that as been so in vogue the last few years. It's clearly burnt into our DNA, lol (shocker!).

    I suspect the "race is a myth" meme is in part the motivation of this confused article (I mean, really; who does the author imagine to be so dense that they'd be 'surprised' that a DNA test of a person with a white mother and black mother that only looked at the maternal DNA would show '100% Caucasian'?). Somebody smack the author for me. Please. Now. Hard. Twice.

    Sure, we make way too big a deal over it, and people of different races still share much common genetic heritage. But they don't share those genes that encode distinct racial characteristics (however small a subset of the human genome they may be). Race is not a 'social construct', it's genetically determined (which in reality I think most of the people who say "race is an illusion" know damn well, they just are bothered by human racial differences and want to deny they exist). It's an admirable sentiment, but at the end of the day, BS is still BS, know matter how well intended.

  • Dana

    This is a really poorly written article that needs lots of editing.

    I came across it looking for information on DNA testing and started off confused by the statement that Obama is 100% Caucasian, to thinking that all tests are just of maternal lineage and therefore useless, to realizing there are many different types of tests possible and that it's important to have a quality test done that incorprates the most relevant of these… and I realized that from the COMMENTS not the article itself.

    The information is really poorly presented and misleading, especially if you skim and/or skip the comments.

    • Mr Olorin

      What do you expect in a nation where the vast majority of people don't understand the difference between a mean and a median as the simplest measures of central tendency?

      Researchers dealing with the arithmetic of population genetics have to try to provide examples that the average person can understand. The point of the example was the limits of mtDNA, inherited solely from one's mother. Many tests only look at mtDNA and in fact would show Mr. Obama's mtDNA as majority white (not entirely–not doubt there were other races in his mother's genome over the past X millennia).

      I think the message here is, when reading about complex scientific matters, don't skim. Be prepared to read more than once, and cross reference as questions occur to you. Ask questions. Look things up. This isn't the latest sports score of celebrity wardrobe failure.

  • Gem615

    Just a minor comment to all the comments as I also pulled this page up searching for something else…. to me it sounds like everyone needs to take a chill pill. I unfortunately have the luck of understanding both sides of the coin – I think the original intention of his blog is to more clearly explain to those in the population who tend to take things a little too much for face value (e.g. dna testing, bigfoot, aliens) – and there certainly are quite a few of them out there. Those of you/ us that may (or not be) slightly more educated and/ or possibly have a better grasp on reality, common sense, street smarts. etc. (or whatever you'd like to refer to it as) need to relax on the read rage here – maybe cough up a chuckle or two instead…. just sayin' ;)

  • Leslie

    You see your(female line) cousins' mitochondrial DNA because we all inherit our mother's mitochondria (female line DNA,) and they, like their mother, if she's your mother's sister, have the same mitochondria from their grandmother, great grandmother, etc. as you. Males inherit their mother's mitochondrial DNA, but they do not pass it on to their kids, who will instead have their own mother's mitochondrial DNA. Which is not to say they don't inherit other genes from their father's mother as well as from their mother's father, etc. that just don't show up in this particular kind of test.

  • Orlando

    Hmmm…? I can see how having to refer to President Obama as "100% Caucasian" would be quite a misrepresentation that would probably have the o'l chap whistling dixie by the end of the day. Lord knows, "we" wouldn't have any of that. He might wake up realizing he even married a black woman, omg! But duly noted, the Author has clarified for "them" African Americans that may not be able to associate the color of their skin to there regional origin i.e. "Africa", or have no clue about the inner workings of the mtDna… By the way I was just thinking, just a thought, if "them" African Americans want discover they're Caucasian, do they have your permission?

  • Orlando

    I mean you know. Let's say a black discovers he/she is "100% Caucasian" and maybe wants to find their white relatives. Can he/she do that, without your permission?

  • Orlando

    …Oh, by the way, Many black creoles, and possibly black Hispanics, who've never heard the word "plantation", might need you clarification, if be the case that the phenomenon of "100 %" Caucasianism in blacks is limited to plantationism from 19th centurism… Which brings my back to President Obama who happens to be "100%" and has no known relation to "plantation life,". I'd guess he's the sole and absolute exception according to the article.

  • Orlando

    Honestly Doctor Starr, why didn't you make the case for the Euro-American who finds out he's 100% African?

  • Orlando

    According to this article, 35% of African Americans are a direct result of Euro-American women of the 19th century having engaged in sexual intercourse with a "plantation" worker of African origin that resulted in pregnancy. Useful huh? Very!

  • Orlando

    I stand corrected. The article was referring to the Y chromosome test that identifies the Caucasian trait in an individual. That being said: This article states that 35% of African Americans are a direct result of Euro-American of the 19th century having engaged in sexual intercourse with a female "plantation" worker of African origin that resulted in pregnancy. Very useful I'd say.

    • Mr Olorin

      More likely the rape of white women by black men. Look at FBI Uniform Crime Statistics for the past decades. White males rarely rape black women; black men frequently rape white women.

  • Orlando

    According to this article, the implications are endless; half sisters, half brothers, rightful heirs.This is going to be fun. A new branch of study, law, and politics. I'd like to try my hand at holding the origin of naming what should be the term for this…. I mean is it an -ism, or an -ology? Hopefully the latter doesn't become the former, God help us all!

    Ecolobiogenology- the study of ancestry through the process of ecological, biological, and geno-logical investigation.

    I chose this order because it's a statement of the order of the origins of our knowledge. We learn from our 'known' environment to our genetic realm. Thus has been our history as a species, thus the order.

    Oh yeah genological, it's going to be coined also, wait for it :)

  • zawadi

    These tests only hand u a slice of a pie… that is how u need to see it. it will only tell u a little of who u are and most do not understand this going into it. As black as my family is. my grandmothers still had blonde hair and white skin ( they were black ). hahahhah

  • Gay Eastman

    This article is incorrect because my mom's side is French and my dad's side is scandinavian and mine came back scandinavian.

    • Susan Fisher

      If you read the entire article, you would find that it reasonable since it reveals mitochondrial DNA that has passed along unchanged for centuries. Scandinavians settled in France as well as England long ago.

  • Samantha

    I don't understand why so many people are offended by the statement "the masses that don't understand" . I mean really this was informative for me because I am one of the masses that did not understand and I happened to come across this article while searching the net trying to decide if this would be right for me and worth the money I spend on having it done. Everything I came across was talking about how wonderful this is and how it will tel you where you come from and where your ancestors came from. So you see I am ignorant on the way DNA testing works and what it can and can not tell. I know my great grandparents on my fathers side came over from Ireland but that is all I am sure of. So I have no problem saying I was ignorant on the subject but now that I have read this article and then all the comments (discounting the argumentative, arrogant, offended statements because I find that more ignorant than anything else and will not listen to it) I can say I am a little less ignorant. I use the word ignorant because it means uneducated and that is what I and most (who are not scientist, biologists, doctors and so on) are because we have not actually studied it in length. Those who are offended by that then may I suggest you change it by studying it in length and depth so that you will no longer be ignorant. Thanks to all who comment on here without emotions or attacking his article because it was not in depth because it helped me to have a better understanding of what these test can and can not achieve.

  • http://www.dnaspectrum.com/ DNA Spectrum

    The article contains an interesting infographic representing the positive attributes and limitations of the differing DNA testing technologies. There is one piece that is missing though; the algorithm or technology that is used to analyze the genetic markers. Sound statistical practices, a solid understanding of convergence, and a large sample size (not necessarily the number of participants, but the number of representative populations) can all add a deeper understanding of ancestry and help with some of the limitations of tracing lineage. A summary of testing technology and the statistical analysis used (usually Random Match Probability for ancestry) would make for a much more thorough discussion and understanding of the contributions of DNA-based ancestry tests.