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I understand that everyone carries autosomal DNA from both their mother and their father. Because there are only a small number of crossovers per chromosome, I see that not everyone would carry any DNA from an ancestor several generations back.
What are the chances of a person carrying any DNA from an ancestor N generations back, where N > 1 and the simplifying assumption is made that no ancestor appears more than once?
This is exactly the question I answered in my blog post about 2 years ago: Probability of No Autosomal Segments Matching
The final table of probabilities I came up with was:
So reading the table:
The probability curve of no segments matching (which is 100% minus the chance that a descendant carries DNA) looks like this:
Here are a couple resources for you:
Concepts – Percentage of Ancestors’ DNA
This chart provides a summary of how many ancestors you have in each generation, an approximate year they were born using a 25 year generation and a 30 year generation, respectively, and how much of their DNA, on average, you could expect to carry, today. You’ll notice that by the time you’re in the 7th generation, you can be expected, on average, to carry 0.78% meaning less than 1% of that GGGGG-grandparent’s DNA.
How much of your genome do you inherit from a particular ancestor?
As a rough rule of thumb the autosomes you received from (say) your mother, k generations back is broken into (22+33*(k-1)) chucks, as your genome comes in 22 chromosomes and there are on average 33 recombination events per transmitted genome. These chunks are spread across your 2^(k-1) maternal ancestors. So, for example, nine generations ago the autosomes you receive from (say) your mum are broke, on average, into 286 large chunks, and these are spread across your 256 ancestors. Thus on average each of ancestors has contributed only a single block to you, and by chance it is possibly that they contribute zero. This gets worse the further we go back in time, your genome is broken up into more and more chunks, but this does not grow as fast as your number of ancestors. This makes it increasingly likely that you inherit no autosomal material from a particular ancestor.
