DNA Relatives Reliability

[geebee]

For autosomal DNA it is really simple.

You get half your DNA from each parent.

In turn they each get half of their DNA from their parents.

If you keep dividing by half for each generation you go back, after 10 generations (approx 300 years), you will only have 0.1% of the DNA from your ancestor.

100 x 0.5 = 50 (parents)
50 x 0.5 = 25 (gparents)
25 x 0.5 = 12.5 (1 x gparents)
12.5 x 0.5 = 6.25 (2 x gparents)
6.25 x 0.5 = 3.125 (3 x gparents)
3.125 x 0.5 = 1.5625 (4 x gparents)
1.5625 x 0.5 = 0.7825 (5 x gparents)
0.78125 x 0.5 = 0.390625 (6 x gparents)
0.390625 x 0.5 = 0.1953125 (7 x gparents)
0.1953125 X 0.5 = 0.09765625 (8 x grandparents)

If you were born in 1980, your earliest traceable direct ancestor would have been born in 1680, 300 years before your date of birth.

Y DNA on the other hand passes from father to son down your paternal line. Your 8 x paternal grandfather would share the same Y DNA as you.

Similarly mT DNA passes from mother to child down your maternal line. Your 8 x maternal grandmother would share the same mT DNA as you.

With y DNA and mT DNA, it is possible to go back much further in time to see where two individuals share a common ancestor beyond 10 generations. Although it is impossible to accurately determine how ancient this connection would be. It could be the 11th generation before you were born or an infinite number of generations stretching back thousands of years.

Hope this clarifies.

Let's see how the above actually compares to a real transmission over three generations.

Bern (my father) to Kim (my sister) - 50%

Bern to Sarah (Kim's daughter) - 20.4%

Bern to Drake (Sarah's son) - 7.34%

By inference (since did not actually test)

Lyn (my mother) to Kim - 50%

Lyn to Sarah - 29.6%

Lyn to Drake - 14.16% (Kim's total contribution of 21.5%, less the 7.34% from my father)

As you can see, my mother's contribution to her great grandson is nearly double that of my father's.

While the above is the only 3-generation transmission I can show you, here are a few more 2-generation examples:

Bern to Stephanie (my brother Bernie's daughter) - 25.4%

Bern to Joseph (Bernie's son) - 18.5%

Bern to Brandon (Kim's son) - 28.0%

Bern to Kathryn (my daughter) - 22.5%

By inference:

Lyn to Stephanie - 24.6%

Lyn to Joseph - 31.5%

Lyn to Brandon - 22.0%

Lyn to Kathryn - 27.5%

Interestingly, only Stephanie has a pretty even inheritance from both grandparents (25.4%/24.6%). Her brother Joseph actually only inherited 2/3 as much from his paternal grandfather as from his paternal grandmother.

[mxcrowe]

Another great example in life of how generalities apply very well to the collective but very poorly to individuals...if only more of us could remember this fact!

[mildlycurly]

The majority of my DNA relatives are of Irish or British ancestry, with all apparently having some of this ancestry. The most "out there" one I can find is a Hungarian with four Hungarian grandparents and some stated Romanian ancestry. Given my matches on other sites I would have expected some more Jewish matches, but the most I've seen is 1/8th Ashkenazi. I also have no known Romani matches, and by extension South Asian matches.