So Ms. Liff did what many others have: she turned to DNA testing and the internet. She found answers, some happy and others heartbreaking, but the information was always revelatory.

Through the genetic testing company 23andMe, which says it has sold 10 million kits and that 95 percent of buyers have connected with relatives who are in its database, Ms. Liff found her parents.

Ms. Liff connected with a biological cousin who linked her with her biological father, Keith Cruz .

Mr. Cruz, 52, was from the Bensonhurst neighborhood in Brooklyn but had moved to Long Island where he lived with his wife and three children.

In February, Ms. Liff called Mr. Cruz after rehearsing with her boyfriend the various scenarios the conversation could take.

Mr. Cruz knew why Ms. Liff would be calling, but he could still not believe what she was telling him. He was so nervous to be speaking with the daughter he had never met, that he “almost fell down a flight of stairs.” But within moments, he said, “It was like I had spent every day with her — I felt instantly at ease.”

They arranged to meet recently near Ms. Liff’s apartment, at Tom’s Restaurant, the diner of “Seinfeld” fame. He showed up with flowers, grabbed a booth near a window and held his breath as he watched her approach.

“My heart stopped,” recalled Mr. Cruz, a trash collector. “This 22-year-old girl — my daughter who I’d never seen before — was crossing the street. My heart was beating in my chest like a hammer. For that brief second, time seemed to stop.”