How Much DNA I Share With Each of My Grandparents

Last week I finally received the results of my first DNA test.  Yay.  I wrote about it in My DNA Results are Back; Break Out the Chocolate!  Like every good genetic genealogist should, I transferred my results to other platforms to connect with other family members and to use their tools.  I transferred my Ancestry results to GEDmatch, FamilyTreeDNA, and MyHeritage.

While looking at GEDmatch, I saw the results for my three tested grandparents on my match list.  Each shared somewhat different amounts of DNA with me.  How interesting!

At the start of my DNA journey, I had assumed that children get 50% of their DNA from their parents and 25% from each of their grandparents.  The first assertion is true, but the second is not.  Half of our DNA is indeed passed to us from each of our parents.  But each of those halves is a scramble of the DNA received from their parents.  On average we received 25% from each grandparent.  Blaine Bettinger collected statistics on observed DNA sharing for various statistics and put them in the ISOGG charts described here: DNA Statistics, Average and Range of Shared DNA for Various Relationships.  For grandparents the range is 1272 – 2365 cM, which is about 37% to 69%.

How much autosomal DNA I received from each of my grandparents:

  • Paternal grandfather               30%
  • Paternal grandmother             20%
  • Maternal grandfather              23%
  • Maternal grandmother            27%

While using GEDmatch:

  • I compared my phased results to the phased results of each of my parents to get the best results possible.
  • The size of the match when using the one-to-one comparison tool (keeping all the defaults) sometimes differs from the match in the one-to-many chart.  I believe the one-to-one comparison is supposed to be more accurate.
  • Using the phased data, the match with my father was the same but the match with my mother became slightly larger.  Must be an artifact of phasing, because no new matching could be created.  Her larger number was closer to my father’s result though, and equal results would be expected.
  • Match sizes generally decreased slightly using phased vs. unphased DNA (except with my parents), showing that I had some identical by state (IBS) vs. identical by descent (IBD) matching with them.  This is why we do phasing, to remove IBS when we can.  However, my results with my paternal grandmother actually increased when I used my paternal phased data and my father’s maternal phased data vs. my paternal phased data and her data.  Likely another phasing artifact.
  • The shared autosomal cM with each of my parents was about 3585 cM.

Genetically, I also discovered that I share more DNA with my aunt than I do with two of my grandparents.  The ISOGG autosomal statistics charts show that the average genetics shared with an uncle/aunt and grandparent are the same.  Theoretically it is expected.  Seeing it helped make it real to me.

I also looked into my X-DNA matching.  How much X-DNA I received from each of my grandparents:

  • Paternal grandfather                 0%
  • Paternal grandmother             50%
  • Maternal grandfather              21%
  • Maternal grandmother            29%

While I shared the least amount of autosomal DNA with my paternal grandmother, I get the most X-DNA from her.  One half of my X chromosome, from my father, matches across her two X chromosomes together.  Theoretically I knew that, but it is so interesting to see.

It is fascinating to see the randomness of inheritance in action.  Happy researching!


Posted in DNA

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