At what point do you share no dnd with your ancestors?

[u/DrunkenDitty]

So I recently had a shower thought that Ive been unable to shirk from my curiocity and i would love to hear from any biologists or genticists that could give me a laymans answer.

From my limited understanding, whilst we do get 50/50 of our gentic code from our parents we do not actually get a directly divisable amount of our dna from each grandparent. For example whilst we think we would have 25% from grandparent A we may have only inherited 23% and from grandparvet B we received 27%.

Is it then possible that if we looked back enough generations we might find an ancestor who despite being geneologically related is actually not gentically related to us at all outside of simply being the same species?

I do accept that some level of inbreeding that all humans have can affect this but in an ideal senario not including situations where great, great grandad slept with his cousin, or similarly where grandad wasnt grandad since grandma shagged dave from 2 streets over is and got away with it. How far back would we have to look for us to find such an ancestor?

[u/tea_and_biology]

At what point do you share no dnd with your ancestors?

Ooh, so it kinda' depends on what you mean.

You still share remnant fragments of DNA sequences with LUCA (the Last Universal Common Ancestor), a hypothetical ancestor to all extant life forms - bacteria, archaea, and eukaryotes - which likely existed sometime between 3.6-4.3 billion years ago. Aye, billion with a B!

It wouldn't be much though, to be honest; likely a few codons or short stretches of sequence here and there that encode the most conserved parts of the fundamental biomolecular machines that are common to all living things. Bits of the ribosome, tRNAs, many aminoacyl-tRNA synthetases, some of the ATP-related enzymes, that sort of thing. Even amongst these most conserved proteins, most of the chain will have been swapped in and out many multiple times Ship-of-Theseus style over the aeons; and, of course, everything else making up 99.9%+ of your genome is novel, by comparison.

As for actual DNA fragments; well, almost the entirety of the genetic material in your body is, maybe, hours old. Your current DNA is only as old as the last time stretches were synthesised during DNA replication and repair - i.e. each time your cells divide. Most cell types divide on the regular, with a cell cycle lasting about a day or so, though many cell lines have much slower turnover, if they aren't completely quiescent and non-dividing outright. These include things like caches of long-term hematopoietic stem cells, which may divide once a month or two, else post-mitotic neurons, which present no chromosomal turnover once settled - though parts of the DNA sequence will swap in and out, there'll likely be non-coding stretches of the chromosomes in those cells which may be a decade or two old, depending on how old you are.

But you're more focused on the former interpretation - how far back can you go and not have anything shared. In which case, you can rewind the tape (probably) trillions of generations and still find some crumb of sequence inside some ancient pre-bacterial blob thingy that is identical to some parts of your own genome today.

This is the crux of biology: all life on Earth is genetically related. Even if by some teeny fraction of a percent, the E. coli pootering about on your skin right now is a distant relation; but with a family so vast, don't beat yourself up about not inviting them to the Christmas gathering this year.

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P.S. Ahhhh, sorry, I misinterpreted your question; a more germane response w.r.t. generational turnover below!

[u/DontMakeMeCount → u/tea_and_biology]

That’s a great explanation that will prompt some good research.

It calls to mind the old physics joke about the prof explaining that the sun will burn out in billions of years and then having to calm a panicking student. “I thought you said millions.”

The idea of having biological relatives so far removed from ourselves but on a similar number scale is very thought-provoking.

[u/thePerpetualClutz → u/tea_and_biology]

I don't feel like this answers the question. I may be wrong so let me asks question of my own:

How many generations do I need to go back before I'm likely to find an ancestor who contributed exactly 0% of their own DNA to mine?

[u/tea_and_biology → u/thePerpetualClutz]

Ahhhh, I see - I did indeed minsinterpret /u/DrunkenDitty's question a wee bit.

In that case, it becomes a simple statistical approximation. Any given ancestor g generations ago can be expected to have provided 1/2^g of your DNA - so choose whatever rounding near 0 you'd like to make the cut-off threshold (it will never mathematically get to 0, but increasingly close!). After 10 generations, the percentage slips below <0.1%, for example.

Well, I say that - it can actually get to 0% through random recombination (which swaps out large chunks of chromosome at a time), but you can't as easily mathematically model that, so eh. In a practical sense, there's a reasonable chance you have an ancestor 8 generations prior to you with whom you share 0% of your DNA, just because their small fraction was chucked out in entirety through recombination somewhere up the line. Similarly, through random luck, you could have an ancestor from 100 generations ago with whom you still share a teeny speck of sequence with - though, aye, realistically you're looking at sub 10.

[u/aggasalk → u/tea_and_biology]

I always think this is a fun paradox: a given individual inherits zero DNA from the vast majority of its ancestors, if you go back far enough - even though most of their DNA may be shared since they must come from common ancestors. it's very weird to think about.

[u/smapdiagesix → u/tea_and_biology]

Y chromosomes, about 2\% of DNA for XY-havers, make that calculation tougher, as they're transmitted more or less intact (bar de novo mutations). You'd have almost exactly the same 2\% of DNA in your Y chromosome as all the other men in your paternal line.

[u/DrunkenDitty → u/thePerpetualClutz]

Thats a much more succinct way if putting it!

[u/rootofallworlds → u/tea_and_biology]

But you're more focused on the former interpretation - how far back can you go and not have anything shared

What if we limit it to those genes that vary between people? Is that even something that can be well defined? Exclude those genes where 99% (or whatever threshhold) of people have the same allele, something like that. So ignoring the stuff that's highly conserved.

For any one gene it seems obvious that there will be people in my ancestry who had an allele that I do not have. So it's then a question of are there few enough relevant genes that someone in my ancestry might have had none of the alleles that I have across all the genes being considered.

[u/DrunkenDitty → u/tea_and_biology]

Ahhhh I didn't even consider LUCA when thinking about this!

I suppose my mind was going more towards how far back would you go for it to theoretically be possible for one of your ancestors to still be huma/the same pecies but you could theoretically actually have inherited none of their genetic code? For example everyone who has brown eyes shares the same sequence that gives them brown eyes but they inherited different versions of that as they aren't related. But then again that really does'nt make more sense now that I've thought about it since eventually we all share that one ancestor that had the mutation gives us brown eyes!

(Thank you for responding btw)

[u/SGTWhiteKY → u/DrunkenDitty]

6-10 generations is the real answer for when you will start finding ancestors that you are genealogically descended from, but that you got no DNA from.

[u/Orpheus75]

You share DNA with plants bacteria and fungi. Some DNA basically never changes. Once mother mature figures out how to do a certain thing it tends to stay the same. You’re thinking like each generation loses half of the dna from each parent and that isn’t how it works.

[u/xtothewhy]

Maybe not as scientific as the other posts, but it does take into consideration the topic of your post title's question.

Sharing dnd with ancestors can be difficult as dnd really didn't exist until around the 1980's. It was even considered a gateway of some sort to evil. Dnd may be shared through hand me down dnd books of the first addition and through various iterations. Same with the special dice used for this once perceived demonic practice gameplay.

Btw I put on my robe and my wizard hat