"Legally"? Really? What statute is that?
My wife has a nephew in Virginia who is as Caucasian as anyone on Earth but his children had to be listed on school records as “Guamanian”. Why? Because his wife is the daughter of a Caucasian man and a Guamanian woman, she had to list her race as Guamanian and the children are listed as Guamanian because their mother is, even though they are three fourths Caucasian. The school system lists students by race and they insist that the children are Guamanian.
The tradition goes back a long way. You are your mother’s race, the roots are probably in the old, “Mama’s baby, daddy’s maybe” line. Before DNA testing it was seldom possible to be one hundred percent sure of fatherhood.
Things are now reversed from the old ways when it comes to black people, “high yellows” used to pass as white if they could, now anyone who has even the slightest hint of black ancestry claims to be black, I saw a man on tv who claimed to be the first “black” something in the military and he looked pure Caucasian to me.