Illustration: Celine Loup

Like almost all African Americans, I have white ancestry. That’s not a surprise; it’s a legacy of slavery. For many African Americans, those white ancestors can’t be named (as is my case, though not for lack of trying). Relations between male slave owners and their female slaves weren’t generally advertised; anonymity is another legacy of slavery.

My secret lies in my DNA. There’s a DNA test called an admixture test, which measures an individual’s percentages of European, sub-Saharan African, and Asian/Native American DNA back to about the time of Columbus. According to this test, the average African American is 24 percent European—in other words, mostly African. (Despite the persistent family myths of Cherokee great-grandmothers, very few African Americans have any discernible Native DNA.) My admixture reveals something about me not evident to the eye, not present in the paper trail. It’s something only science can see. My percentages of European and sub-Saharan African DNA are almost exactly equal. DNA tells me I’m as white as I am black. Society, however, has something else to say.

What I’ve learned through hosting my genealogy series on PBS, Finding Your Roots, is that most of us know precious little about our ancestors. If there’s any way that the American people are monolithic, regardless of their backgrounds, it’s in their secretiveness. From African American guests, Ashkenazi Jewish guests, guests of Irish and Italian and Chinese descent, I hear variations on the same line again and again: “Why didn’t anyone tell me?” “I never heard that.” “I asked, but they didn’t answer.” The actor Aziz Ansari was shocked by what had been kept from him. “I’m telling you, man, these people don’t talk!”

Sometimes the pain of remembering a traumatic past—one of slavery or of pogroms, of poverty or abandonment or illegitimacy—might be unbearable if put into words. I can only assume that some family stories are swept under the rug out of embarrassment or a sense of shame. Just as the roots of a plant are often hidden out of sight, deep underground, and reached only through copious digging, in genealogy, sometimes we get our hands dirty, too. Slave owners on the family tree don’t always get a seat at the table, and many of my white guests have been known to brace themselves before they meet with me. Maybe people who have a painful past simply prefer the present. As I told the actor Christopher Walken, whose mother never uttered a word about her own father, who as a young man was a convicted criminal in Scotland, some people just want a tabula rasa; they want to write their own story.

But what happens when the present becomes painful and the future uncertain? Our world feels unstable right now; each day, the ground seems to shift under our feet. People need a firm foundation, and they look to their roots to provide it. Their history is the history of this nation. I receive emails all the time from viewers who are investigating their own ancestry, anchoring themselves in their pasts to gain perspective on the present and a foothold on the future. This is what I strive to do for my guests. Upon learning the names of his slave ancestors, who were brought to America on the very last slave ship to reach these shores, Ahmir Thompson, best known as Questlove, leader of the aptly named band the Roots, said, “I’ve been waiting for this all my life, to literally have roots. Like, what tree do you know can really thrive without any place in the ground?”

As a society, our foundations have often been the opposite of nurturing. I find myself telling my guests the same thing I tell students in my lectures at Harvard, that under the floorboards of Western culture run twin streams, ever-present and always ugly: one is anti-black racism, and the other is anti-Semitism. You could say they’re as American as apple pie. In our current environment, room has been made under those floorboards for other sibling streams, more recent additions to the wretched family: homophobia, Islamophobia, anti-Hispanic discrimination, to name just a few. To all these streams there’s an ebb and flow, but in times of economic uncertainty, stereotypes offer consistency and even comfort. And when these stereotypes are trotted out — or tweeted out — from the highest office in the land, all bets are off.

In the normalization of bigotry, immigrants and anyone considered “the other” are targets. But here’s the great irony: There’s no such thing as racial purity. Never has been. I’m a black man with white blood. The writer and director Ava DuVernay is black like me, with slightly less than half of her DNA European, but it is her African American identity, as she said to me (and as I would say myself), that “is very much part of my heartbeat.” The journalist Bryant Gumbel, who has been accused of being too white, too black, and not black enough, is 65 percent European, with 7 percent of that being Ashkenazi Jewish DNA. His last name reveals his Jewish ancestry; his skin color and his self-identification as an African American do not.

Racial purity is a fever dream of the far-too-far right, and the rabid, rising fervor for this false idea has become a national nightmare for the rest of us. “We are all a product of this big melting pot,” said the actress Scarlett Johansson, the daughter of a Jewish mother and a Danish father who holds dual American and Danish citizenship but is in fact — and much to her surprise — descended from Swedish nobility. “These two sides couldn’t be more different. It’s very much an American story.” As we close the book on 2017, it’s a story that the white supremacists, the nationalists putting all their eggs in one lily-white basket, need to hear, loud and clear.