April 17, 2014

How Much DNA Do You Share With Your Blood Relatives?

How much genetic material do you share with your biological relatives?  Take a look at the family tree below and the figure in the red box is the percentage of genes your body has in common with your blood relatives.  For example, your first cousin has 12.5% of the genes you do (implying, inversely, that 87.5% of their genes are different).  Your third cousin twice removed, on the other hand, would have only 0.195% of the same genes, meaning 99.805% of their genes would be different.  This assumes, of course, that you have no double relations in your family tree (e.g., sharing a great-great grandmother from two sides of your family tree).  

DNA Shared Between RelativesThe chart also ignores relatively rare phenomenon such as the elusive double cousin.  These relationships arise when two siblings of one family reproduce with two siblings of another family.  This results in the children being related to each other through both parents, and sharing the same grandparents.  As a result, double cousins are genetically equal to half-siblings, sharing double the genetic material normally seen in first cousins.  

  • Doug

    Not sure I understand. I would expect to share well over 99% of DNA with my immediate relatives. How do you calculate that I share only 50% DNA with them?

    • Joshua Kennon

      Your body was created by taking roughly 50% of the genes from your father and 50% of your genes from your mother.  If you have siblings, different genes in different combinations were passed onto then due to sexual reproduction creating a roulette effect, resulting in siblings sharing roughly 50% of the genetic material, as well.  

      The exception is identical twins, who share 100% of DNA due to coming from the same egg and the same sperm, which split and began reproducing independently from the same gene pool combination.

      On a related note, a common misconception is that identical DNA must result in identical outcomes.  Identical twins, for example, have different fingerprints, which you can read about here: http://www.nytimes.com/2009/10/06/science/06qna.html.  If one twin suffers from autism, there is only a 90% chance the other will, too (not 100%).  If one twin is gay or lesbian, there is only a 50% chance the other is too (not 100%), etc. etc.  This is because of, in simplified terms, the fact that our genes have ‘on/off’ switches attached to them that can be flipped through conditions in the womb, environment throughout life, exposure to certain chemicals, and a host of other factors.  For a very basic explanation of how genes get flipped on or off, read http://www.thetech.org/genetics/ask.php?id=63

      And, of course, it’s fascinating how our ancestral DNA plays a role in our day-to-day lives.  Most of the humans alive on planet Earth cannot drink milk; the figure is around 70% for adults.  If you live in the United States and are white, though, you wouldn’t realize this because your ancestors developed a mutation that allows us to process cow’s milk.  This explains more: http://www.thetech.org/genetics/ask.php?id=250

      If you prefer a hierarchy written out, Genealogy.com has that at http://freepages.genealogy.rootsweb.ancestry.com/~laetoli/degree.html

      • Doug

        Ahh ok.. it is just a semantic difference then. I was comparing your percentage to statements like “Humans share 98% of their DNA with chimps”. That percentage is about how much of the data contained in the DNA is the same regardless of ancestry, where you are talking more about the bloodline path of where it came from.  Thanks for the clarification!

        • http://andrewdouglass.com/ Andrew Wells Douglass

          I agree the “DNA shared” figures are misleading. They make it sound as if there were an infinite numbers of genes, when of course there are only so many ways to code for whatever protein. Perhaps it would be better stated as “probability you share a given gene because of shard ancestry” for, say, eye color (which is itself polygenic, not blue-or-brown as taught in grade school!). Some genes also tend to be transmitted together. You share some/many genes by pure coincidence. Thanks.

      • TL Howard

        Actually, the most recent and large sampled research (Australian Twin Registry) plus another (forgot its name, sorry) shows that the concordance of monozygotic for homosexuality (male) is only 20%. There is obviously a biological cause/trigger (with only a genetic susceptibility to that environmental trigger) to male homosex.

      • Drakkar77

        The gene that processes lactose can be switched back on by consuming enough milk, at least in some people. Epigenetics is still a rather new science and so it’s not understood as well yet.

      • Matthew

        Am I 50% my father and 50% my mother then? Am I not unique? Is there anything unique about me? I dont want to be 50% someone and 50% of someone else :(

        • http://www.joshuakennon.com/ Joshua Kennon

          Your genetic material is but the combination results in a completely new, never-before created outcome that is entirely unique.

        • Matthew

          But still that combination is 50% my mother and 50% my father??? It makes me feel sick.

        • -A

          I actually think it’s a beautiful thing. You come from both your parents, but are made from a very unique combination of their DNA – as well as that of your grandparents, great grandparents, etc.

        • Matt

          Are you serious? How does that make you feel sick?? That’s the entire point of replication.. survival. We are the result of everyone who has come before us. (see what I did there?)

        • Matthew

          It makes me feel sick because it means there is no me. If im half my mother and half my father, wheres the me part? See what I didnt do there…?

        • Frank

          No I don’t. By your definition no organism that has ever lived is much of themselves. Only thing that are more “themselves” than the rest are tumours. Trying to relate the “you” as a philosophical term to genetics is just plain dumb.

        • Matthew

          Tw*t. Off course you didnt see what I DIDNT do there.
          Im glad my question revealed your non-existent sense of self, and you then reintergrated yourself by making the source (my question) invalid. Too “philosophical”?

        • Jeff

          It makes me feel sick how dumb you really are, Matthew.

        • Matthew

          There we are then thanks very much.

        • Matthew

          Did you read Franks comment before you responded? And dont use my name like you know me, Jeff/tw*t.

  • K

    What are the figures for half-sibling relationships?

    • http://www.joshuakennon.com/ Joshua Kennon

      Cut the sibling percentages in half.

  • Tashina

    I found out I have a cousin whos dad is my mom’s scound cousin. And his mom is my dad’s scound cousins aswell, how much DNA do we share?

  • http://www.joshuakennon.com/ Joshua Kennon

    This is a joke, right? Please tell me you realize the numbers shouldn’t add up to 100% …

  • http://www.joshuakennon.com/ Joshua Kennon

    Again, this is a joke, right? …

    • Frank

      There are people believing Santa Claus is invented by Coca Cola and the moon is made of cheese, what can you do.

  • John Tate

    Why would the numbers add up to 100

  • jjay09

    why is it that even charts displaying the family tree is done so in a such a sexist way

    • http://www.joshuakennon.com/ Joshua Kennon

      Given that there were two possible ways this chart could have been presented by whomever designed it, it is impossible to say whether or not the chart is presented in a sexist way as there was a 50% probability of either the paternal or maternal lineage being used as the illustration (a complete family tree would have been more difficult to read and therefore not as optimal to utilize as a visual aid so we will exclude it from the analysis).

      Absent other information, a determination of sexism is impossible. For example, had a maternal family tree been used, a man could have claimed it was sexist but it would not make it so. There simply isn’t enough evidence to indicate either way.

      Without said evidence, or any additional supporting data, the conclusion that the chart is sexist is a form of subconscious confirmation bias meant to strengthen a present conviction in the observer that reinforces preconceived ideas.

      (An an interesting discussion of second and third order effects, one could argue, upon studying, say, 100 textbooks with family trees in them, that a distribution that represented more than an approximate 50/50 split would be sexist but even this would be problematic as people are influenced by their life circumstances. In the lower and middle classes, nearly 1 out of 2 boys are now born to a mother out of wedlock, which would theoretically lead to a higher probability of them using her bloodline if they were preparing a chart such as this, skewing the results in favor of women. On the other hand, given past legal precedence based on the nearly now-defunct patriarchal structure of marriage a century ago, names are passed down from the male in almost all cases as a result of cultural practice, meaning that a person might be more interested in knowing where his or her namesake originated due to a myriad of subconscious forces, such as mere association tendency (which is so powerful, there is overwhelming statistical evidence that boys with names like “Dennis” become dentists far more likely than should be the case; it’s the way the human mind pulls us in certain directions). Therefore, you could expect the distribution to favor males. Still yet, whomever prepared the chart could be a medical student studying Y-STR on the Y-Chromosome and want to illustrate the unique situation found exclusively in males that causes each and every person on the male only chart to share an identical copy of nearly half the genetic material that composes their body.)

      TL;DR: It is mathematically impossible based on present evidence to conclude whether there was any form of gender preference in the creation of the chart.

      • Jonelle

        try this article – Charlemagne’s DNA and Our Universal Royalty

        We’re all probably related!

  • stacie

    I have a first cousin that i believe is my half brother what would the percentage be if we did a dna test?

    • http://www.joshuakennon.com/ Joshua Kennon

      If he is your cousin, 12.5%. If he is your half brother, 25%.

  • Jay Sullivan

    This chart is completely wrong. This is not taking into account that the most unrelated people in the world share about 99.5% of their DNA. Taking that into account, siblings share about 99.75% of their DNA ((100% + 99.5%) / 2). Cousins share about 99.615% ((99.75% + 99.5%) / 2). Second cousins share about 99.5575 ((99.6125%+99.5%) / 2), third cousins 99.52875, etc.

  • Melissa Ramirez

    What is he if we have the same Grandmother but different grandfathers? Do we have the same blood?

    • http://www.joshuakennon.com/ Joshua Kennon

      You would cut the appropriate figures in the chart in half (divide by 2) as you share half the common ancestors.

  • wayne

    I assume when you say siblings share 50% genes that this is for all practical purposes an approximate figure. If I have 4 brothers, they all share 50%, yet a different combination of the original pool?

  • Rae

    Not sure I am understanding. I met someone I enjoy spending time with and recently was told that my great grandfather and their biological great grandmother are brother and sister. I was devastated and was trying to figure out are we really genetically related and if so, what is the relationship.

    • http://www.joshuakennon.com/ Joshua Kennon

      That means your shared common ancestor is on the great-great grandparent level. You are 3rd cousins with only 0.781%, at most, shared genes. You are practically strangers. None of the 50 states even ban second cousins and you are even more distant that that – I know of nowhere in the world, throughout all of history, where such a congress would be forbidden.

      Frankly, I can’t name a single one of my third cousins and wouldn’t know them on the street if I met them. I imagine there are a significant amount of people in the United States married to their third cousin and don’t have a clue. This is not something that should cause you any emotional distress.

  • bianca

    Is the DNA percentage less for second cousins who share the same great grandmother but different great grandfather? would it be next to nothing? I apologize for my ignorance in advance.

    • http://www.joshuakennon.com/ Joshua Kennon

      Yes, the number would be cut in half as you only share a single common ancestor instead of two common ancestors. You’d only share a maximum of 1.5625% of your genes. There should be no issues with birth defects were such people to meet randomly, get married, and have a child, nor would such a union be prohibited anywhere on the planet to my knowledge.

      • kim

        I didn’t grow up with my boyfriend or know him
        but found out that my dad and his mom are 1st cousin s and we share the same great grandparents…..what are we 3rd cousins?

        • AntonDubinksy1960

          You are first cousins once removed, so share about 6% of the same genes.

  • JD

    I don’t understand what they mean with “first once cousin removed” or “second cousin once removed”. Why does removed mean there? Why remove them?
    I would appreciate if someone could explain please. Thanks very much.

    • JD

      apparently I made typo: I meant: “first cousin once removed” and what does “removed” mean there? and the rest of the questions…
      Thanks very much for your help.

      • Frank

        Weird English family member classification that basically

        screws up anything to do with generation. Without the removed and stuff basically your father’s cousin is your cousin too……

  • Brad

    I was adopted at 3 days, I know my mother, I think I know my father, Could I DNA test my possible sister’s daughter and match to mine to see if we match. (Sister would have different mother). Would it be enough DNA match to see if I am part of that family?

    • JD

      why not compare your DNA with possible half sister then you get better results to show you are or not part of that family. There should be about 25% match between you and her. With her daughter the percentage obviously would go down more to 12.5% match or even less I think…

      Best wishes…

  • Libby Hanna

    i’m a double cousin, mums brother married dads sister, its super interesting to know how genetically similar I am to my cousins!!!!

  • kristy

    Why would there not be a 50% in the mothers box that makes no sense to me

    • Frank

      Assuming either parent is the same? Duh?

  • Tom

    In the context of us sharing 96% of our genes with chimpanzees how does this chart make sense? Do I share more genes with a chimp than my sister?

    • Caihlyn

      100% of your genes are from ‘humans’. 50% of your genes are from a specific human being, your mother and the other 50% are from a specific human being, your father. You are 100% made of human genes. Chimps share 96% of the same genes as ‘humans’ … not a percentage of a specific human being’s genes.