pretty much just think of human genetics/family background. for instance my great grandmother was full italian, married a "normal american guy" with not full known genetic background. anyway, they had kids (my grandmother), they were all 1/2 italian. my grandmother married a german heritage guy. they had kids (my mother), that makes her 1/4 italian. then my mom married my dad (my dad of no italian descent that we know of, so assuming he has none). they had me, so i am 1/8 (or 12.5% italian)