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Five years ago, Alice Collins Plebuch made a decision that would alter her future — or really, her past.

She sent away for a “just-for-fun DNA test.” When the tube arrived, she spit and spit until she filled it up to the line, and then sent it off in the mail. She wanted to know what she was made of.

Plebuch, now 69, already had a rough idea of what she would find. Her parents, both deceased, were Irish American Catholics who raised her and her six siblings with church Sundays and ethnic pride. But Plebuch, who had a long-standing interest in science and DNA, wanted to know more about her dad’s side of the family. The son of Irish immigrants, Jim Collins had been raised in an orphanage from a young age, and his extended family tree was murky.

After a few weeks during which her saliva was analyzed, she got an email in the summer of 2012 with a link to her results. The report was confounding.

About half of Plebuch’s DNA results presented the mixed British Isles bloodline she expected. The other half picked up an unexpected combination of European Jewish, Middle Eastern and Eastern European. Surely someone in the lab had messed up. It was the early days of direct-to-consumer DNA testing, and Ancestry.com’s test was new. She wrote the company a nasty letter informing them they’d made a mistake.

But she talked to her sister, and they agreed she should test again. If the information Plebuch was seeing on her computer screen was correct, it posed a fundamental mystery about her very identity. It meant one of her parents wasn’t who he or she was supposed to be — and, by extension, neither was she.

Eventually, Plebuch would write to Ancestry again. You guys were right, she’d say. I was wrong.

In a family photo, Alice Collins Plebuch’s father, James “Jim” Collins, poses with his children. In the second row: Jim Collins, John Collins, Bill Collins, Brian Collins and Ed Collins. In the third row: Alice Collins Plebuch and her sister, Gerry Collins Wiggins.

We are only just beginning to grapple with what it means to cheaply and easily uncover our genetic heritage.

Over the past five years, as the price of DNA testing kits has dropped and their quality has improved, the phenomenon of “recreational genomics” has taken off. According to the International Society of Genetic Genealogy, nearly 8 million people worldwide, but mostly in the United States, have tested their DNA through kits, typically costing $99 or less, from such companies as 23andMe, Ancestry.com and Family Tree DNA.

The most popular DNA-deciphering approach, autosomal DNA testing, looks at genetic material inherited from both parents and can be used to connect customers to others in a database who share that material. The results can let you see exactly what stuff you’re made from — as well as offer the opportunity to find previously unknown relatives.

For adoptees, many of whom can’t access information about their birthparents because of closed adoption laws, DNA testing can let them bypass years, even decades, of conventional research to find “DNA cousins” who may very well lead them to their families.

But DNA testing can also yield uncomfortable surprises. Some testers, looking for a little more information about a grandparent’s origins, or to confirm a family legend about Native American heritage, may not be prepared for results that disrupt their sense of identity. Often, that means finding out their dad is not actually their dad, or discovering a relative that they never knew existed — perhaps a baby conceived out of wedlock or given up for adoption.

In 2014, 23andMe estimated that 7,000 users of its service had discovered unexpected paternity or previously-unknown siblings — a relatively small fraction of overall users. The company no longer provides data on surprise results. However, its customer base has more than doubled since 2014, and now contains more than 2 million people — and as more people get involved with recreational genomics, bloodline surprises are certain to become a more common experience. The 2020s may turn out to be the decade that killed family secrets, for better and for worse.

“We see it every day,” says CeCe Moore, a genetic genealogist and consultant for the PBS series “Finding Your Roots.” She runs a 54,000-person Facebook group, DNA Detectives, that helps people unravel their genetic ancestries. “You find out that a lot of things are not as they seem, and a lot of families are much more complex than you assume.”

Alice Plebuch found herself in this place in the summer of 2012. To solve the mystery of her identity, she needed more help than any DNA testing company could offer. After all, genetic testing gives you the what, but not the why.

Plebuch would turn out to be uniquely suited to the role of private eye in her own detective story. Now living in the suburbs of Vancouver, Wash., she worked as an IT manager for the University of California before her retirement. “I did data processing most of my life, and at a fairly sophisticated level,” she says. Computers do not intimidate her, and neither do big questions that require the organization and analysis of complex information. She likes to find patterns hidden in the chaos.

Just the skills necessary to solve a very old puzzle.

Jim and Alice Nisbet Collins on their wedding day in the 1940s.

After the initial shock of her test results, Plebuch wondered if her mother might have had an affair. Or her grandmother, perhaps? So, she and her sister, Gerry Collins Wiggins, both ordered kits from DNA testing company 23andMe.

The affair scenario seemed unlikely — certainly out of character for her mom, and besides, all seven Collins children had their father’s hooded eyes. But she couldn’t dismiss it. “My father, he was in the Army and he was all over the world, and it was just one of those fears that you have when you don’t know,” she says.

As they waited for their results, they wondered. If the Ancestry.com findings were right, it meant one of Plebuch’s parents was at least partly Jewish. But which one?

They had a gut sense that it was unlikely to be their mother, who came from a large family, filled with cousins Plebuch and her siblings all knew well. Dad, who died in 1999, seemed the likelier candidate. Born in the Bronx, Jim Collins was a baby when his mother died. His longshoreman father, John Collins, was unable to care for his three children and sent them to live in orphanages. He died while Jim was still a child, and Jim had only limited contact with his extended family as an adult.

But still, the notion Jim could somehow be Jewish seemed far-fetched. His parents had come to the United States from Ireland, and that history was central to Jim’s sense of himself. “He was raised in an orphanage; he didn’t have anything else,” Plebuch says. “He had his Irish identity.”

She plunged into online genealogy forums, researching how other people had traced their DNA and educating herself about the science. She and her sister came up with a plan: They would persuade two of their first cousins to get tested — their mother’s nephew and their father’s nephew. If one of those cousins was partly Jewish, they’d know for sure which side of the family was contributing the mysterious heritage.

The men agreed. The sisters sent their kits and waited.

Then Plebuch’s own 23andMe results came back. They seemed consistent with her earlier Ancestry.com test, indicating lots of Ashkenazi Jewish ancestry from areas such as Belarus, Russia, Ukraine and Lithuania. She also discovered that her brother Bill had recently taken a 23andMe test. His results were a relief — sort of.

“No hanky-panky,” as Plebuch puts it. They were full siblings, sharing about 50 percent of the relevant DNA, including the same mysterious Jewish ancestry. This knocked out another theory they had considered — that Plebuch might have been adopted.

Plebuch found a feature on 23andMe’s website showing what segments along her chromosomes were associated with Ashkenazi Jews. Flipping back and forth, comparing her DNA to her brother’s, she had a sudden insight.

There was a key difference between the images, lurking in the sex chromosomes. Along the X chromosome were blue segments indicating where she had Jewish ancestry, which could theoretically have come from either parent because females inherit one X from each. But males inherit only one X, from their mothers, along with a Y chromosome from their fathers, and when Plebuch looked at her brother’s results, “darned if Bill’s X chromosome wasn’t lily white.” Clearly, their mother had contributed no Jewish ancestry to her son.

Father Mother Passes down only X chromosomes to all children Can only pass down a Y chromosome to sons and an X to daughters X X X Y X X X Y Son Daughter Daughter’s genome Genome screenshot of Alice Plebuch = Ashkenazi Jewish ancestry X chromosome showing Jewish ancestry, which theoretically could have come from either parent Son’s genome Genome screenshot of Alice’s brother Bill X chromosome, which had to come from his mother, without any Jewish ancestry Source: Screenshots from © 23andMe, Inc. 2007-2016. All rights reserved; distributed pursuant to a Limited License from 23andMe SHELLY TAN/THE WASHINGTON POST Father Mother Can only pass down a Y chromosome to sons and an X to daughters Passes down only X chromosomes to all children X X X Y X X X Y Daughter Son Daughter’s genome Genome screenshot of Alice Plebuch = Ashkenazi Jewish ancestry X chromosome showing Jewish ancestry, which theoretically could have come from either parent Son’s genome Genome screenshot of Alice’s brother Bill X chromosome, which had to come from his mother, without any Jewish ancestry Source: Screenshots from © 23andMe, Inc. 2007-2016.All rights reserved; distributed pursuant to a Limited License from 23andMe SHELLY TAN/THE WASHINGTON POST = Ashkenazi Jewish ancestry X chromosomes showing Jewish ancestry, which theoretically could have come from either parent X X X X Daughter Mother Passes down only X chromosomes to all children Genome screenshot of Alice Plebuch Genome screenshot of Alice’s brother Bill X chromosome, which had to come from his mother, without any Jewish ancestry X X Y Y Son Father Can only pass down a Y chromosome to sons and an X to daughters Source: Screenshots from © 23andMe, Inc. 2007-2016. All rights reserved; distributed pursuant to a Limited License from 23andMe SHELLY TAN/THE WASHINGTON POST

“That was when I knew that my father was the one,” Plebuch says.

The next day, her sister Gerry Wiggins’s results came back: She, too, was a full sibling who also displayed significant Jewish ancestry. Then, Plebuch got an email from a retired professor known for his skill at interpreting ancestry tests, to whom she’d sent hers. “What you are is 50 percent Jewish,” he wrote. “This is in fact as solid as DNA gets, which in this case is very solid indeed.”

But how could their father have been Jewish? Could Jim Collins’s parents have been secret Irish Jews? Or maybe Jews from Eastern Europe who passed themselves off as Irish when they came to the country as immigrants?

Now they really needed the data from the cousin on their father’s side. If he also had Jewish ancestry, Plebuch figured, that could point to a family secret buried in Europe.

They waited for months, through a series of setbacks — problems in the lab, problems with the mail. Meanwhile, the sisters emailed back and forth.

Plebuch asked her younger sister: Did this revelation about their father’s ethnicity unnerve her? They’d been so certain of their family roots, and “now we know nothing,” she wrote.

“It is the first thing I think about when I wake up in the morning,” Wiggins replied, “and the last thing I think about as I drift off to sleep.”

At last, Plebuch was alerted that her cousins’ results were ready. The data from their mom’s nephew revealed that he was a full first cousin, as expected — sharing about 12.5 percent of his DNA with Plebuch.

But the results from her dad’s nephew, Pete Nolan, whose mother was Jim Collins’s sister, revealed him to be a total stranger, genetically speaking. No overlap whatsoever with Plebuch — or, by extension, with her father.

In other words, Plebuch’s cousin wasn’t actually her cousin.

And her dad’s sister wasn’t actually his sister.

Jim Collins, his wife, Alice, and John Collins, the brother he was raised with, in a circa 1944 photo. John was nearly a head taller than Jim.

Plebuch was devastated. This finding knocked out the secret-Jews theory — but if it put Plebuch closer to the truth, she still felt unmoored. She was deeply fond of Nolan, with whom she shared a birthday. “I was afraid he was going to reject me because we were no longer biological cousins.”

She called Nolan to share the results of his DNA test. “He was sad,” Plebuch says, “but he also told me I was the best cousin he ever had.”

Plebuch and Wiggins came to the stunned conclusion that their dad was somehow not related to his own parents. John and Katie Collins were Irish Catholics, and their son was Jewish.

“I really lost all my identity,” Plebuch says. “I felt adrift. I didn’t know who I was — you know, who I really was.”

For Wiggins, the revelation confirmed a long, lingering sense that something was amiss with her father’s story. Studying the family photographs on her wall, she’d thought for years that their paternal grandfather looked like no one in her immediate family. Visiting Ireland in 1990, she had searched the faces for any resemblance to her 5-foot-4, dark-haired father. “There was nobody that looked like my dad,” Wiggins says.

The sisters set about methodically pursuing several theories. With Jim Collins and his parents long dead, Plebuch knew she needed to unravel his story through the living. She signed up to take a class in Seattle on how to use DNA to find her father’s relatives.

If the woman Jim called his sister was not his sister, was there evidence of an actual sibling out there somewhere? Might that sibling have children? Might Plebuch and her siblings have first cousins they’d never known about?

The family tree Alice Collins Plebuch grew up with began to look different.

The dystopian novelist Margaret Atwood is fond of saying that all new technologies have a good side, a bad side, and a “stupid side you hadn’t considered.” Doing DNA testing for fun can carry consequences few of us might anticipate. It requires little investment at the outset, but it has the potential to utterly change our lives.

After researching her family history, Laurie Pratt decided five years ago to enhance her genealogical knowledge by testing herself and her parents. This was how she discovered that her dad was not related to her.

Pratt, 52, an airline ground operations supervisor in Orange County, Calif., went to her mother, who at first said the results were “impossible.” But over time, her mother divulged hazy memories of a short-lived relationship during a period when she and her husband were briefly separated.

Her mother couldn’t recall a name before she died. The man who raised Pratt also died; she never told him he was not, biologically speaking, her father.

She searched over several years, eventually identifying a potential candidate within the family tree of previously unknown cousins she found through DNA matching. She sent this man a letter — and days later, in February of this year, he suddenly popped up in the Ancestry.com database, identified by a saliva test as her biological father.

The man called her, and they spoke briefly on the phone. Though he was unmarried when Pratt was conceived, he fretted over the idea that he had abandoned a baby without knowing it. Pratt asked if they could meet, and the man agreed, but asked if he could take some time first to process the news and tell his wife and daughter.

Two days later, Pratt logged onto Ancestry.com and discovered that the man’s test had been deleted.

Reactions to DNA testing surprises vary dramatically. Moore, the genetic genealogist, says that, in her experience, even those who are initially dismayed end up glad that “they learned about the truth of themselves.”

But seekers may be a self-selecting bunch, and those who find the truth thrust upon them by someone else’s quest are not always happy about it. Gaye Sherman Tannenbaum, an adoptee who spent decades searching for her birthparents and now helps others on their quests, says in some instances, people are “outright hostile” when they learn of a newly discovered relative.

The reaction is understandable: DNA surprises often imply extramarital affairs, out-of-wedlock births and decades-old secrets.

Researchers from the University of Leuven in Belgium recently examined the English-language websites of 43 direct-to-consumer DNA testing companies and found that few companies warn consumers about the possibility of discovering “misattributed paternity.”

23andMe is unusual in offering multiple warnings. (“Unexpected relationships may be identified that could affect you and your family.”) “We are as transparent as possible,” says Kate Black, the privacy officer for 23andMe, brought on in 2015 after the company was criticized for failing to prepare consumers for such surprises. “We try to educate and inform people in every tool.”

Still, consumers may skim those warnings, or refuse to believe such surprises might lurk within their own families. Jennifer Utley, the director of research at Ancestry.com, says that even though she had seen many cases of surprise relatives in her work, she still found herself in “complete shock” when she tested her own DNA and discovered a first cousin she hadn’t known existed.

“I had no idea who this person was,” says Utley, who has since learned that her cousin was the product of a teenage relationship, raised by an adoptive family. Of her family, she now concludes: “We’re the best secret-keepers on the planet.”

Pratt says she doesn’t regret testing her DNA. She found herself both “devastated and curious” after the initial discovery about her genetic heritage. But, of course, that discovery was not hers alone, because her genes are not hers alone. Cases of unexpected paternity and secret adoptions implicate other people.

“I think this jars him,” she says of her biological father. “He goes to bed the good guy — he’s always been very religious, very Catholic. And he wakes up, he’s Mick Jagger. He has a baby. It blew his mind a little bit.”

In late April, Pratt sent the man another letter. She had “no desire to push myself into your family,” she wrote, nor make a financial claim. What she sought were stories about him and his family, to help her build a sense of where she came from. Just one meeting, a few hours, was all she asked.

She still hasn’t heard back.

The Collins children — from left, Kitty, Jim and John — with their longshoreman father, John Josef Collins, in 1914. Collins, a widower, was unable to care for his three children and sent them to live in orphanages. He died while Jim was still a child.

By early 2013, the Collins children were hot on the trail of a hundred-year-old mystery.

They had their father’s birth certificate, indicating that he’d been born on Sept. 23, 1913. They wrote to his orphanage and learned that their dad had been sent there by the New York Society for the Prevention of Cruelty to Children.

Plebuch wondered if Jim Collins, just a baby at the time, had somehow been confused with another child when he was taken from his father’s home.

She found a forensic artist said to be skilled in understanding how faces change over time. She sent her a picture of her dad sitting on his father’s lap when he was about 11 months, along with photos of him as an adult. Were these of the same person?

Probably, the forensic artist ruled. The ears hadn’t changed, and the mouth, chin and facial proportions seemed the same.

If the mystery of their father didn’t begin with his parents’ life in Ireland, nor with his own time in the orphanage, Plebuch and her sister concluded it must have happened shortly after Jim was born. Unusually for the era, his mother gave birth not at home but at Fordham Hospital in the Bronx.

Could something have happened there?

Fordham Hospital in the Bronx, where Jim Collins was born in 1913. (Wurts Bros./Museum of the City of New York)

By this time, the sisters were using techniques developed by Moore and others to help adoptees try to find relatives in a vast universe of strangers’ spit. Every time a site like 23andMe informed them of what Plebuch calls a “DNA cousin” on their Jewish side — someone whose results suggested a likely cousin relationship — they would ask to see that person’s genome. If the person agreed, the site would reveal any places where their chromosomes overlapped.

The idea, Plebuch explains, was to find patterns in the data. A group of people who share segments on the same chromosome probably share a common ancestor. If Plebuch could find a group of relatives who all shared the same segment, she might be able to use that — along with their family trees, family surnames, and ancestors’ home towns in the old country — to trace a path into her father’s biological family.

The work was slow and painstaking, complicated by the fact that Ashkenazi Jews frequently marry within the group and often are related in multiple ways. This can make distant relatives look like a closer match than they actually are. But the sisters forged on, sending at least 1,000 requests for genome-sharing to DNA cousins through 23andMe. It became Plebuch’s full-time job.

Some ignored their overtures, while others were drawn in by the saga and devoted their own efforts to helping the sisters untangle it. It was as if the Collins sisters had plugged into a larger family, a web of strangers who wanted to help because generations before, their ancestors had shared soup, shared heartache, slept in the same bed.

One DNA cousin made a clever suggestion: Why not search for evidence of a baby born around the same time under a common Jewish surname, Cohen? He reasoned that the nurses, perhaps relying on an alphabetical system, might have confused a Collins baby with a Cohen baby. Cece Moore was by now volunteering to advise Plebuch, and with additional help from Tannenbaum and the New York City Birth Index of 1913, Plebuch found a Seymour Cohen born in the Bronx on Sept. 23. DNA cousins fanned out on the Internet, tracking down a descendant of Seymour’s sister.

Plebuch wrote to the woman, a professor in North Carolina, and offered to pay for her test kit if she’d contribute something completely free and absolutely priceless: her saliva. The woman agreed.

Weeks later, the results came back. No relation.

After that red herring, Plebuch decided to dive deeper into the 1913 birth index, to find babies who were in the hospital at the same time as her father. It was no easy task: The list of children born in the Bronx in 1913 ran 159 pages, was not ordered by date, and didn’t distinguish hospital births from home births. But she managed to isolate all the male children born on Sept. 23, as well as the day after and the day before. She further narrowed the list to names that sounded either Jewish or ethnically neutral — 30 babies in all.

Her hope was that one of those babies would share a surname with one of the people that the DNA matching sites identified as a likely relative. So she searched methodically.

“Appel” — nothing. “Bain” — nothing. “Bamson” — nothing.

It was another dead end.

The sisters went back to the chromosome segment matching, both at 23andMe and Family Tree DNA, where they had also uploaded their genetic data. They bought at least 21 DNA test kits for themselves, relatives and strangers suspected of being relations. Plebuch found she and her siblings matched to 6,912 likely DNA relatives, with 311,467 “segment matches” among them — segments along the chromosomes that overlapped with those of the Collins children. Which is to say, 311,467 potential clues.

The data they had kept on spreadsheets quickly became overwhelming, so their brother Jim, a retired software and systems engineer who had worked on NASA supercomputers, designed an iPad app called DNAMatch to help them and other seekers keep their data straight.

Plebuch was determined, and unusually well suited to the task of solving a puzzle hidden in big data. She and Wiggins searched this way for two and a half years. But she was having no luck finding someone closely related to her father’s biological family — they simply weren’t in the system.

Perhaps they didn’t know about DNA testing, or couldn’t afford it, or weren’t interested.

All the sisters could do was keep working and waiting, hoping the DNA testing revolution would make its way to strangers who shared their blood.

To solve her father’s mystery, Plebuch explored a web of “DNA cousins” who testing suggested were blood relatives.

Ultimately, the crack in the case came not through Plebuch’s squad of helpful DNA cousins, but through a stranger with no genetic connection.

It was Jan. 18, 2015, a Sunday, and Plebuch was feeling down. She was writing an email to her cousin Pete Nolan — the beloved relative it turned out she wasn’t really related to — to update him on her stalled search.

As administrator of his 23andMe account, she had permission to check the list of his DNA relatives yet rarely did so, since new relatives rarely showed up. But she decided to check it this day — and this time, there was a new person. A stranger had just had her saliva processed, and she showed up as a close relative of Nolan.

Plebuch emailed the woman and asked if she would compare genomes with Nolan. The woman agreed, and Plebuch could see the segments where her cousin and the stranger overlapped. Plebuch thanked her, and asked if her results were what she expected.

“I was actually expecting to be much more Ashkenazi than I am,” the woman wrote. Her name was Jessica Benson, a North Carolina resident who had taken the test on a whim, hoping to learn more about her Jewish ethnicity. Instead, she wrote, she had discovered “that I am actually Irish, which I had not expected at all.”

Plebuch felt chills. She wrote back that her father had been born at Fordham Hospital on Sept. 23, 1913. Had anyone in the Benson family been born on that date?

Jessica replied. Her grandfather, Phillip Benson, might have been born around that date, she wrote.

Plebuch began to cry.

She started combing through her list of baby names from the 1913 Index. No “Benson” born that day in the Bronx. But then, well after midnight, she found it:

The New York City Birth Index had a “Philip Bamson,” born Sept. 23 — one of the names she had searched among her DNA cousins. This had to be Phillip Benson, his name misrecorded on his birth certificate.

Plebuch knew in her bones what had happened. This was no ancient family secret, buried by shame or forgotten by generations.

This was a mistake that no one had ever detected, a mistake that could only have been uncovered with DNA technology. Someone in the hospital back in 1913 had messed up. Somehow, a Jewish child had gone home with an Irish family, and an Irish child had gone home with a Jewish family.

And the child who was supposed to be Phillip Benson had instead become Jim Collins.

Sitting, from left, are Phillip Benson’s first wife, Esther Abolafia Benson, their son Kenny, and Phillip Benson. Behind them are Ida Cott Benson and Sam Benson, the parents Phillip Benson grew up with.

Pam Benson was stunned by what this stranger was telling her over the phone.

“I said, ‘You’ve got to be kidding me,’ ” says Pam, who is Jessica’s aunt and the daughter of the late Phillip Benson.

The Lawndale, Calif., woman sent off for her own DNA kit and discovered that, rather than being part Jewish as she’d long thought, she was part Irish, and first cousins with a man she’d never heard of — Plebuch’s “Irish cousin,” Pete Nolan.

A copy of Phillip Benson's birth certificate, in which his name is misrecorded as “Philip Bamson.” Born the same day as Jim Collins, he was sent home with the Collins family in an accidental switch. A copy of Jim Collins's birth certificate, one number apart from Phillip Benson’s and signed by the same doctor, suggesting they were processed close together in time.

The families compared the birth certificates for Jim Collins and Phillip Benson and found they were one number apart and signed by the same doctor, suggesting they were processed close together in time. Plebuch began to research the ways an earlier generation of hospitals kept track of their littlest charges. In the book “Brought to Bed: Childbearing in America, 1750-1950,” she found an astonishing picture, taken at a Manhattan medical institution the year before her father was born. It shows at least a dozen newborns piled on a cart like so many cabbages.

“Every time I show it, when I give lectures, the whole audience gasps,” says author Judith Walzer Leavitt, a childbirth historian and a retired professor at the University of Wisconsin-Madison. “You can understand how possible it was to switch babies inadvertently.”

In 1913, hospital births were still unusual, and procedures to identify babies were inconsistent. Some hospitals kept babies sleeping in cots by their mothers’ beds, while others kept them in nurseries, increasing the chances of a mix-up. While it’s hard to know what practices were in place at Fordham Hospital, which was shuttered in 1976, Leavitt says it was not until the 1930s or ’40s that it became standard for hospitals to give babies and their mothers identifying wristlets or anklets. In 1913, they more typically “just depended on mothers’ recognition or nurses’ remembrance.”

The families exchanged photographs. Pam Benson saw Plebuch’s short, dark-haired dad Jim Collins, who looked far more like Benson’s 5-foot-4 grandfather and 4-foot-9 grandmother than did her own blue-eyed, 6-foot father, Phillip.

Plebuch’s search led to a family she never knew about that was inextricably linked to her own. A Collins baby was raised a Benson and a Benson baby was raised a Collins. Here are the biological lines revealed by DNA testing.

Plebuch’s search led to a family she never knew about that was inextricably linked to her own. A Collins baby was raised a Benson and a Benson baby was raised a Collins. Here are the biological lines revealed by DNA testing.

“My grandfather came to my dad’s shoulders,” she says. She had once asked her dad how he could be so tall. “He said, ‘recessive genes.’ ”

The Collins sisters had long had their own explanation for why their father didn’t seem to resemble his siblings. Roger Wiggins, Gerry’s husband, recalls meeting Jim’s tall, lanky brother in the 1970s and asking Gerry about it. “She said, ‘Well, my dad was in the orphanage, and when he was in the orphanage he was malnourished.’ ”

Plebuch and Pam Benson took to calling each other “swapcuz,” though in fact they share no genetic relation. And now Plebuch discovered she had a real new first cousin: Phylis Pullman, the daughter of the biological sister Jim never knew. In late 2015, Plebuch flew to Florida to meet her. Sitting at opposite ends of a couch, the diminutive women were like mirror images; they could have been sisters.

Pullman told her the family story of how, when her tall Uncle Phillip was courting his first wife, her observant Jewish parents didn’t believe he could possibly be a member of the tribe.

“He had to bring his birth certificate,” says Pullman. “Little did we know it wasn’t his birth certificate.”

Phylis Pullman, left, first cousin to Alice Collins Plebuch, and Alice chat with Alice's second cousins Dan Klein and Jerry Klein while looking over photo albums of their families. (Yana Paskova/For The Washington Post)

In January, several members of the Collins family joined Pullman and Pam Benson on a cruise. It was oddly comfortable, Pullman says — no strangeness among strangers, as if blood recognized blood. Even Pam Benson, the daughter of an Irishman raised Jewish, who didn’t share genes with any of them, felt at ease. “It was like we were all one big swap family,” she says. She and Plebuch have been working together to try to get New York State to annotate their fathers’ birth certificates, to reflect their true parentage.

But the revelations have also felt like a loss. Pam Benson’s late father was a Jew, only he wasn’t, and sometimes her daughter would come home and catch Pam crying over what he would have thought of this. How were she and Plebuch to reconcile that their fathers weren’t what they thought they were? And, for that matter, what were they? Was Jim Collins a Jewish man because he was born that way, or an Irishman because he was raised one?

Plebuch has come to agree with her younger sister that if their dad were alive, it would be right to tell him the truth about his birth. But she considers it a mercy that Jim Collins didn’t live through the era of recreational genomics. This was a man so proud of his heritage that his children gave him an Irish wake, with Wiggins singing his favorite song, “Danny Boy.”

“My dad would have lost his identity,” Plebuch says. “He’s been kind of spared that.”

She and her siblings also think about what would have happened if Jim Collins remained with his biological family, and had become Phillip Benson, as he was supposed to. As the two families exchanged old photos, Plebuch came across one of a young Phillip sitting on a horse and felt a pang of jealousy. She wouldn’t begrudge Phillip for those happy childhood days — but it should have been her dad on that horse.

In looking over Benson family photos, Alice Collins Plebuch was struck by a childhood photo of Phillip Benson on a horse. She wondered what kind of life her father, Jim, would have lived had he been raised with his biological family, the Bensons.

If not for the switch, Jim would have been raised in an intact home. He almost certainly would have completed high school and might have done something with his gift for mathematics. Instead, he served in the Army and later as a California prison guard — spending his career in institutions like the one that defined his childhood. He made a decent life for himself, but his kids still grieve for the losses of that little boy. “In the orphanage, my father got an orange for Christmas,” Plebuch says.

And yet, were it not for what happened in 1913, Alice Collins Plebuch would not exist. The Collins children owe their lives to an administrative oversight. A nurse’s momentary lapse of attention, perhaps. It was a terrible thing, and yet, how can they resent that it happened?

It is astonishing what DNA testing can do. The same technology can cleave families apart or knit them together. It can prompt painful revelations, and it can bring distantly related members of the human family together on a quest, connecting first cousins who look like sisters, and solving a century-old mystery that could have been solved no other way. It can bring to light a split-second mistake committed by someone long dead, in a city across the country, in a building that no longer exists. It can change the future and it can change the past.

It can change our understanding of who we are.

Plebuch says she and her siblings decided as a family “we were not going to be bitter.” It is a complex feat, made necessary by old-fashioned error and modern-day technology, to grasp that a terrible thing happened, and that you are grateful for it. Nor does Plebuch regret what she’s learned. She does not regard DNA testing as a Pandora’s box better left closed, though this thing she undertook casually turned out, she says, to be “the biggest deal in the world.”

It is the truth, after all.

Alice Collins Plebuch poses for a portrait after meeting relatives in Seaford, N.Y., on June 24. Of her father, she says she is glad he did not live into the age of recreational genomics. “My dad would have lost his identity,” Plebuch says. “He’s been kind of spared that.” (Yana Paskova/For The Washington Post )

Libby Copeland is a former Washington Post staff writer. She is collecting interesting stories about DNA testing. To share yours, contact her at DNAtestingstories@gmail.com.