There is a significant amount of controversy about the genetic origins about the people who self-identify as "Melungeon,"and variously claim Portuguese and Native American roots.
Last year an NPR segment addressed research pointing to African ancestry and a recent DNA study published in the Journal of Genetic Genealogy pegs many as the offspring of sub-Saharan African men and women from North or Central Europe.
So what you suspect about your own ancestor being born to a family that was passing for white is entirely possible. Given how people of African descent were treated at the time of the 1880 census, it's likely that many Melungeon families made every attempt to have the community classify them as "white," rather than "mulatto."
However, the only way to ascertain the ethnic mixture of your own ancestry is to take an admixture test from a DNA testing company such as Family Tree DNA, 23andMe or Ancestry.com. Good luck!
Henry Louis Gates Jr. is the Alphonse Fletcher University Professor and the director of the W.E.B. Du Bois Institute for African and African-American Research at Harvard University. He is also editor-in-chief of The Root. Follow him on Twitter.
This answer was provided in consultation with Eileen Pironti, a researcher from New England Historic Genealogical Society. Founded in 1845, NEHGS is the country's leading nonprofit resource for family history research. Its website, AmericanAncestors.org, contains more than 300 million searchable records for research in New England, New York and beyond. With the leading experts in the field, NEHGS staff can provide assistance and guidance for questions in most research areas. They can also be hired to conduct research on your family. Learn more today.