From: rokimoto557@gmail.com   
      
   On 9/11/2025 4:09 PM, 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?   
   >   
   > 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 ?   
   >   
      
   I was involved in genomic selection.  How it works is you figure out the   
   genomic relationship between all individuals back to around 3 or more   
   generations.  You use the genetic variation segregating in the   
   population to determine how much of their genomes they have in common.   
   Since the lines had been closed for decades all the animals are related   
   to each other in some way so you get a genomic relationship with animals   
   not derived from the same families just 3 generations back.  It is a   
   better and more accurate means of determining how related each   
   individual is instead of relying on pedigree that relies on probalistic   
   outcomes of vagaries of segregation.  On average you would inherit about   
   25% of the genetics of one grandparent from one of your parents, but   
   because of random segregation from that parent one of the parent's   
   parents can get short changed and you inherit more than 25% from one and   
   less than 25% from the other.   
      
   In the case of cousin matings the progeny can end up looking like they   
   inherited more than 50% of one of their parents genetics.  This is   
   because some of the same genetics are coming from both cousins involved   
   in the mating.  The progeny end up with an average of 6.25%   
   homozygousity from their parent's shared ancestor for first cousin   
   matings.  Since the cousins share 25% of their genetics their progeny   
   only inherit on average 12.5% from one parent, but some of the 25% that   
   is not transmitted from one parent can be transmitted from the other   
   parent.  This makes the progeny of cousin matings look like they   
   inherited more than 50% of their genetics from one parent because both   
   cousins have genetics in common.   
      
   That is what you are going to be seeing in your backtrack analysis if   
   you could obtain DNA from those individuals.  For your scenario of   
   cousin matings among one set of grandparents you still have 8 great   
   grandparents, but two of the great grandparents are sibs so you would   
   have 14 in stead of 16 great great grandparents.  One of your parents   
   would be inbred.  By the time the genetics segregated down to you it   
   would look like the inbred grand parent mating contributed more in your   
   genome than 25% each.  You would have inherited some of the inbred   
   genetics from both of them making it look like your inbred parent had   
   inherited more than 50% of their genome from their cousin parents, and   
   passed half of that more than 50% down to you.   
      
   Ron Okimoto   
      
   --- SoupGate-Win32 v1.05   
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