Identifying someone with a really large DNA match

[LizzieL]

  why do you  think  your  match  is 20 years old?  Is it just  because  their profile says  20 years  and under?

He has a tree on MyHeritage with only himself on and has his name and year of birth on it (2002). That's why I think he's in his 20s. I'm surprised MH shows living people, but it seems to be general thing on MH trees.

[Biggles50]

If we then presume that your Paternal Grandfather’s Mother had an illegitimate child which was adopted, lets call them X

Then your Paternal Grandfather would have had a 1/2 sibling and they would share c1900cM with X.

This is what one of my DNA matches that I am in contact with shares with their own 1/2 sibling. 

X has a child Y and Y would have c950cM of the original 1900cM

Y has a child Z and Z would have about 1/2 of Y’s DNA or c475cM

There is a BUT with this hypothesis …………………

[Cell]

I have tried to contact, but I am not a subscriber to MH, just uploaded data (and possibly paid a small amount at the time), so I would have to take out a sub. 

You should be able to message them. I am not  a subscriber  either I( except  that I paid a once off amount  for the unlock features, chromosome  browser  and so on a couple of years  ago) I can  use the message system  freely.
Do you have  a contact  button under  your matches names?
Kind regards

[LizzieL]

There is a contact button, I pressed it and got an invitation to subscribe, but just done it again and it comes up with a message box!.
Very odd, but thank you - without your message I would not have tried again.
Maybe it had logged me out the first time, and I've just logged back in just now.

[Cell]

There is a contact button, I pressed it and got an invitation to subscribe, but just done it again and it comes up with a message box!.
Very odd, but thank you - without your message I would not have tried again.
Maybe it had logged me out the first time, and I've just logged back in just now.

Myheritage always has glitches. They are terrible for it.
Hopefully  your match will reply
Best of luck
Kind regards

[LizzieL]

If we then presume that your Paternal Grandfather’s Mother had an illegitimate child which was adopted, lets call them X

Then your Paternal Grandfather would have had a 1/2 sibling and they would share c1900cM with X.

This is what one of my DNA matches that I am in contact with shares with their own 1/2 sibling. 

X has a child Y and Y would have c950cM of the original 1900cM

Y has a child Z and Z would have about 1/2 of Y’s DNA or c475cM

There is a BUT with this hypothesis …………………

That's interesting, I hadn't realised the figures could be so high. My paternal grandfather's mother married at 17, so an illegitimate child before marriage seems a bit unlikely. But if her parents had a child I didn't know about (a full sibling) the figures could be similar.
My paternal grandfather's mother's birth was never registered, but some of her older siblings were. There could have been another child after her who was not registered either.

[Biggles50]

665.7cM is close.

With DNA inheritance being what it is you will have about 887cM from your Great Grandmother.

Hence a distant MRCA in this case is not likely to be that distant.