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Peter

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Reply with quote  #61 
A grandson through a daughter would definitely not share his grandfather's Y chromosome, unless his father happened to through some remoter link, and could like a granddaughter in theory share no genes at all. The theoretical possibility is so remote that it probably wouldn't happen in a million years, but exists.
BaronVonServers

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Reply with quote  #62 
You're right I was working with 'direct line male descent' in mind. 

I was, I think, getting to the fact that while a son (or daughter) has 1/2 from Dad, that son could pass everything from his mother, except the 'Y' to the grand son (the daughter could pass everything from the mother).  In other words 'grandson' could be almost nothing like grandpa, even with 'direct male lineage'.



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Peter

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Reply with quote  #63 
As you say, with minor qualifications everyone gets half of their genetic heritage from each parent, or to put it another way everyone gets half of each parent's genes. The "selection" is fairly much random, or so it appears. Dominance and recessiveness have nothing to do with what genes you get, only with how genes express themselves. So it is even a technical possibility that you could get unrelated siblings, that is they got a different half of each parent's genes. or genetically identical siblings that are not identical twins (they could even be a different sex), that is they both got the same half of each parent's genes. Neither one would actually happen in a billion years, the odds against are so great, but the theoretical possibility is still there.
Windemere

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Reply with quote  #64 
Thanks for the above postings.

Indeed, the statement that 8 English monarchs were succeeded by eldest surviving sons isn't accurate, and should state that 7 monarchs were succeeded by eldest surviving sons. I've edited my original post, so that no one later on will repeat the error.   And indeed, it was the succession of William II to his father William I  that accounts for the discrepancy.  Inheritance by a "younger son" is a more apt description of that succession. Actually, I've always been fascinated by the historical period that includes the interactions and rivalry between Robert Curthose, William Rufus, and Henry Beauclerc. All 3 sons were alive and vigorous at the time of their father's death. Robert inherited Normandy while William inherited England, and in due time  Henry wound up with both. I suppose that's what happens when one becomes too preoccupied with statistics, and forgets the actual historical background.

One example of biological restoration of a Y-chromosome in royal history occurred when Isabel, daughter of Emperor Pedro II of Brazil, married Gaston of Bourbon-Orleans. The Braganzas of Brazil  and the Bourbon-Orleans of France were both branches of the Capetian dynasty, and shared its Y-chromosome.  Emperor Pedro had no surviving sons, and his heir was his daughter Isabel. But his grandsons acquired the same Y-Chromosome that he carried , although it was transmitted to them by his son-in-law Gaston. While the biological Y-chromosome restoration was a somewhat unique occurence, the union was certainly a pre-arranged one, and didn't happen at random. The Braganza and Bourbon-Orleans lineages had been separated for almost 800 years, though they both maintained the Capetian Y-chromosome.

Another example may have occurred when Queen Regnant Isabella II of Spain , heiress of her father Fernando VII, married her cousin Francisco de Asis of Borbon, and hopefully maintained the Borbon agnatic lineage. Again, that was a planned, not random, union. Though in that case, one or more generations of non-paternity events have been surmised. Again, this involved the Capetian Y-chromosome mentioned above.

Even identical twins, coming from the same fertilised zygote, may not always be 100% identical. When the fertilised zygote splits into two, while both parts receive the vast majority of the same genes, there are likely a few genes that  remained on one side or the other. 


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