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ancestors and a more realistic model of per-chromosome recombination, and thus his results should be considered to have superseded mine.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

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319 Responses to How Many Ancestors Share Our DNA?

  1. Thynaya williams

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  8. A broken heart can be one of the most painful things to heal from and can take a very long time to heal. During these times it can feel like the whole world could light up in flames and it still couldn’t compare to the pain inside. Of course, admitting a broken heart can be a difficult thing to do and most of us try to continue on with life masking the pain in our heart. With this pain comes many emotions. I was also trying to masked my pain, until i found help, here is my story. My name is Hope Wilian from USA, My Wife and I are both directors and shareholders in our family business. We have been married for 20 years but my wife is now going for a divorce, i agreed in the first place but i later realized what happens to my family and what rights will I have in the business if my wife divorce me, and i have worked so hard for my family and this business, i was incomplete and wondered what to do because he already went to court. I was browsing on the internet when i saw a post on how a spell caster called Dr lugard the great spell caster that helped someone bring back her ex, so i wondered if he can help me, i gave him the try and contacted him at his email { endofpainspelltemple@gmail.com or endofpainspelltemple@outlook.com, drlugardendofpainspelltemple@yahoo.com }, he told me not to worry that everything would be alright, i gave him the chance to cast the spell, he told me that in 48 hours my wife will come back and beg me, i never believed it at first, surprisingly completing 48 hours he came back and started to beg me not be to be annoyed about what he did, and now we are happy again. Thank you Dr lugard today my home is restored back if you are going through similar problem you can contact him at his private Email: endofpainspelltemple@gmail.com or endofpainspelltemple@outlook.com, drlugardendofpainspelltemple@yahoo.com you can also call +2347089339977 Whats-app HIM if you wish on+2347037098000, thanks for sharing from my experience.. Thank you Sir, i will always be testifying about your good work

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  12. Be careful about love spell comments here I got scammed by three of them,I lost over $1570 until a friend direct me to Dr Amina who help me get back the father of my two kids after he left me for over 2 years.and now my ex is fully back and he love me and my kids like never before.if you need help with love spell then mama Amina is the solution to your problem she charge me only $135 and her love spell work like magic.she can be reach at (draminalovespell@gmail.com

  13. Victory at last after i almost lost am man to another woman and all my saving to fake spell casters.
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  16. Am Tanya Albert, Dr. Ekpiku God will continue to bless you more abundantly, for the good works you are doing in peoples life, I will keep on writing and posting testimonies about you on the Internet, I’m Tanya Albert, I was a HIV patient, I saw a blog on how Dr. Ekpiku cured someone, I contacted him and also got my healing, kindly email him now on ekpikuspelhomeofgrace@gmail.com or ekpikuspelhomeofgrace@hotmail.com email me on tanya.albert58@yahoo.co.uk. THESE ARE THE THINGS Dr. Ekpiku. . HERPES . HIV/AIDS . CANCER…

  17. I want to give thanks and I will always give thanks to DR.OKITI who brought back my ex lover that has left me for 3years within 48hours, I have said about this last week but I promised to always tell people about this every week end so that those that did not read about it last week will read about it this week, I have been looking for how to get this boy back to my life because I love this boy with the whole of my heart, I could not replace him with any body,one day I was watching my television when I saw a lady giving thanks to DR.OKITI and telling the world how he helped her I was so shocked I could not believe it because I never taught that there are powers that can bring back lost lover, then that was how I decided to contact him too because I do really need my ex lover back,when I contacted him I told him everything and he told me not to worry that my ex lover will surely be back to my arms within 48hours at first I could not believe because I was thinking how could somebody that has gone for 3years come back within 48 hours,so then I decided to watch and see,unbelievable within the next 48hours I got a call from unknown number so I decided to pick the call the next thing I could hear was my ex lover voice he was pleading and begging me on the phone that I should forgive him that I should forget all that have happened that he did not know what came over him,he promised not to leave for any reason, that he was really sorry for what he did,i was so surprised because I never believed that this could happen,so that was how I accepted his apology and the next morning he came to my house and still pleading for me to forgive him I told him that everything is okay that I have forgiven him, that was how we started again and now we are married, I promised to say this testimony in radio station, commenting this testimony is still okay but before this month runs out I promise to say this in radio station and I will,sir thank you very much.World please am begging you people to try and thank this man for me,or if you need his help here is his email address:Dr Okitispell@gmail.com

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  19. Stephen wathan

    Dr. khumalo God will continue to bless you more abundantly, for the good works you are doing in peoples life, I will keep on writing and posting testimonies about you on the Internet, I’m faith Stacey, I was a HBsag patient, I saw a blog on how Dr. Khumalo cured someone, I contacted him and also got my healing, kindly email him now on healthmedlab@gmail.com THESE ARE THE THINGS Dr. Khumalo DO. HERPES, WITCH CRAFT PROBLEM,HIV/AIDS. CANCER. ALS Hepatitis B/C .NEED YOUR EX BACK, PERFECT CHARM, NEED A BABY, thank you and God bless you for saving my life

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