154 cM Match

[Kaybron]

One of my top matches is 154 cM.  The person I match with does not have many people in his tree but I have been in touch with the person who manages his tree. From our shared matches I have established that we must be connected through my father, not my mother.  My father's family largely lived in the Croydon, Surrey area and this person's mother (I shall call Mary) was also born in the Croydon, Surrey area.  Through contact with the tree manager I have been told Mary does not know who her father is. I have looked at the DNA Painter tool and think my 154 cM match could be a half second cousin.  If he is a half second cousin, what common ancestor would we have.   
 

[davidft]

For a second cousin the persons you would share in common is one of your great grandparents couples

It goes cousin match common persons equals a grandparent couple
second cousin match common persons equals a great grandparent couple
third cousin match common persons equals a great great grandparent couple, and so on

The half bit as in half second cousin means that rather a couple match at say great grandparent level you just match one of the great grandparents

[Kaybron]

Thank you for explaining.  Is it possible with this cM of 154 to be a half first cousin match, a child of my grandfather.  When you put in this cM into the DNA painter tool it mentions that the probability of this is 0%.   

[davidft]

Whilst that would be possible it would be extremely unlikely, a bit like winning the lottery!

Have you considered it might be half first cousin once removed as that has a possibility of 25 per cent?

The diagram here

https://dnapainter.com/tools/sharedcmv4

shows how a half first cousin once removed fits in

[Gadget]

Have you considered a difference in generation so that the 'removed ' option comes in to play? Do  you know 'Mary's' age? Is she the same generation as your father or you? 

Gadget

[Craclyn]

You may be able to narrow down the options further by analysing shared matches.

[Kaybron]

Thank you for the responses Craclyn, Gadget and davidft.

My grandfather was born in 1898 and his last child with my grandmother was in 1939.  My father was born in 1920.  Mary, the mother of my 154 cM match was born in 1931.  My grandfather has 2 possible brothers that could be Mary's father. 

I am of a similar age to the person I match with.

Kaybron

[sugarfizzle]

You appear to be the same generation as your match, though not definitely.

Without half or removed coming into it at all, you are possibly 2nd cousins, i.e. shared great grandparents, possibly 3rd cousins, shared 2G grandparents.

That would be Mary's father was either a brother of your grandfather's, or a brother of your great grandfather's.

As Craclyn said, shared matches could give further clues.

Regards Margaret.

[Kaybron]

Thank you sugarfizzle for this information.

Mary's father could only be a brother of my grandfather.  My great grandfather's brothers had all did well before Mary was born.

I think I am on the correct track with possible fathers for Mary.  There are 2 brothers of my grandfather, either one could possibly be the father.

Thank you for suggestions.  I am still learning how to analyse shared matches.

Regards Kaybron