DNA matches - am I being illogical?

[ali.aitch]

Alison, Josie, Pat, Marion and Barbara all have DNA in common with each other, sharing between 39 and 903 cMs. Alison and Josie are first cousins, descended from John Horlock and Mary Johnson. Pat and Marion are second cousins, descended from John Horlock and Mary Poole. Barbara's family tree going back to her great-grandparents does not include John Horlock. Barbara's ancestors came from Scotland, Ireland, Dorset, south London and the home counties. John Horlock spent his adult life in Islington, north London. Barbara's grandfather and great-grandfather (and their wives and daughters) lived part of their adult lives in Islington.

In order for Barbara to share DNA with four people all descended from John Horlock, I think one of Barbara's female ancestors must have had a child by John Horlock or one of his close male relatives. Am I right, or could there be another explanation? All ideas and suggestions gratefully received. Thanks!

[rsel]

Hi,
    Have you looked at the DNA Painter web site ?  They have a tool where you can enter a family tree for a group of individuals and enter the DNA match sizes to a person that you are trying to find the relationship to.  This will give you a series of possible relationships, which will then give you an idea of the generation you are looking for as the MRCA.  This should give you a better idea on where to look for the overlap.

Richard

[ali.aitch]

Richard - Thank you. I knew you could enter the number of shared cMs, but was quite unaware of the whole family tree thing ...! I will go and give it a try. Reading back my own message and looking at the figures in the matrix, it did occur to me that perhaps the 'conjugal relationship' happened much earlier.
Thanks again, much appreciated - Alison

[ktk8]

Also the possibility that someone in Barbara's tree may have been adopted?