Generations and Ethnic Percentages

[darkerhorse]

My understanding is that since we have 128 5x great-grandparents, you'd expect to inherit about 1/128 or 0.78% of your DNA from any given 5x great-grandparent.

Now, if a given 5x great-grandparent was the only one of your direct ancestors with , say, North African ancestry, and he was full-blooded, then you can expect to have 0.78% of your DNA to be North African (in a perfect world).

Is that correct?

If this example applied to two 5x great-grandparents who were each full-blooded then the expected percent North African would be double, to 2/128 or 1.56%^.

Is that correct?

I suspect my father's family has North African blood on his father's maternal side but no one in the immediate family has taken a DNA test, and I wanted to know what percentage to expect, were it to be true. The paper trail shows no evidence of anything but pure Sicilian ancestry, and it ends at about the 5x great-grandparents. So, I figure, if there was a full-blooded North African then he/she would have to be beyond that generation.

Would only 0.78% or 1.56% North African likely show up or be reported on a DNA test?

I would add that many Sicilians have some North African ancestry but my suspicion is based on looks, which I know is unreliable. I'm just wondering if DNA testing would even pick it up if it's that far back.

[MarcuccioV]

The way I understand it, in mathematical theory, yes, but in reality it doesn't work that way, as siblings can get various percentages of DNA from the same parents, altering there ethnic makeup from each other. I guess you'd call it a "crapshoot". Just like the pic you posted of the 3 brothers; each looks different & likely has different percentages in their DNA makeup.

I've even read where one can receive DNA from 3 grandparents, but not the 4th. It's completely random...

[MarcuccioV]

This MIGHT help to explain it a little better. The science is a little bit above my pay grade...

https://www.ancestry.com/cs/dna-help/ma ... 20you%20do.

[darkerhorse]

Yeah, I was just looking for the theoretical percentage, to see if it was expected to be large enough to show.

I suppose it's a crap shoot in any one case, but over infinite cases the average percentage should approach the number(s) cited above.

At the 5x great-grandparent generation, it might be a needle in a haystack (which would have been a better title for this thread).

[darkerhorse]

In other words, in this example, North African ancestry may very well not be detected by DNA testing, even if it were in the family tree.

[MarcuccioV]

In other words, in this example, North African ancestry may very well not be detected by DNA testing, even if it were in the family tree.

That would be the short answer, yes...

[perhaps]

my Italian Sicilian friend tested with myheritage and has this percentage: 47% Italian; 28.5 % greek and south Italian, 10.7% sephardic Jewish-North African, 5.1% North African, 8.3% other: that is: 4.8% w Asian, 1.3% ashkenazi Jewish, 1.2% Nigerian, and 1.0 % melanesian. as he said: some of everything! he is 92 yr old this year.

[amis89]

You can inherit more ethnic dna from one ancestor and less from another. My mother is half Italian, so I should be something around 25%, but when I add up all of the percentages that come from southern Italy (middle eastern, Greek, Sardinian, etc.; I come out to have approximately 35% “Italian” dna. My full sister ended up with less than 20%.

[parkergambino]

All,

I'm no geneticist, but...

The packaging of DNA into inheritance units known as chromosomes cannot be ignored. Think of our chromosomes as 46 sacks of DNA. Each sack's contents travel(s) together through the generations; sacks do not burst open and spill contents randomly.

(Let's put aside for now the Y-chromosome, which traces the paternal lineage).

You inherit twenty-three chromosomes from mom; where did she get her 46 from?

Twenty-three from maternal Grandma, Twenty-three from maternal Grandpa.

Each child receives from its mother a blend of some of maternal Grandma's chromosomes and some of maternal Grandpa's chromosomes, adding up to 23. In an extreme outlier case, a child might receive all 23 maternal chromosomes of her grandmother, and none of her grandfather's! More likely, the child would receive a mixture that is close to equal representation (though randomly shuffled) of the grandparents' chromosomes. Voila, Independent Assortment!

This is all the same for the paternal line EXCEPT for the Y-chromosome, the one chromosome whose grandparental origin is certain: received from the father, and also from the paternal grandfather.

You basically cannot have chromosomes from more than 46 ancestors; this, too, is an unlikely number; you probably received chromosomes from a lower, even much lower, number of ancestors.

By the time you go back 6 generations (to gggg-grandfather, n=64), at least 18 (64-46) ancestors of that generation are essentially already shut out from your chromosomes.

When you wrap your head around the fact that you inherited some blend of grandparent chromosomes, it helps make sense of why some reproduction-system-related traits (such as twinning) might run in a family but skip generations.

Yes, I admit that there is some fudging in the system, because chromosomes are not forever stable and are subject to change.


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Mitochondrial-DNA (= mtDNA) is not covered by the above discussion. mtDNA is not part of the "2n=46" system of autosomal & sex chromosomal inheritance. mtDNA has its own separate inheritance rules.

Parker

[MarcuccioV]

All,

I'm no geneticist, but...

The packaging of DNA into inheritance units known as chromosomes cannot be ignored. Think of our chromosomes as 46 sacks of DNA. Each sack's contents travel(s) together through the generations; sacks do not burst open and spill contents randomly.

(Let's put aside for now the Y-chromosome, which traces the paternal lineage).

You inherit twenty-three chromosomes from mom; where did she get her 46 from?

Twenty-three from maternal Grandma, Twenty-three from maternal Grandpa.

Each child receives from its mother a blend of some of maternal Grandma's chromosomes and some of maternal Grandpa's chromosomes, adding up to 23. In an extreme outlier case, a child might receive all 23 maternal chromosomes of her grandmother, and none of her grandfather's! More likely, the child would receive a mixture that is close to equal representation (though randomly shuffled) of the grandparents' chromosomes. Voila, Independent Assortment!

This is all the same for the paternal line EXCEPT for the Y-chromosome, the one chromosome whose grandparental origin is certain: received from the father, and also from the paternal grandfather.

You basically cannot have chromosomes from more than 46 ancestors; this, too, is an unlikely number; you probably received chromosomes from a lower, even much lower, number of ancestors.

By the time you go back 6 generations (to gggg-grandfather, n=64), at least 18 (64-46) ancestors of that generation are essentially already shut out from your chromosomes.

When you wrap your head around the fact that you inherited some blend of grandparent chromosomes, it helps make sense of why some reproduction-system-related traits (such as twinning) might run in a family but skip generations.

Yes, I admit that there is some fudging in the system, because chromosomes are not forever stable and are subject to change.


---------------------

Mitochondrial-DNA (= mtDNA) is not covered by the above discussion. mtDNA is not part of the "2n=46" system of autosomal & sex chromosomal inheritance. mtDNA has its own separate inheritance rules.

Parker

Absolutely correct. Makes me think, though. On my maternal (Italian) side, the ethnic mix includes other Mediterranean locales. If accurate (and quite a few matrices agree, so there must be SOME truth to it), then how far back would you need to go for these to start dropping off -- and, conversely, why are some remaining..? Are they simply stronger & more likely to recombine, or are they more recent..?

I my case my paper trail on the Italian side goes back almost 200 years to the same commune -- so when were these other ethnicities more prevalent..?

On my father's side, there seem to be Italian traces as well (as well as a few physical attributes that back it up), yet no documentation. Same question applies.

I wish genetic science wasn't quite so complicated...