First cousin marriage and DNA

[gazania]

I am pretty new to understanding my DNA results.  I understand the randomness of inheriting DNA from parents, grandparents etc.  However, according to all documentary proof, my gggrandparents were first cousins.  is it reasonable to expect that a first cousin marriage would affect the degree of inherited DNA?  Has anyone had any experience of interpreting the matches from a first cousin marriage?  Thank you. Gazania

[Guy Etchells]

It might do or it might not.

DNA confuses many people even those who claim understand DNA often fail to take into account DNA does not necessarily contain any traces of a specific ancestor or even a complete lineage.

We inherit all our DNA from our parents but we do not inherit all our individual parents DNA.
This means that siblings, in most cases do not have matching DNA (though the possibility cannot be ruled out).

The further back we go the possibility of inheriting any DNA from a particular line diminishes as more and more lines are added.

You state your gggrandparents were first cousins.
That means instead of inheriting DNA from their 4 parents and their 8 grandparents they could inherit DNA from their 4 parents but their 8 grandparents would not be the same people and at least 4 of their grandparents would not have the same DNA to pass on as it would come from brothers and their spouses or sisters and their spouses.

The more brothers or sisters your ggggrandparents (the parents of your gggrandparents) had the less chance of the cousins DNA being the same, because those brothers and sisters would not have the same DNA to pass on.

In terms of the current generation any lines not affected  by this cousin marriage may not carry the same DNA as any line affected by the cousin marriage.

Cheers
Guy

[JayNQ]

  Thank you, Guy, for the very clear explanation.  My cousin gggrandparents only had one child (my ancestor) who went on to have children.  But in turn, this child had a half sister (his mother's illegitimate daughter)  who named her father as another cousin of the above pair (which cannot be proved either way) but perhaps did it for respectability reasons.  My self and the same generation descendant of the half sister are matched at the 68/23 level.  We will never prove who the father of the half sister was, let alone all the other complicated relationships in this family who lived in the same village for many generations. Perhaps we should be grateful that we matched at all.  Thanks again, Guy.

[hurworth]

It would have a small effect.

Assuming no other consanguineous relationships in your ancestry (and in most parts of the world that is a big assumption) then instead of having 64 different gtgtgtgt-grandparents you have 62, and two of them feature twice. 

It does mean that your gt-grandparent would have been homozygous on a few DNA segments (i.e. the strand from the mother and the strand from the father for a segment were exactly the same).  So for that segment no matter whether they passed on their maternal or paternal segment all their children would have received the same segment at that place.  So this would make your gt-grandparent and their siblings share slightly more DNA than usual for siblings, because they are full siblings AND second cousins.  Full siblings share about 1/2 their DNA, and 2nd cousins share about 1/32.  So they'd share 16/32 + 1/32 = 17/32 of their DNA.


Likewise their children (1st cousins) could share slightly more DNA than usual for 1st cousins, and so on down the generations, but the effect would decrease in each successive generation as it's watered down by the other lines.

[gazania]

Thank you for the explanations. Much appreciated.  Gazania (aka JayNQ)