Wednesday, July 18, 2018

Genealogy Explained: "Do Siblings Have the Same DNA?" Graphic

Mark Orwig is the writer of the Genealogy Explained website, and he has created a neat graphic to explain the answer to the question posed  in "Do Siblings Have the Same DNA?"

I encourage all of my readers to read the entire article because the science is interesting and the "deck of cards" example is familiar to most of us.  The graphic is:

Pictures are always better than many words, and this graphic shows why siblings (assuming they aren't identical twins) may not have the same ethnicity estimate even though they have the same parents.

In the graphic, there are four grandparents who have perceived ethnicities of:

*  Paternal grandfather:  50% Scottish, 50% Irish
*  Paternal grandmother:  100% Italian

*  Maternal grandfather:  100% German
*  Maternal grandmother:  50% Italian, 50% Irish

Based on the percentages, one would expect the ethnicities of the parents to be:

*  Father:  50% Italian, 25% Scottish, 25% Irish
*  Mother: 50% German, 25% Italian, 25% Irish

Based on those percentages, one would expect the ethnicity of a child to be:

*  37.5% Italian, 25% German, 25% Irish, 12.5% Scottish.

However, the graphic shows that the two children have significantly different ethnicity estimates (based on a DNA test) than the expected ethnicity based on the expected ethnicity of the parents.  This occurs because of  the random nature of admixture - how the DNA segments are passed down to the next generation.

For me, I have noted that the six different ethnicity estimates I have received are completely different, and do not match my perceived ethnicity based on classical genealogy research.  See Comparison of My Autosomal DNA Ethnicity Estimates.

I understand better now that I need to test my two brothers and both of my daughters to get a better handle on the ethnicity estimates for my family members.  Unfortunately, I did not have the opportunity to test my parents (who died in 1983 and 2002) and grandparents (who died in 1942, 1962, 1976, 1977).

Like I said - Read the entire post!  Thank you to Mark for permitting me to use the graphic in this blog post.


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Eileen said...

Thanks Randy. This is a really great graphic and an excellent way to explain ethnicity estimates to a beginner or even those that have been working with DNA for a while.

Karla said...

Randy, your graphic is really good, and so is the explanation of siblings having very different ethnicity, but the problem here is that the granddaughter is not going to inherit her maternal grandfather's German dna intact. It just will not happen, so this chart is also misleading to the very beginners you are explaining this to.