From: {$to$}@meden.demon.co.uk   
      
   On 11/09/2025 22:09, Martin Harran wrote:   
   > I understand that we get half our DNA from each of our parents. How   
   > does that work backwards? I am into family research and have   
   > identified  all 8 of my great-grandparents; did each of them   
   > contribute exactly 1/8 of my DNA or is that an approximation?   
      
   It's an approximation. There's a number of factors that mean that the   
   50% is an approximation. What is exact, in almost all humans, is that   
   the same number of nuclear chromosomes is inherited from both parents.   
      
   Firstly, if you're male you inherit less nuclear DNA from your father   
   than from your mother (a Y chromosome rather than X chromosome). This   
   amounts to about 1.5% less.   
      
   Secondly, mitochondrial DNA is uniparentally inherited (though there is   
   some paternal leakage); you inherit that from your mother only. But this   
   is only 0.005% of genome. There is also the potential complication of   
   heteroplasmy.   
      
   Thirdly, chromosomes are not the exact same length in all individuals,   
   with insertion, deletion, duplication, ... variants segregating in the   
   population. I suspect that we don't have good data on the degree of   
   variation in genome size; the difficulty of sequencing repetitive DNA   
   means that the exact size of a genome is not precisely known, and I   
   suspect that measurement noise blurs the data from flow cytometry. (Flow   
   cytometry estimates the mass of DNA per cell.)   
      
   Fourthly, microchimerism means that some children contain maternal   
   derived cell lines, though this is less common than the occurrence of   
   fetal derived cell lines in mothers.   
      
   The number of genes inherited from each parent is a closer match, but   
   there's still the matter of the mitochondrial genes, the smaller number   
   of genes on the Y-chromosome than on the X-chromosome, and copy number   
   variation in nuclear genes.   
      
   When it comes to multigeneration inheritance, ignoring crossing over   
   each of the 23 chromosomes inherited by a parent from one grandparent   
   has a 50% chance of being passed onto a grandchild. So the amount of the   
   genome inherited by the grandchild could be between none and a 1/2, with   
   1/4 being the most probably result. The chance of none is 1 in 8   
   million. When crossover is taken into account the result is that there   
   are more genetic segments involved, inheritance is more strongly peaked   
   around 25% of the genome, and the chances of 0 or 50% are so low that it   
   may not have happened prior to the last couple of centuries.   
      
   Excluding identical twins and chimaeras from consideration   
      
   Most everybody of European descent is descended from Charlesmagne, but   
   that doesn't mean that they've inherited any of his genome.   
      
   >   
   > In my case, I have 8 distinct great-grandparents What happen if there   
   > is some cross-breeding? Let's say that two of my grandparents were   
   > cousins so I only have seven different great-grandparents, Would that   
   > mean one of the 7 would have contributed 1/4 of my genes and the other   
   > six contributing 1/8 ?   
   >   
      
   On average 1/4, but varying between 0 and 1/2. I want to say more   
   strongly peaked around 1/4 than grandparent contributions, but I haven't   
   done an analysis to confirm that.   
      
   --   
   alias Ernest Major   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)