I read something while researching my DNA results that made me realize how little my ancestors have to do with me. Maybe somebody here can tell me if it's actually true.
You share about 0.5^n percent of your DNA with each member of generation n, where n = 1 are your parents, n = 2 your grandparents, and so on. The explanation that was given to me is that each of your parents contributes 50% to your genes. Depending on which of their parents' genes they gave you, you share anywhere from 0 to 50% of your DNA with each of your grandparents, but for simplicity's sake let's call it 25%.
The point is, most of the people we're trying to find out more about through ancestry websites and DNA tests are barely related to you. Searching for your great, great, great grandfather from the old country? Fine, you only share about 3% of your DNA with him.
Genetics has something to do with my interest in genealogy, but honestly the direct familial connection is what is so interesting to me. Sure, I never met my great-great-great grandfather, and his genes probably have a negligible effect on my life now, but it's fun to think about his connection to me: My dad was really close with his grandfather Henry, who died when my dad was 12. Henry, in turn, had been very close with HIS grandfather, John, who died when Henry was 12. John fought in the Confederate Army for four years, got shot twice and stabbed once, including at Gettysburg.
My thoughts and beliefs have been greatly shaped by my dad, and I'm positive my dad is who is is at least partially because of his relationship with his grandfather. Carry that backward, and I'm only three degrees removed from a soldier in the Civil War -- it's not impossible to think, completely independent of genetic inheritance, that some of my personality quirks or principles worked their way straight to me from someone who was born in the 1840s.
It's always fun to think about how the cute little family traditions you grew up with might have originally just been the quirk of an ancestor from 200 years ago.
It wasn't until we learned my great-grandmother's Hebrew name (last year!) that we made a connection between that and a Yiddish lullaby my grandmother used to sing to my sister. Long story short our best theory is that it was passed down from my great-great grandmother, who was born in the 1870s.
It's just a simple matter of distance, as things get farther away, they are farther away.
But in this case, not actually as far away as you are thinking, because of something called pedigree collapse. For example, the second row in your family tree, your grandparents, has 4 people, 2^2. The 20th row, only about 500 years ago, has mathematically 2^20 = 1,048,576. In reality far fewer, because the same people fill multiple spots. Only 1,000 years ago, 2^40 = 1,099,511,627,776.
A trillion ancestors 1,000 years ago? Doubtful.
So you can see, this exponential explosion is not an accurate way of thinking about ancestry.
Pedigree collapse within a certain area is what leads to the various human races. The distinction of race is simply the description that someone is descended from some localized subset of the global population.
You're far more closely related to your g^40-parents, than the g^40-parents of someone half-way round the world because your g^20-parents are more closely related to your g^40 parents. And so it goes up and down the tree.
So yeah, maybe only ~3% at a certain level, but that is as closely related in terms of match-able DNA sequences that it is possible to be at that distance in time. You're still very closely related in terms of the overall DNA "color palette."
And it is likely somewhat more than 3%.
Many Ashkenazi Jews have a 'genetic relation' that is in fact higher than their 'genealogical relation' would suggest because there has been such a high degree of intermarriage among a small population over the years. This is the case for all races, just maybe more pronounced in Jews because of the smaller population.
There's something sad about such a statement, and it epitomizes a certain cause of present day problems in society.
Are your genes the only things that "has to do with you"?
What about the built environment and society in which you exist with all of it's inventions and innovations from a strictly primal state of affairs?
Who built that society and environment around you? If you're like most people, your ancestors.
What aspects of your ancestors guided their building and structuring of society? Their genes. Your ancestors have very much to do with you, and you have much to be grateful to them for.
Edit:
p.s. As a corollary addendum, if your ancestors have nothing to do with you, what of your descendants? Why bother leaving an orderly and healthily functioning world to people "who have nothing to do with you"?
Safe to say, that we should pray for the well-being of future generations that most people today don't think this way, and be thankful that most people in the past didn't think this way.
What surprised me is when I realized that it was entirely possible (though maybe not probable) for siblings to share 0% of their DNA. That is, if one sibling received the "first" 50% of the father and mother's genes, and the other sibling received the "second" 50%, they would have 0 overlapping genes.
Wouldn't some of the genes be identical, ie you randomly get one of two identical copies? I'm thinking some of the essential genetic code sections might be of a nature where having them damaged would kill the kid.
That isn't possible. You got your Y from your dad if you are male. You got an X from your mom. So all siblings share mom's X and all sons share dad's Y. You also got your mitochondrial DNA from mom.
This 3% or whatever from 0.5^n is a lower bound. To get close to it, all your other relatives would need to come from completely somewhere else. But in reality, many of your other ancestors were probably related to him, because they had lived in his valley since they invented farming, or whatever. So you probably have some more of his genes from several great, great, great grandmothers who were related to him.
Seems like it is assuming all of those ancestors had unique DNA when in fact they were probably all 99+% similar if you look at it from the perspective of the whole genome. I'm no biologist though, would be interesting to see a real explanation.
I don't know either, but I read on HN some time ago that the size of a CD were comparable to ... I don't even know, a human genome. So, assuming that's a lower bound, 1% of 600MB is still 48 million bits. 2^48 would be the number of different strings representable with 48MB, but maybe not all of those will be viable.
I cannot say why he is downvoted but there are some mistakes in his comment. First, there would be 47 shareable elements as you have 46 chromosomes plus the mitochondrial genome. And then, any of the 46 chromosomes (besides the Y chromosome in males) can and do exchange segments with its homolog/sister chromosome of the same type (crossing over or recombination). While this does not happen all the time, it does happen, so you from some points in history you are inherating your genome from many more ancestors.
Chromosomes don't stay intact like (it sounds like) jonsen is assuming. Instead they cross-over or re-combine to make never-before-seen strands -- the new strand has one end from one copy & the other end from the other copy.
You share about 0.5^n percent of your DNA with each member of generation n, where n = 1 are your parents, n = 2 your grandparents, and so on. The explanation that was given to me is that each of your parents contributes 50% to your genes. Depending on which of their parents' genes they gave you, you share anywhere from 0 to 50% of your DNA with each of your grandparents, but for simplicity's sake let's call it 25%.
The point is, most of the people we're trying to find out more about through ancestry websites and DNA tests are barely related to you. Searching for your great, great, great grandfather from the old country? Fine, you only share about 3% of your DNA with him.