Although technically/legally, the term "free white person" included Eastern Europeans, Irish, Italians, and Jews, ethnically those groups were certainly seen as different & less than other white ethnicities, and were treated accordingly i.e. subject to gross generalizations/racialization and excluded from holding sociopolitical power. The idea of "whiteness" here is that white anglo-saxon protestants held the most power for the longest period of time in the US. Not so much about genes but about (perceived) ethnicity.