This story is excerpted from the new book “The Lost Family: How DNA Testing Is Upending Who We Are” (Abrams Press), which goes on sale March 3 and grew out of a 2017 Washington Post story.

The questions started for Roberta Estes when she was 18 months old, though she wouldn’t know it until later. That was when her father was in a car accident, and the hospital called his wife, and then the hospital called his other wife. Both wives showed up at the hospital with babies, which is how they found out about each other. Roberta’s mother dumped her father.

Roberta did not unravel the full story of her father’s other family until she was an adult, and had become a genealogist. Over the past 20 years, commercial DNA testing with mail-in spit kits has helped genealogists make major breakthroughs more easily. But at first, Roberta had little more than word and paper — and her ultimate journey was revealing of how the work of amateur ancestry sleuths has evolved.

Roberta wanted to find her half brother, the little boy she’d crossed paths with in 1956, when both of them were too young to know what was happening. She tried city directories, obits, marriage records and divorce records and eventually consulted a private investigator, who found a police record for Roberta’s half brother and, through that, located the man’s address. Roberta wrote him a letter.

David Estes turned out to be a long-haired, tattooed long-haul truck driver, and Roberta was struck both by his rough-around-the-edges demeanor and by his loyalty to those he loved and those who needed his love. The first time they spoke by phone, his deep voice was everything to her, and at first, she could barely speak.

They became close. David was the kind of guy who rescued dogs that had been abandoned at truck stops, and once saved a Rottweiler puppy by slugging the guy who was abusing it (hence, that police record). He lived in Ohio, about three hours from where Roberta lives in a rural part of Michigan outside Ann Arbor, but he would take the worst trucking runs just because they took him near her house. Roberta loved it when David called her “Sis.” She was his only biological family aside from his children. He did not talk about feelings easily, which made it all the more meaningful when he told her he loved her.

Over the years, Roberta gained a lot from genealogy. She learned she was descended from a white man who owned a slave named Harriett and fathered children with her, and she found descendants of Harriett, distant cousins with whom she grew close. She learned that, through her paternal side, she had ancestors from sub-Saharan Africa. And she learned a lot about her father that was difficult to reconcile: how he drank, how he squandered his intelligence, and how he deceived two families.

As a child, Roberta had idolized him, much to her mother’s annoyance, and as an adult, she had to honestly look at the whole man. But genealogical research also allowed her to peel back the context for his life: The son of an impoverished bootlegger who was fed alcohol when he was hungry, he went into the hospital at least twice in an attempt to stop drinking. She wonders whether his pain may have driven him to take his own life in a second car accident, when Roberta was 7.

“Both David and I unquestionably knew that man loved us,” she said. “I have really good memories with my father, and so did David. There is just no black and white; there’s just a really big range of gray.”

For Roberta, genealogy solved mysteries; it gave her empathy and understanding for life’s grays. And most of all, genealogy gave Roberta her brother David.

Of course, she couldn’t know her family history hobby would give all that when she first got into it in the late 1970s, when she was pregnant with her first child. Back then, genealogy was as fast as polishing rocks.You asked family members for their stories. You wrote letters, actual letters sent through the mail. You traveled to small towns and chatted with clerks and went to libraries to scroll through microfilm until your eyes were red. Genealogy was for the few, the dedicated, the serious family historians.

“The slowness of the process was part of it,” Roberta said by phone from her home. She had long silver hair and was wearing a T-shirt that said, “I’m a genealogy rebel — I research past my bedtime.”

Since the first genetic genealogy tests became available in 2000, DNA testing gradually has become much cheaper and much better at finding close relative matches. Now, for $99 (and less during flash sales), the idly curious can stumble into genealogy; within weeks of submitting saliva they’ll have an ethnicity pie chart, as well as a list of perhaps thousands of relatives who match them in a company’s database.

They can quickly find out more about those relatives through family trees, Facebook, Google, LinkedIn, and people-search websites. They can see billions of the world’s genealogical records instantly via a subscription at Ancestry and through the free resources at FamilySearch.org, which is run by the Church of Jesus Christ of Latter-day Saints. And they can join groups, mainly on Facebook, for all the different kinds of genetic genealogy seekers.

Which means that whereas at the dawn of the consumer genomics era, when Roberta was searching for David, a person had to work years to solve a mystery, while now, the answer can often be found within days.

For nearly eight years, Roberta had David in her life. Then David got sick.

Many years before, when David had served as a Marine tail gunner in Vietnam, he’d been shot down and received a blood transfusion; almost three decades later, he developed hepatitis C from that transfusion, and then liver cancer.

As part of Roberta’s research into the Estes line, over the years she’d asked David to take a Y-DNA test, which looks at the sex chromosome a man inherits from his father. But it wasn’t till shortly before David died in 2012 that, after ordering tests to examine his autosomal DNA and her own, Roberta came to understand why David’s Y chromosome didn’t match those of other Estes men she’d studied.

Autosomal DNA testing — which now dominates the market for at-home genetic ancestry tests but was then relatively new — looks at the 22 pairs of chromosomes (known as autosomes) that don’t determine a person’s genetic sex. Unlike Y-DNA testing, it considers the genetic material contributed by both parents and their parents before them, and it can reveal relatives along both the maternal and paternal sides.

After Roberta got back the results from the newest kind of DNA testing and saw that she and David didn’t share genetic material, she realized that she and David were not, in fact, biologically related. It turned out, her father — the man who’d had two families — was not actually David’s biological father, a fact her father probably never knew himself. David’s mother had had a relationship with someone else.

Roberta tried to talk to David about what she’d discovered, but he made it clear he didn’t want to know the results from the DNA test, so Roberta never told him. The knowledge would have wounded him, and he was sick. And besides, David was her brother, in all the important ways.

At David’s funeral, the room was filled with his trucker friends, who left their big rigs parked around the funeral home with engines running in a tribute to David. His wife was sobbing. The preacher didn’t show. Roberta realized there was no one else to give the eulogy. So she did it.

For years after that, Roberta searched for David’s family, wanting to give David a kind of closure, wherever he was. She vowed to find his biological father. She was still administrator of David’s 23andMe account, so she searched among his matches for close relatives.

In time, Roberta made some discoveries, and posted about them on her blog, in a series titled “Dear Dave.” “Dave, meet the man we believe is your father,” she wrote, below a grainy black-and-white image of a man with a narrow face and deep-set eyes, who looked exactly like his son. “Dave, meet your half sister Helen,” she wrote, posting pictures of herself with Helen, the two women smiling and hugging.

Funny thing about Roberta and Helen. Because they were both David’s sisters — though one woman never had the chance to meet him in life and the other loved him even though they weren’t genetically related — it seemed to Roberta as if they were each other’s sisters, too.

When I spoke to Roberta, her depiction of David was so real, I felt like I knew him — his rough edges, his warm heart. She told me that once, when he came by her house for an impromptu visit along a trucking run, he saw some medical paperwork spread out on the table — Roberta thought she might have cancer, though she’d later turn out to be fine.

He confronted her: “Were you going to tell me?” If she was sick, David said, he’d sell his house and come take care of her. She could tell he meant it.

“You can’t do that,” she said.

“You can’t tell me what I can and can’t do,” he replied.

Roberta cried when she told me this story. “I don’t know if I’ve ever had anybody in my life love me like that,” she said.

The story of David was not an indictment of DNA testing, Roberta told me. Genealogy had given David to her; DNA could not take him away. The legacy of her father — his brilliance and his dysfunction — had taught her that genetics only goes so far. “Your genes neither guarantee you nor condemn you to anything,” she said. And genetics is not the only measure of family. DNA is just DNA — it could not account for her and David.

“It’s not love,” she said.

Libby Copeland will be in conversation with The Post’s Amy Argetsinger on March 6 at East City Bookshop in Washington, and with Jennifer Mendelsohn on March 5 at the Ivy Bookshop in Baltimore.