DNA Calculator: Another reason to IGNORE Small Segments

23/12/14 edit to add: From Felix Chandrakumar, ” I just updated the DNA calculator to customize thresholds/settings along with 2 predefined options for total and recent ancestry. Hope this will avoid confusion. I also made the other 3 tools obsolete.”

You will no longer be  able to replicate my experiment, but you will still be able to calculate your total recent ancestry.  Enjoy!

Felix Chandrakumar has come out with another gadget to amuse the DNA aficionados over the holiday season called the DNA Calculator.    According to Felix, “DNA Calculator will accurately tell you how much percentage of DNA is shared between two people. The accuracy is within the range of 1-2%. It supports FTDNA, 23andMe and Ancestry files.”

As an administrator for a few family members, I was able to run our raw data through the DNA Calculator and determine our shared percentages:

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The results were odd for several reasons.  To begin with,the percentage of DNA shared between close relatives was not what one expects to see.  ISOGG has published the following statistics:

  • Grandparent and Grandchild on average share 25% DNA
  • Great-grandparent and great grandchild on average share 12.5% DNA

Why was I sharing 39-45% of my DNA with my grandmothers?  Why were my sons sharing between 35-41% of their DNA with their great grandmothers?

In addition to the seemingly weird shared percentages, the entries marked with asterisks aren’t related in a genealogical time frame.  They don’t appear on each others match lists at FTDNA or Gedmatch, nor do they come from similar geographical areas.

  • Maternal Grandmother and Paternal Grandmother sharing 31.67%

I was surprised enough by the high percentage of shared DNA among unrelated people to not only re-run their files but to also re-download the raw data and make certain it had not been mis-labeled.

At Gedmatch I compared my grandmothers head to head:

Comparing Kit F364959 (*Duffy) and F372234 (*Kern)
Minimum threshold size to be included in total = 700 SNPs
Mismatch-bunching Limit = 350 SNPs
Minimum segment cM to be included in total = 7.0 cM
Largest segment = 0.0 cM
Total of segments > 7 cM = 0.0 cM
(1813) No shared DNA segments found

When I dropped their default values at Gedmatch to 100 snps/1 cm I began to get an idea as to why they had such strange shared percentages.  Across all of their chromosomes, the results were similar to the figures below:

Featured imageFeatured image

A person’s total amount of DNA is 6766.2 cm.  Gedmatch showed my grandmothers sharing 1253.3 cm, with the vast majority of their shared segments  in the 1-2 cm range, and a handful up to 4.8 cm. With my grandmothers sharing about 16% of their DNA at mostly 1-2cm segments, the rest of their shared DNA is beneath the 1 cm  default I had set.

22/12/2014 edit to add:  Felix Chandrakumar confirmed the conclusion I was coming to, “If I want to be clear, the tool is meant to know the shared DNA which tells your total percentage of shared ancestors in each others pedigree.”  None of these ancestors are recent.  The shared ancestry isn’t even population based.  It is human ancestry.  We all descend from the same original population and are of course going to have many shared ancestors in the human pedigree.

The DNA Calculator. is a fun enough bit of kit for immediate relatives or even unrelated individuals, as long as the user is aware that all DNA is being measured, not just segments above 7 cm/700 snps and the total shared DNA % will reflect that.  Given the ongoing debate of how low you can go in setting default values when comparing kits, this tool is a useful illustration that those very small segment matches are in everyone’s DNA, not just your matches.

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4 thoughts on “DNA Calculator: Another reason to IGNORE Small Segments

  1. You said “The DNA Calculator. is a fun enough bit of kit for immediate relatives or even unrelated individuals, as long as the user is aware that all DNA is being measured, not just segments above 700 cm/700 snps and the total shared DNA % will reflect that.”

    It’s not really important, unless you’re a perfectionist but I think you might have meant 7 cM, not 700 cM. Great article though!

    Liked by 1 person

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