Is my surname Bernatowicz Polonized Russian surname? I know that my grandad was born in Belarus and that I am 1/8 Belorussian but can it be Russian or Belorussian? Russian would be Bernat-ovich Belorussian would be Bernat-ovic.
If your grandad was born in Belarus, whatever it means, maybe you think about nowadays borders, that is not proof he was Belorussian. He can be as well Russian, Polish or Jew.
But if your parents said your grandad was Belorussian it doesn't matter where he was born.
Below some links about Barnatowicz in Poland.
According to same of them surname Bernatowicz become from first name Bernat ( Bernat - Bernatowic - Bernatowicz ). It was known even in the XII Century.
But there was as well the Armenian families with this surname.
And there are more then 1 000 people in Poland now with such surname.