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deepseas72's avatar

To what extent is my twentieth great grandfather REALLY related to me?

Asked by deepseas72 (1076points) March 21st, 2008

I’ve been playing around with my genealogy lately, and I’m not entirely clear on the concept. I’ve traced a couple of lines back to a twentieth great grandfather. Is this so far back that there’s almost no genetic connection?

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19 Answers

oneye1's avatar

you could trace it all the way to noah if you don’t beleive in the bible then to the gow on the bottom of my shoe

omfgTALIjustIMDu's avatar

He would be your 20th-great-grandfather.

mcw's avatar

without him, there wouldn’t have been a you. So it seems like he’s pretty significant. :)

squirbel's avatar

If you are male, your Y chromosome is nearly a direct link. You also share similar mitochondrial DNA.

squirbel's avatar

By the way, be proud in the simple fact that you can trace back that far. It is an awesome feeling, or should be.

nayeight's avatar

yeah there’s alot of people who can’t trace back that far, a good majority of african americans being one example.

squirbel's avatar

Or Puerto Ricans…

nayeight's avatar

but I agree with mcw that without him there would not have been a u. Isnt it strange to think about what might have happened if by one thing in the past changing, it could change your existence? I often wonder about things like that….

nayeight's avatar

I wonder what race/nationality/people can trace back the farthest?

gailcalled's avatar

Both my grandfathers (born in Jewish schtels in Lithuania and the Ukraine) left autobiographies written in English w. details (dates, locations, names of ships, employment, etc) So I have been able to find an amazing amt .of info on sites like I have also found tons of stuff on the US census’ of 1900, 1910, 1920,1930; archived old phone books.

My great-grandfather on my paternal grandmother’s side was kicked in the head by a horse (info provided by a cousin)

Thus our family goes from Oscar Finkel in the mid 1800s to the delicious Madeline, aged two. And I just found a patent for a spring-opening umbrella from 1894, invented by Benjamin Finkel and two others. Good genes, that’s for sure.

That’s seven generations.

srmorgan's avatar

20 Generations is really quite an accomplishment, especially if you characterize yourself as merely “playing around” with the subject. I have been doing family research for over ten years and the furthest I can go back on any one branch is nine generations and for my particular ethnic heritage, that’s quite an achievement.

Recently I found a connection between my wife’s paternal grandmother to a family whose history had been documented in a 700 page book written in 1903. That family (Cheseborough, Cheesbro, Chesborough, and various other spellings) can be traced back to the arrival of the “patriarch” in 1630 along with the original boatload of Puritans who founded Boston Mass. and even further back by 100 years in Boston, Lincolnshire in England. I have not counted the generations.

Moreover, I can’t take credit for research done by the author of the above-mentioned book who did all of that research without tools like the internet, without the work done by the Church of Jesus Christ Latter-Day Saints and without access to microfilm copies of the US and UK censuses and various public and church records.

As far as your relationship to this ancestor, you carry his genes and that is about it.


nikipedia's avatar

Genetically? You have approximately 1/2 your father’s DNA, who had 1/2 your grandfather’s DNA, who had 1/2 your 1st great grandfather’s DNA.

So you share approximately 12.5% of your 1st great grandfather’s DNA (half times half times half). Let’s do that 20 more times: 0.125^20 = 8.67×10^-19. So you share:

0.0000000000000000087% of your 20th great grandfather’s DNA.

deepseas72's avatar

Nikipedia: thanks so much. That’s just the answer I was looking for. Kudos!

nikipedia's avatar

Oops, just realized my mistake and can’t figure out how to edit! It should really be (1/2)^23, or .000012%.

ordosingularis's avatar

The issue gets a little more complicated. We (humans) share between 1 and 5 (rough) percent with another random human sampled from the plant. So if you’re German, you share 50% with your sibling, 25% with your grandfather, 12.5% with your uncle, perhaps 5% with another German and perhaps 1% with an African. The gene pool simply isn’t that large to truly share only .000012% of one’s genome with another.

ordosingularis's avatar

To squirbel: In humans, mitochondrial DNA is inherited EXCLUSIVELY from the mother. And as for the Y chromosome being a “direct link”, that is not true. If at any point in his ancestry the genetic link is through a woman, that Y chromosomal DNA would be completely lost (well except for the recombining portion of the Y chromosome which is minimal and most of the genetic data would not be recomined into the Y chromosome anyways). I’m not trying to be a dick, but don’t pollute a discussion with misinformation.

ordosingularis's avatar

To clarify my first point, deepseas72, if a DNA test was done between you and this ancestor, it would reveal no more of a relationship to him than someone completely random such as myself. Nikipedia’s math is correct; that is how much of this ancestor’s genetic information was directly passed to you, but you incidentally (due to the relatively small human global gene pool) have several percent of the the same genetic information as I stated in my first post. If you care about this, I can give you some links. Let me know if I can help (I used to be a biochemist, though now I’m an engineer – more lucrative :)

srmorgan's avatar

I am not trying to refute anything you might have written, frankly I don’t try to understand the details and statistical of genetics.
I have read however of a DNA study concerning male Jews who are Cohanim, or descendants of Aaron, the brother of Moses. Belonging to this tribe passes exclusively from father to son to son, down through the generations. It would not pass to an adopted son or a step-son among other restrictions. The conclusion would be that all Cohanim are descended from Aaron and would therefore carry similar DNA in some way, shape or form.

The last item I read showed that 30% of the men who claimed to be Cohanim submitted their DNA in this study and there was a 30% correlation amongst the samples. I am ont sure what that means but this is certainly not a random result.

I wish I could be articulate about this study but I don’t have the background to convey this to you in a more coherent manner.


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