DNA Inheritance Is Passed Down Randomly. So Randomly That I Am 24% More Irish Than My Brother.

DNArec (1)I bet you thought that you were 100% related to your full biological siblings.


Siblings share, on average, about half their DNA. The reality is, however, we can actually be anywhere from 0 – 100% genetically related to our siblings! When looking at DNA test results, you could, theoretically, be totally unrelated to a sibling, though the percentage usually falls in the 50% range.

To confuse you more, your ethnicity results in an Autosomal DNA (atDNA) test can be quite different from sibling to sibling, as we each inherit unique combinations of DNA from our parents that present different parts of our genetic history.

Photo credit- www.genetics.thetech.org

DNA recombination from parents to their children. Photo credit- www.genetics.thetech.org

This stems from how DNA is passed from one generation to the next for the majority of our genome. You are unique, having received 50% your DNA from each of your parents. Your parents received 50% from each of their parents, and so on. The 50% passed to you from each of your parents was a shuffled combination of genetics so, unless you and a sibling are identical twins, you can expect your results to be different than your siblings. Recombination is purely random, so one sibling could inherit substantial chunks of DNA that the other sibling did not inherit—or vice versa. Sometimes, the differences in results can be surprising.

My brother and I each tested with AncestryDNA, a company that offers the Autosomal DNA SNP test (we have also tested with competitive companies, which offer different types of tests and differing abilities to analyze resulting data). A genealogical DNA test studies a person’s genome at specific locations, and several different types of tests are available. The Autosomal test utilizes DNA from the 22 matched pairs of autosomal chromosomes we all have, or “autosomes.” For the record, you actually have 23 pairs of chromosomes. The remaining pair, called the sex chromosome, determines your gender, male or female. An Autosomal DNA test may be taken by either a male or female, and is often used to search for relatives (on either side of the family tree), called “DNA cousin matches,” up to a maximum of 6 – 8 generations back.

The Ancestry test also provides participants a colorful pie chart, which gives you a percentage breakdown of your ethnicity by region. It is called an Ethnicity Estimate or an admixture test. Ancestry, the company, divides the world into about 25 modern reference populations, or regions, and the approximate percentage of DNA inherited from each is provided. Sections of your DNA are identified that best match the reference databases. However, the reliability of the results is dependent on a number of variables, such as comparative population size (which can be limited), the number of markers tested, and the degree of admixture in the person tested. Distinguishing between populations within continents can be difficult; as well, genetic ancestry does not respect country borders, which change often, or the migration of ancestors long ago. A person with “known” German ancestry may find zero German DNA in his/her results. This issue, most likely, is that at some point in history the ancestors of this family, who did not originate in Germany, moved to Germany and “became” German. But that does not make them genetically German.

Fortunately, as science and technology improves accuracy of the ethnicity reports continues to improve and to provide greater depth of detail. For example, results previously defined as Western European are now being broken down into subgroups such as English and French or Irish, German, etc. Results only defined as African previously are starting to break down into specific countries. Southern Europe is broken off from the Iberian Peninsula. As new algorithms are developed providing more accurate results, these improved results will show up in your account at no additional charge.

The test results for my brother and me state that we are “immediate family”—in other words, full bio brother and sister. That’s good to know! But then the results follow different paths. My brother’s DNA test results provided the following Ethnicity Estimates:

91% Great Britain; 5% Ireland; 3% Trace Regions (Italy/Greece)

According to the current AncestryDNA algorithms, my ethnicity admixture results are:


My Ethnicity Estimate from AncestryDNA

37% Great Britain; 29% Ireland; 21% Europe West; 8% Italy/Greece

Since I now know that I’m 24% more Irish than my brother, I will celebrate a bit harder at St. Patrick’s Day this year!

My admixture notes a 21% Europe West contribution, which doesn’t show up in my brother’s estimates at all. Europe West is defined as primarily Belgium, France, Germany, Netherlands, Switzerland, Luxembourg, and Liechtenstein. On our maternal side, we have three lines that trace back to France and two lines tracing back to Germany. Somehow, more of our maternal line DNA was passed to me versus my brother. I guess random genetics at work.

The bottom line of those results: Ethnicity estimations remain a science that are still a bit fuzzy. Humans have moved too far over too many thousands of years for it to be reliable history of where ancestors originated. And as each parent may pass a different and variable percentage of an ethnicity down to their children, results can be surprising. So, be open minded to the results received. As well, get as many of your siblings tested as possible. Doing so opens up new insights into your full ethnicity story, as well as the possibilities of finding additional cousins.

Do you have any unusual or surprising ethnicity estimates between you and siblings? If so, let me hear about them!


  1. Lisa

    Hello –
    Thanks for writing this. I do not have my own experience but this helps explain a LOT as it pertains to DNA testing. I told my father that he didn’t need to do a test because his brother did it, but now I think I will go ahead and have him move forward on it.
    Thanks again – I will try to stop back and let you know how it goes!

    1. Julie (Post author)

      Oh please do! I’d love to hear what you find out. And thanks for stopping by and commenting. It’s nice to know that my humble little blog is helping people discover their heritage.

  2. Anne

    Beautifully explained in laymans language. Your blog post should be required reading for anyone who does the AncestryDNA ethnicity test. Apparently there have been incidents where children accused their mother of having affairs because they failed to understand the complexities of DNA. Well done.

    1. Julie (Post author)

      I’m humbled– thank you for the kind words! I’m always happy to hear that my blog is being read, is helpful to those who stopped by, or is distributing accurate information!

  3. Jess

    I just recently got my results and they look completely different than my brothers.. completely different regions and all..also i have 22% italy/greece and he only has 7%..
    I don’t know what to think..

    1. Julie (Post author)

      Those percentage differences aren’t that unusual. It’s amazing how different two siblings can be.

    2. Maureen

      Well Julie, I am amazed at how different my sister’s and my DNA are:
      SISTER: ME:
      Europe 98% 99%

      Great Britain 40% 3%
      Scandinavian 30% 12%
      Ireland 12% 34%
      Europe West 10% 48%
      Italy/Greece 4% 1%
      Iberian Pen. 1% 1%
      European Jewish <1% 0%
      West Asia Caucasus 2% 0%
      South Asian 0% 1%

      When one takes the two most common DNA origins composing the majority of our DNA, my sister is basically British/Scandinavian and I am Western European/Irish. If siblings, on average, share 50% of their DNA, we share only 39% of our DNA.

      1. Anne

        Yours is extremely different for full siblings. While ‘possible’ it seems statistically very unlikely. I would contact the lab and explain the situation and see if they can give you a deal on a re-test. (Small percentages can be virtually ignored with these tests, and probably should be except for conversational value perhaps.)

        1. Julie (Post author)

          Anne, thank you for your advise back to Maureen. I agree with it.

        2. Maureen

          Thanks Anne for your comments. Ancestry did identify my sister as a first degree relative, however. I think that means a full sister… The results are surprising though. My first cousin was closer in composition to me!

  4. Sue

    Well, I feel a bit disappointed. I had been very excited and looking forward to my results, but if it isn’t going to tell me my ethnicity I wish I had saved my money! I am super bummed. I had thought I would be able to tell my son that he had half my dna.??? But he may not? I could be Irish and my son not Irish? Guess I am just stupid. I feel like I spent a hundred bucks for something I doubt I will even look at now.

    1. Julie (Post author)

      Oh, please look at it! Depending on what test you took, you will receive a breakdown of your ethnicity; that pie chart will provide an estimate of what locales your ancestors came from and approximately what percentage you inherited from each region. Yes, your son inherited 50% of your genetic makeup but WHAT 50% can’t be controlled. But he will still have fun learning what his own personal ethnicity is likely to be.

  5. Diana

    We just got the results of my children, and one of our kids has more Western Europe than my husband and I combined. How can THIS happen?


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