How Many Ancestors Share Our DNA?

This post was written four years ago, using a quick-and-dirty model of recombination to answer the question in the title. Since then a more detailed and rigorously tested model has been developed by Graham Coop and colleagues to answer this same question. You can read more about the results of this model on the Coop Lab blog here and here. Graham’s model is based on more accurate data, more careful tracking of multiple ancestors and a more realistic model of per-chromosome recombination, and thus his results should be considered to have superseded mine.

Over at the Genetic Genealogist, Blaine Bettinger has a Q&A post up about the difference between a genetic tree and a genealogical tree. The destinction is that your genealogical tree is the family tree of all your ancestors, but your genetic tree only contains those ancestors that actually left DNA to you. Just by chance, an individual may not leave any DNA to a distance descendant (like a great-great-great-grandchild), and as a result they would not appear on their descendant’s genetic tree, even though they are definitely their genealogical ancestor.

At the end of his post, Blaine asks a couple of questions that he would like to be able to answer in the future;

  • At 10 generations, I have approximately 1024 ancestors (although I know there is some overlap). How many of these ancestors are part of my Genetic Tree? Is it a very small number? A surprisingly large number?
  • What percentage, on average, of an individual’s genealogical tree at X generations is part of their genetic tree?

I think that I can answer those questions, or at least predict what the answers will be, using what we already know about sexual reproduction.

Simulating Sex with Recombination

We can give a simple answer by assuming that each chromosome is passed on intact, with a 50% chance of getting either one from a pair. This gives us a maximum of 44 genetic ancestors, and means that the probability of being related to any particular ancestor N generations ago is 1 – (1 – 0.5N – 1)22. We’d have about 43 genetic ancestors out of 1024 genealogical ancestors after 10 generations.

This is an underestimate; outside of the Y chromosome and the mitochondria, DNA is not passed down as whole chromosomes; recombination occurs, in which chromosomes come together and swap DNA. This mixes up DNA and stop is getting lost, and lets you have DNA from more than 44 genetic ancestors.

To answer Blaine’s two questions, we need to take recombination into account. To do this, I put together a computer simulation of recombination, using data from Chowdhury et al‘s large population study of recombination; they found that recombination rates vary from person to person, and especially between genders, with significantly more recombination in women than in men (see this graph for the data I used). I simulation individuals with the 22 non-sex chromosomes, and each person had their own recombination rate, chosen at random, dependent on their gender. I simulated sexual reproduction with unrelated individuals, and checked whether their DNA was present in their descendants N generations in the future.

The results

Here are the results of the simulation:

sharingprob

This graph shows the probability that an ancestor of mine from N generations ago will pass DNA onto me, i.e. the probability that a genealogical ancestor is also a genetic ancestor. The black line is the simulation with recombination, the red line is our prediction without recombination; as we guessed, recombination increases your chance of being genetically related to your ancestors, though it still drops of pretty dramatically; after 10 generations, only about 12% of your genealogical tree is in your genetic tree.

The probability of having DNA from all of your genealogical ancestors at a particular generation becomes vanishingly small very rapidly; there is a 99.6% chance that you will have DNA from all of your 16 great-great grandparents, only a 54% of sharing DNA with all 32 of your G-G-G grandparents, and a 0.01% chance for your 64 G-G-G-G grandparents. You only have to go back 5 generations for genealogical relatives to start dropping off your DNA tree.

We also care about how many genetic ancestors we have after a certain number of generations, shown below:

Nancestors

The number of genetic ancestors starts off growing exponentially, but eventually flattens out to around 125 (at 10 generations, 120 of your 1024 genealogical ancestors are genetic ancestors).

As a final note, there is an interesting effect of the larger recombination rate in women; you are, on average, slightly more closely related to your maternal line (your maternal grandmother, your mother’s maternal grandmother, etc) than you are to you paternal line (your father’s paternal grandfather, etc). We can see the sharing probability for the maternal line and the paternal line show:

mvflines

You are about 30% more likely to be genetically related to your maternal-line ancestor 10 generations ago than you are the corresponding paternal-line ancestor (14% vs 11%).

My not-very-well-documented R code for this simulation can be found here

Share and Enjoy:
  • Digg
  • Reddit
  • StumbleUpon
  • del.icio.us
  • Facebook
  • Twitter
  • Google Bookmarks
  • FriendFeed

28 Responses to How Many Ancestors Share Our DNA?

  1. How do the simulations change when there are 2, 3, 4…. common ancestors present within specific generations? What you provide is the theoretical minimum.

  2. Great, thanks! Your first chart above addresses one side of the issue: Given recombination, what is the probability you will inherit NO DNA from a given ancestor x generations back?
    The other side: Given recombination, and given that you KNOW you inherited y cM of autosomal DNA from a particular line of ancestors, what is the probability you will inherit AT LEAST y cM of DNA from an ancestor x generations back? This plot should look quite different, and I guess it should depend on the length of the chromosome involved, right?

    If so, can you at least give a simplified answer, assuming an average autosome length?

  3. Doh, can you tell it has been a while since my last genetics class? I guess the answer to my question above (which describes the situation of many 23andMe /DecodeMe customers) basically comes directly from the definition of a centimorgan. Feel free to delete the question.

  4. “….you are, on average, slightly more closely related to your maternal line (your maternal grandmother, your mother’s maternal grandmother, etc) than you are to you paternal line (your father’s paternal grandfather, etc).”

    Cool.

    But, aren’t most people going to be more like their fathers, then, than their mothers since they should inherit more directly *groups* or *packages* of genes that did not recombine (i.e. that remain linked)?

  5. Was pleased to read about girls being closer to the maternal line. . .

  6. I am a female and more closely related to my paternal line, 53.13%, according to Relative Finder. Almost everyone of my matches comes from my father’s side.
    Some of his relatives, I am even more related to because of the recombination. On the average, it looks as though I may lose a few cm’s with most; although, many of our matches have remained exactly the same.
    One particular high/distant cousin match for him, totally disappears for me.
    All very interesting!

  7. Pingback: Using Genome-Wide SNP Scans to Explore Your Genetic Heritage

  8. You are showing a point estimate but it would be nice to see the range of values. Can you repost the plots with 95% prediction intervals?

  9. Does it mean that ethnic affinities derived from autosomal DNA data are also that recent? (10-12 generations)

  10. This information really revolutionizes our thinking about our ancestors, and it has been fun to study what you have done with this simulation.

    It is known that recombination is not equal across the genome, and there are known recombination hotspots. It would be interesting to see how this analysis would look if you incorporated recombination rates across the genome. All of the data is accessible from HapMap’s website http://hapmap.ncbi.nlm.nih.gov/downloads/recombination/latest/rates/

    Also, it is strange to me that you see such a leveling off in number of ancestors at 125, it even looks like the number of ancestors drops a little at 15 generations. Does your code include a minimum threshold for being identified as a relative? Otherwise, I don’t think that is possible.

    Nice work, I hope you could generate a simulation with the recombination rate data, because it is way out of my league to try something like that.

  11. We have about 3.08 billion base pairs. Therefore, we have about 6.16 billion bases and over 30 generations before we have to make a choice between one ancestor and another, one allele and another, at a particular base pair.

    About 30 generations at about 30 years per generation is about 900 years. 900 years ago our world population was between 310 million and 790 million. It is likely no more than 2/3rds of those have descendents to the present, given the effect of the plagues that happened between 900 years ago and now.

    Without inbreeding, however, at 30 generations you have 1,073,741,824 ancestors. If the whole world were in your ancestry, then you would still have many cases of “inbreeding” with sixth cousins, seven times removed.

    However, even if there were only 30 ethnic groups in relative reproductive isolation from each other, until the last 200 years there has been very little genetic sharing between groups. There was not much interbreeding, for instance, between native Americans and Mongolians 200 years ago. Genealogical ancestry does not extend much beyond 400 years ago.

    Therefore, we can say that your genetic ancestors and your genealogical ancestors are in the same set and equivalent.

  12. @John Lloyd Scharf

    We have about 3.08 billion base pairs. Therefore, we have about 6.16 billion bases and over 30 generations before we have to make a choice between one ancestor and another, one allele and another, at a particular base pair.

    No that’s not how it works. You don’t have 3.08bn independent base pairs, you have 22 independent autosomes, the bases within which have a correlation structure dependent on recombination.

  13. SO THIS MEANS IF A WOMAN WERE ABLE TO TRAVEL BACK IN TIME AND HAVE SEX WITH A DIRECT ANCESTOR AND BECOME IMPREGNATED BY HIM SHE COULD THEORETICALLY HAVE A COMPLETELY NORMAL CHILD?

  14. @Luke: This to me seems one of the main blog entries about genetics. I think a work-up of the issue in a scientific paper would be worth. So many write about Admixture between populations and do PCA etc, but the basic knowledge about the statistical correlations in autosomal DNA inheritance seems to me still rudimentary.
    A similar study related to the X chromosome (X-DNA) would be very interesting. The number of potential ancestors indeed follows an interesting pattern and only 2/3 recombine: 1 (male), 2 (female), 3, 5, 8, 13, 21, 34, 55, 89, 144

    @Hanna: Already by the known simplified inheritance models you share ca. 12.5% of the DNA with first cousins. The same like with your Great-grandparents. So I guess there will be the same probability of normal childs if timetravels would be possible or the use of gametes of ancestors.

  15. Martha Lyle

    I halfway think I understand this! I’m wondering how the genetic inheritance and Y-DNA are related. My brother has been tested for Y-DNA, and matches 67 markers, confirming that our ancestor, who came to America in 1656, was from Sweden (95% accuracy). Does this high marker match have any relationship to the percentage of inherited genetics. As I read the material above I think I am seeing that my brother has only 11% of the ancestor’s DNA inheritance. I guess my question is, would this percentage be any higher based on the Y-DNA?

  16. Helena Oksanen

    Hi, This article make sense and I have match for one father but not to their children and we don´t share X -chromosome at all! So it seems to be like your results says.

  17. There are four Canadian Algonquin Indian Natives in my ancestry from the 1600′s to 1700′s. Is it possible that a DNA could register an ‘Indian’ presence in my genome from that long-ago time? Not that I can justify the cost of DNA however, the data I have comes from reliable archival journals written and kept by Jesuits in Church records and libraries and in Franco-American Society records in cities where there is a strong number of Franco-American residents. Thank you. AliceMary

  18. gene are the base of life whose contain the informations. deos it is possible to know who were the first human race (means african or american or indians or egyptians)

  19. i wanna to know that after the origin first the algae orignates ,and from there all the species has originated but why only aur ancestors has evolved not the other species espacially in mental level .only the humans had evolved at the mental level that is only one species but from the origin of life many species had come earlier and after us ,but our ancestors the aps had developed only at that time the they were animals too.what was the especial in the aps.

  20. I want to ask, if my great grandfather is a Portuguese, will I still b a Portuguese if their son and granddaughter married a Chinese?

  21. I was wondering if someone is directly of royal descent from thier paternal grandfather, and paternal grandmother, into them by way of thier father (3000 years ago), is the king’s dna still inside of them

  22. I did the family finder test. I got over 150 matches, including 4 women who “supposedly” are 2nd cousins!!! Well, I know who my 2nd cousins are on both sides, and these 4 women are not amongst them. Furthermore, our connection must be way further back than 2nd cousins, because their ancestors are from places that are nowhere near my ancestral locations, and my tree goes back a long way. Does anyone have an explanation for these 4 matches? When I called the company I was assured their results are accurate and if not 2nd cousins, they should at least be 3rd cousins – not a chance! Of course, since I am 100% Jewish, the matches were about 95% all Jewish, and 5% with some Jewish ancestry.

  23. What percent of my great great grandfather’s genes do I have?

  24. My maternal side, the results are B2 with a direct line from myself (male) going straight to my 3rd great grandmother (Native American) born in 1833. There are no males in the line until myself. Therefore, how much of this Native American connects to me and how far back does it go? The various DNA sites have me with matches Eastern/Southern Asia and the entire Americas. How would this look on a line graph? I have many 1st thru 5th cousins plus two of my mother’s sisters on my relative list. Some of these I have never met but we know our common ancestors. on my mothers side.

    I am now beginning to work on the paternal of which I have many 3rd thru 5th cousins who I chat with. I will likely need a half sibling, aunt or uncle to confirm both my father and common ancestor with those on this line.

  25. Rhonda Parker

    I am interested in hearing the answer to James’ question above about royal or any other lineage 800 to 1000 years ago. I have a ancestry.com issue with many people highjacking a prominant line of an ancestor of mine. Our haplogroup doesn’t even match them, but we are seeing the family names from our DNA matches on 12 to 24 markers that would confirm the family names and geographic locations also demonstrated by our documentation. I am now trying to go all the way back to Devonshire England and even the Norman invasion to reconfirm my family tree. Do these further back matches count? How much?

  26. My name is Mrs maria i am from Chicago am 25
    years old i got married at the age of 23 i
    have only one child and i was living
    happily .After one year of my marriage my
    husband behavior became so strange and i
    don’t really understand what was going on,
    he packed out of the house to another
    woman i love him so much that i never
    dream’s of losing him, i try my possible best
    to make sure that my husband get back to
    me but all to no avail, i cry and cry seeking
    for help, i discussed it with my best friend
    Allix and she promise to help me. She told
    me of a man called DR omosel, she told me he
    is a very great man and a real man that can
    be trusted and there is nothing concerning
    love issues he cannot solve and she told me
    how he has help countless number of
    people in restoring their relationship. I was
    really convince, I quickly contacted him dromosel .
    I explain all my problem to him, he told me
    that i should not worry that all my problems
    will be solved immediately. He told me what
    to do to get my husband back and i did, he
    said after 3days my husband will come back
    to me and start begging, and it really
    happen as he said, i was very surprise, this is so amazing.
    To God be the glory our relationship is now
    very tight and we both live happily again. If
    you having similar problem, Contact him
    now(dromoselspellcaster@gmail.com) and
    get your problem solve once and for all. i am living testimony to it.
    1) If you want your ex back.
    (2) if you always have bad dreams.
    (3) You want to be promoted in your office.
    (4) You want women/men to run after you.
    (5) If you want a child.
    (6) You want to be rich.
    (7) You want to tie your husband/wife to be
    yours forever.
    (8) If you need financial assistance.
    (9) How you been scammed and you want
    to recover you lost money.
    (10)if you want to stop your divorce.
    (11)if you want to divorce your husband.
    (12)if you want your wishes to be granted.
    once again the email address, dromoselspellcaster@gmail.com

  27. My name is bella my husband is John i have a word to all my viewers in the world After being in

    relationship with John for seven years,he broke up with me, I did everything possible to bring him back

    but all was in vain, I wanted him back so much because of the love I have for him, I begged him with

    everything, I made promises but he refused. I explained my problem to someone online and she suggested

    that I should rather contact a spell caster that could help me cast a spell to bring him back but I am the

    type that never believed in spell, I had no choice than to try it, I mailed the spell caster his name is Dr

    okogbo,and he told me there was no problem that everything will be okay before three days, that my ex

    will return to me before three days, he cast the spell and surprisingly in the second day,. My ex called me,

    I was so surprised, I answered the call and all he said was that he was so sorry for everything that

    happened, that he wanted me to return to him, that he loves me so much. I was so happy and went to him,

    that was how we started living together happily again. Since then, I have made promise that anybody I

    know that have a relationship problem, I would be of help to such person by referring him or her to the

    only real and powerful spell caster who helped me with my own problem and who is different from all the

    fake ones out there. Anybody could need the help of the spell caster, his

    email:drokogbolovespell@yahoo.com you can email him if you need his assistance in your relationship or

    anything. YOU CAN EMAIL HIM WITH THE EMAIL ADDRESS:drokogbolovespell@yahoo.com

  28. monica morgan

    After being in relationship with him for 3 years, he broke up with me, I did everything possible to bring him back but all was in vain, I wanted him back so much because of the love I have for him, I begged him with everything, I made promises but he refused. I explained my problem to someone online and she suggested that I should rather contact a spell caster that could help me cast a spell to bring him back but I am the type that never believed in spell, I had no choice than to try it, I mailed the spell caster, and he told me there was no problem that everything will be okay before three days, that my ex will return to me before three days, he cast the spell and surprisingly in the second day, it was around 4pm. My ex called me, I was so surprised, I answered the call and all he said was that he was so sorry for everything that happened, that he wanted me to return to him, that he loves me so much. I was so happy and went to him, that was how we started living together happily again. Since then, I have made promise that anybody I know that have a relationship problem, I would be of help to such person by referring him or her to the only real and powerful spell caster who helped me with my own problem and who is different from all the other ones out there. Anybody could need the help of the spell caster, lotto, his email is DRAISEDIONSPELLCASTER@OUTLOOK.COM you can email him if you need his assistance in your relationship or any other problem like wining lottery.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>