Hi,
The points made by RogerK have gone some way to suggesting an answer to a problem I have come across in DNA results.
I am a long-time genie researcher, but I haven't posted here in a long time. At present I am doing a DNA research project to try and identify the birth parents of a cousin's father here in Australia. Four female cousins/nieces have done Ancestry DNA tests. I have used the Shared Matches in Ancestry to build a spreadsheet of 3rd-4th cousin matches and how they all connect. There have been several very kind and generous matches who have helped me along the way. I believe I have now identified very probable birth parents.
The maternal line has been a little easier because we always had a possible candidate for the mother, and DNA testing and matches seems to have confirmed that line as correct.
However, on the paternal line it has been far more complicated. We have had many matches who all have come from a common ancestor, and replies from very helpful 3rd Cousin Matches have allowed me to build a tree down to the point of either grandmother or great-grandmother. Looking at opportunity (a male being in the right place at the right time), we have identified two males as possbile father candidates -a 17 year-old or his 41 year-old father. There is some conjecture regarding which generation male was the possible father. The older man had three wives, and the younger one married once but had several other partners. I have done a lot of research on both men, who were larger-than-life characters here in Western Australia.
The complication occurs because the possible grandmother had two spouses. The putative father is a descendant from the second spouse. I have ten confirmed matches on the left side of my spreadsheet, which all point to a common ancestor of the grandmother. Of these matches, 8 are in USA, 1 is in England and 1 in Australia. Then in the centre of the chart, I have 5 matches from Australia. These matches are descendants from the putative grandmother and from her second spouse. They obviously have DNA from both the grandmother's side (the left of the chart) and the 2nd spouse's side on the right of the chart.
But on the right of the chart I have only 3 matches that have shared ancestors with the second husband, and two of them are aunt and niece. I have been very concerned that there is a lot of flow of matches from the centre of the chart (where the matches all connect to my 4 cousins) to the left, most of whom are from USA. But I have so little connection from the centre to the right (2nd spouse) side, and those matches are in England. I have been starting to doubt if my proposed paternal line was correct after all.
However, RogerK's comments have eased my mind a little. I guess the huge proportion of Ancestry DNA testing has been in USA, and so that is where the bulk of my matches have come from. I will have to be patient and hope that the latest publicity push by Ancestry will persuade more of you Brits to do a DNA test and give me more matches!