What percentage of Spanish would I be if my 21st great-grandpa was 50% Spanish?

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So I'm wondering, what exact percentage of Spanish I am, my great grandpa 21 times "Anastasio" was 50% Spanish. Would there still even be Spanish blood in my DNA, or would it just cancel out?

Ryan Conors

Posted 2017-06-19T21:43:39.447

Reputation: 9

Answers

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You are effectively 0% Spanish.

Theoretically, you can perform some calculations. You are descended from 223 = 8,388,608 twenty-first great grandparents. Of course, due to inbreeding there will not be that many (i.e. some will appear in your tree more than once), but let's just carry on with the calculations ignoring that fact.

Now, if you work under the assumption that you inherited exactly the same ammount of DNA from each of your 21st great grandparents, then you inherited exactly 1/8388608 or 0.0000119% of your DNA from Anastasio. If you'd like to you can half that number to determine a percent Spanish.

But that's simply not true, because you do not inherit DNA from every one of your ancestors. Due to genetic recombination, you inherit more DNA from some ancestors than others. While you inherit exactly 50% of your autosomal DNA from your mother and father, your maternal and paternal gametes had already undergone genetic recombination, meaning you might have inherited 22% from one grandparent and 28% from another. Now amplify this over 23 generations, and you'll find that you inherit no DNA from some ancestors. This is why when testing autosomal DNA and comparing with others it becomes increasingly difficult to identify genetic links beyond 6 or 7 generations back. See the blog post titled How much of your genome do you inherit from a particular ancestor? for more information on this topic.

In summary, the chances are extremely high that you inherited no DNA from that particular 21st great grandparent, therefore you cannot claim any Spanish DNA via this line.

If you are descended from this ancestor in the patrilineal (male) line, then this calculation will be different as the Y-chromosome is transmitted father to son mostly unchanged, even over tens of generations. In such a scenario your patrilineal 21st great grandfather has more significant represention in your overall genetic makeup.

Given the date that ancestor must have lived, I would also be skeptical as to whether you can provide any substantial evidence confirming that that you are descended from that particular 21st great grandfather, but that's neither here nor there.

For a more general discussion of ethnicity percentages see Determining ethnicity according to ancestry?

Harry Vervet

Posted 2017-06-19T21:43:39.447

Reputation: 17 763

Great answer! Totally agree. The only thing is left, which worries me - how probable are mutations in 400000 SNPs tested by Illumina chips (FTDNA, 23andMe and so on) in each generations? – GEORG GAAL – 2017-06-20T05:52:35.317