Originally Posted by
PeterThaGreat 
Lingua Franca, sure for the rulers, not for undereducated peasants that were 98% percent of etnic Finns. Even in the 17th, the number of etnic Finns was only couple hundred thousand. Probably 25% of contemporary Finns speaks swedish in rudimental level and they for sure are not Finnishswedes, do not engage yourself into worthless speculations. You shouldn't use upper class as reference population, since we cannot always consider members of Swedish upperclass as etnically Swedes either.
WTF? Maybe I was unclear! I just said that upper class isn't a good reference. I agree with youon that one.
Contemporary Finns don't need to identify as swedish speaking to obtain rights for trade or shopkeeping nor do they have to move to areas where swedish is the dominant language. Swedish was lingua franca in the cities! If you want to get an education or become a bourgeoisie you had to learn swedish to get by in the cities. Finnish was only peasant dialect for country hicks. Remember that it had no official status nor was there any schooling in it.
I'd go as far as to say that Finno-Swedes are a distinctive hybrid group. Composed of ethnic swedes who migrated here and diluted by Finnish blood so much that they are neither Swedes nor Finns anymore.
BTW The finnish museum institute claims 75% of swedish finns genes are similar to finns.
Museovirasto
Here's a link to a genetic study about regional differences in Y-Chromosome. If I read it correctly then it says svenskatalande ostrobothnians have more of the eastern N3 haplogroup than finnish speaking southern ostrobothnians and satakunta folk.
http://vetinari.sitesled.com/finns.pdf (Check out pages 2 & 3)
I'll see if I can dig up more info about this.