Science shows no basis in fact for the structure or make up of racial groups as we know them. Race classification systems, from a biological, taxinomical perspective are tautologies. They must be used in and of themselves to define themselves, a scientific and logical fallacy. Racial categories are not socially fixed. Some people can move back and forth from one to another, often against their will.
People can pass from one to another and one can be ostracized from one's racial group.
It is a social construct and membership is socially determined. The construct of a white race did not exist as such until the seventeenth century in America with the creation of race laws which created social differences between indentured or poor Euro-Americans and enslaved Africans. In colonial Virginia and other colonies from one half to two thirds of European immigrants arrived in chains. Up to eighty percent of them died in the first twelve months. As an economic issue it became expedient to find ways to separate, and pit European and African servants against each other and thereby better control these two groups.