Shared Ancestors

Genetic genealogy is the application of genetics to traditional genealogy. Genetic genealogy involves the use of genealogical DNA testing to determine the level and type of the genetic relationship between individuals.
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darkerhorse
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Shared Ancestors

Post by darkerhorse »

You have 64 4th great-grandparents, including 32 from your paternal side and 32 from your maternal side.

If your paternal grandparents were 3rd cousins to each other then, starting at your 4th great-grandparents, half of your 32 paternal 4th great-grandparents are shared, resulting in only 16 unique individuals on your paternal side and 32 on your maternal side for a total of only 48 unique individuals.

If all of your 4th great-grandparents were in the same room, you'd need 32 chairs for your maternal side but only 16 chairs for your paternal side.

So, aren't you expected to inherit twice as much from these 16 compared to if you had the full complement of 32 unique individuals on your paternal side?

In other words, aren't you expected to have inherited more from them than from the others of that generation?

I know you still only inherit half of your DNA from your paternal side but aren't you expected to inherit a disproportionate amount from those 16 shared 4th great-grandparents compared to the other 32 -
2/64 or 3.125% for each paternal vs 1/64 or 1.5625% for each maternal?

The 50% you inherit from your maternal side would come from 32 persons, while the 50% you inherit from your paternal side would come from only 16 persons.

Since one of those 16 carries your surname, does that mean you are more your direct surname line than persons who don't have shared ancestors?

For example, if your surname was Russo, then, would you be more a Russo than other Russos who don't have shared ancestors?

In general, would you be more your surname than others, who don't have shared ancestors, are their surnames? For example, more a Russo than they are a Morello?
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MarcuccioV
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Re: Shared Ancestors

Post by MarcuccioV »

I understand what you're driving at because I share the same situation. I have a pair of the same 4GG's on both my grandfather's maternal and paternal sides, so only 62 4GGp's. In theory your hypothesis would make sense, but in the real world the #'s wouldn't be that precise. The further back you go, the less of each generation you receive. It all depends on what the generations before you received, and throw in the randomness of recombination and I think it could vary widely between individuals.

For example person "A" (in the same hypothetical equation) might get more DNA from the shared 4GG's, and person "B" much less. It's altogether possible to get NO DNA from these individuals (the 4GG's).

I think it's really a roll of the dice in any case...
Mark

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darkerhorse
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Re: Shared Ancestors

Post by darkerhorse »

I think I made a math error. It should be 30, not 16 4th great-grandparents on the paternal side. Only one couple is shared.

The principle is the same.

I was asking about "expected" not "actual" inheritance.

By the way, In my case, they are paternal cousins so the surname is shared.
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MarcuccioV
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Re: Shared Ancestors

Post by MarcuccioV »

darkerhorse wrote: 21 Nov 2021, 14:01 I think I made a math error. It should be 30, not 16 4th great-grandparents on the paternal side. Only one couple is shared.

The principle is the same.

I was asking about "expected" not "actual" inheritance.

By the way, In my case, they are paternal cousins so the surname is shared.
I guess theoretically, then yes, I suppose the odds would be in favor of slightly higher inheritance. And yes, 30, not 16. Unsure the shared surname has a place in the equation except for identification, though...
Mark

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darkerhorse
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Re: Shared Ancestors

Post by darkerhorse »

I stressed the surname line due to the subjective factor - we identify with that line, having that surname - even though it's only one of 64 (62) surnames we have at that generation.

Again, in my case, that's also the duplicate couple.
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MarcuccioV
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Re: Shared Ancestors

Post by MarcuccioV »

I think another factor to throw in would be how the tree goes from the shared ancestors -- in my case, since they are on different lines (one maternal and one paternal), their DNA was also shared by 2 separate 3GG's, again by 2 GG's and finally by 2 GG's before reaching my grandfather, so in that case, I think the case for a higher % of their DNA is justified.

If they were all along the same branch, I'd think maybe at some point they'd cancel each other out.

Maybe someone else has a few pennies they'd like to throw in...
Mark

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MarcuccioV
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Re: Shared Ancestors

Post by MarcuccioV »

I went through the list of ALL my 4GG's.

There are 2 surnames that appear 3x at that level (2 of which are the same people).

2 other surnames appear twice (different individuals).

16 surnames appear once (that I know of).

There are 5 yet-unknowns.

That makes 20 different surnames plus the 5 unknowns (which may be the same as some of the knowns) making 25 max out of the 32 possibles for that side...

I already have a headache thinking about it...
Mark

If you ignore your foundation, your house will eventually collapse...
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