Many people know that Thomas Jefferson had a long-standing relationship with his slave, Sally Hemings. But fewer know that they had four children, three boys and a girl, who survived to adulthood. Born into slavery, Sally’s daughter Harriet boarded a stagecoach to freedom at age 21, bound for Washington, D.C. Her father had given her $50 for her travel expenses. She would never see her mother or younger brothers again.

With her departure from Monticello in 1822, Harriet disappeared from the historical record, not to be heard of again for more than 50 years, when her brother told her story. Seven-eighths white, Harriet had “thought it to her interest to go to Washington as a white woman,” he said. She married a “white man in good standing” in that city and “raised a family of children.” In the half-century during which she passed as white, her brother was “not aware that her identity as Harriet Hemings of Monticello has ever been discovered.”

So how did we lose a president’s daughter? Given America’s obsession with the Founding Fathers, with the children of the Revolution and their descendants, why did Jefferson’s child disappear? As it turns out, America has an even greater obsession with race, so that not even Harriet Hemings’ lineage as a president’s daughter was sufficient to convey the benefits of freedom. Instead, her birth into slavery marked her as black and drove her decision to erase her family history.

Harriet Hemings passed as white to protect her fragile freedom. Jefferson had not issued her formal manumission papers, so until the abolition of slavery in 1865, by law she remained a slave, which meant her children also inherited that condition. But in a society that increasingly associated blackness with enslavement, Hemings used her white skin not only to ensure her children’s freedom, but to claim for them all the rights and privileges of whiteness: education, the vote, a home mortgage, any seat they chose on a streetcar. To reveal herself as the daughter of Jefferson and his slave would have destroyed her plans for a better life for her descendants.

Since Harriet’s time, science has proved there is no difference in blood as a marker of “race.” As a biological category, racial difference has been exposed as a sham. Even skin color is not a reliable indicator of one’s origins. As one study calculated, almost a third of white Americans possess up to 20 percent African genetic inheritance, yet look white, while 5.5 percent of black Americans have no detectable African genetic ancestry. Race has a political and social meaning, but not a biological one.

This is why the story of Harriet Hemings is so important. In her birth into slavery and its long history of oppression, she was black; but anyone who saw her assumed she was white. Between when she was freed in 1822 and the ratification of the 13th Amendment in 1865, she was neither free nor enslaved – yet she lived as a free person.

She does not comfortably fit any of the terms that have had such inordinate power to demarcate life in America. Her disappearance from the historical record is precisely the point. When we can so easily lose the daughter of a president and his slave, it forces us to acknowledge that our racial categories are utterly fallacious and built on a science that has been thoroughly discredited.

Yet as political, economic and social categories, racial difference and its consequences remain profoundly real. White privilege has been much on display in our own day, as armed white men proclaiming white supremacy marched unmolested in the streets, while unarmed black men are shot down by police who are rarely held to account. Politicians run successful campaigns on platforms of racial hatred.

This is why, by one estimate, between 35,000 and 50,000 black Americans continue to cross the color line each year.

As I poured through hundreds of family genealogies, searching for more details about the life of Harriet Hemings, I saw that all families have invented stories: details that have been embellished over time, or perhaps altered by accidental errors. Descendants of immigrants Anglicized their names; information in census records is inconsistent from one decade to another; genealogies are altered because of confusion with recurring favorite names over multiple generations.

Those families who pass as white most definitely have such invented stories. It is what they had to do to authenticate a white lineage, to be recognized as fully human and fully American, with all the rights and privileges thereto – rights and privileges not even a lineage as honored as Jefferson’s can match.

Nations, as well as families, invent stories about themselves. In both cases, we will run into characters we would rather not admit as being one of us, and stories we would rather not tell about ourselves. That the president’s daughter had to choose between her family and living a life with the dignity only whiteness can confer is one of those stories. But without them, we will never truly know where we’ve come from; and without them, we will never be able to chart out a path for a better family and national life.

Kerrison is an associate professor of history at Villanova University, and the author of the forthcoming book “Jefferson’s Daughters: Three Sisters, White and Black in a Young America.”