Hi Guy, thanks for sharing your thinking on this. But I think you may have some misunderstandings about DNA. I am certainly not an expert, but here is a little of what I have learnt from others.
Basically as you go back in time there is far more DNA available than is used to "build" a human body
We don't have to go back in time for that. All of us have a genome where
maybe 85% is "non-coding", which means geneticists believe that those segments aren't used to make proteins or perform other useful functions.
But that isn't relevant for genetic genealogy. Testing companies typically test somewhere around 700,000 locations out of more than 3 billion. That is only about 0.02%, but these are the locations which are known to vary. All the rest stay pretty much the same. So even if all of them were tested, they'd tell us very little.
we each inherit 50% of our DNA from our mother and 50% of our DNA from our father but that does not mean we each inherit 50% of our father's DNA and 50% of our mother's DNA.
We do indeed inherit exactly 50% of our DNA from each parent, because we inherit one of each chromosome pair from each. And that inevitably means that we inherit half of each of their DNA.
The only exception to this is the 23rd chromosome, which determines gender. The X chromosome, which both genders have is larger than the Y chromosome which only men have, so a man inherits a longer length of DNA in the 23rd chromosome from his mother than his father. But that fact isn't significant to what we are discussing here.
We inherit approximately (but not exactly) 25% from each of our 4 grandparents (i.e. 2 generations back), approx 12.5% from 3 generations back, approx 6.25% from 4 generations back, approx 3.12% from 5 generations back, etc. The further back we go, the smaller the percentage and the greater the potential for variation.
Which if you understood DNA you would realise that you do not inherit DNA form the bulk of those ancestors.
So, yes, if we go back far enough we will likely not inherit discernible DNA from some ancestors. That was why I limited myself to 5 generations back, so that it is most unlikely that we don't inherit from all of those ancestors. But even if we go further back, we will still inherit discernible DNA from most of them - our DNA has to come from somewhere!
So there are always uncertainties in DNA matching.
The experts say that above about 30 cM, we can be 90% confident of a match within 6 generations, whereas down at 12 cM there is little chance (5%) of such a match.
The various testing companies use different algorithms to calculate matches - they tend to err slightly on the side of giving you a match to check out even if it is uncertain. But with thousands of matches, there will be many genuine matches, and most of us think it is worthwhile having to dismiss some matches that don't work out for the sake of finding the others that do work out.
So I can only conclude that you have taken an unnecessarily sceptical and not fully accurate view of DNA testing. I hope you find the links I have given helpful in building your understanding, as they have helped me. Thanks.
