gwilym'smum wrote: ↑08 Aug 2020 08:10
... Not sure if I am following you about DNA that you do not share with parents. On the DNA courses I have done it has been highlighted in capital letters that you cannot inherit DNA which your parents did not have. ...
Oh gosh... Let me see if I can explain what's in my head.
Every single bit of DNA that you have, did indeed come from your parents. But think about it, if you have siblings and you're not identical twins (
and I think that there may be a rabbit hole of detail about what that means that I don't want to go down), then the two of you do not have exactly the same DNA. The string of DNA that you got from your mother is
not identical to the string that your (theoretical) sibling got from your mother. If it were, you'd be identical twins.
The differences come from the fact that when your mother (or father) created the relevant egg (or sperm) there is a degree of shuffling that goes on in the DNA at that exact point in time. The DNA that's in the egg (or sperm) has
all come from the parent - but
it's no longer necessarily in the same order. (An
analogy might be - "I'm playing all the right notes - just not necessarily in the right order!")
This shuffling is why we're not all identical twins and why our parents match someone in Ancestry DNA but we don't. It's also why we and our siblings won't have the same matches because matches are driven by that order. Of course, if the match that our parent has to someone is 100 cM say, then chipping
a bit away won't make a lot of difference - the analysis will still see the match - it's just shorter for us.
Now think of a 5 cM segment in your parent's DNA. The mistake is to think that your version of that segment must always be shorter. It won't be - it's just different. And since there's only a limited number of letters in the DNA alphabet, it's perfectly possibly that the random change to that 5 cM segment actually results in a 7 cM segment that matches someone else who's totally unrelated to you since the Wars of the Roses. This sort of change results in a match that's regarded as being "Identical By Chance". You might think - but it's uncovered a common ancestor in the Wars of the Roses! Except that if you go that far back, or certainly to 1066, anyone with a substantial part of British descent is almost certainly related to everyone else with a substantial part of British descent - we don't need DNA to confirm that.