(CNN) -- Looking at them, you can see the similarities: the nose, the cheekbones, the dimpled chin and long, brown curls.
But what are the chances, right? What are the chances that two California teens would meet online in a roommate hunt, cross the country to attend Louisiana's Tulane University and learn a semester later they were half-sisters, the daughters of the same Colombian sperm donor?
Emily Nappi, 18, of San Francisco, and Mikayla Stern-Ellis, 19, of San Diego, learned exactly that January 7, when, acting on a suspicion they'd joked about since Father's Day, the women asked their mothers to hunt down their sperm donor numbers from the Los Angeles-based Calfornia Cryobank.
Stern-Ellis was in a doctor's office. She had asked her mother to send her the number. Nappi had done the same.
"They both text me at the same time, and it was the same number," Stern-Ellis said. "I was just staring at my phone. I didn't know what to do. I think the only way to describe it is mind-blowing."
Now that they know they're sisters, it all makes sense. They have the same build and are able to share clothes -- but, sadly, not shoes -- and they're science majors, with Stern-Ellis focusing on animals, Nappi on psychology.
They both sleep-talk and sleepwalk, reported The Advocate in Baton Rouge, and during a Black Friday shopping trip over Thanksgiving break, they independently purchased the same sweater.
It was their commonalities that drew them to each other in April. While preparing for college, they took a roommate survey and found they both had lesbian parents and a passion for theater, The Advocate reported.
"We had so much in common that I decided to e-mail her, Facebook message her and see if we could be roommates," Stern-Ellis told CNN.
That didn't pan out, but the two stayed in touch, and in June, Stern-Ellis posted a tongue-in-cheek Facebook message on Father's Day, thanking her Colombian sperm donor father for the X chromosome.
Whoa, Nappi thought. Her mother, too, had used a Colombian sperm donor. She replied to Stern-Ellis to let her know, saying she didn't want to be "creepy."
The two had fun with it and "went all first semester going, 'That girl might be my sister,' " Nappi said. "We just thought it was a joke."
Over winter break, their curiosity got the better of them. They had to know, so they reached out to their mothers and asked them to find the sperm donor numbers, and they learned in the first week of the new year that they had a new sibling, just seven months separating them.
"There's something so familiar in her face when I look at her," Nappi said.
Added Stern-Ellis, "Everything you don't see in our moms you see in us."
According to The Advocate, Nappi's mother said she selected the 19-year-old sperm donor because he was handsome, tall, athletic and interested in ecology. Stern-Ellis' mom had similar reasons and was drawn to his Colombian heritage and dark skin, the newspaper reported.
The sisters are now playing catch-up. They're learning the basics about each other -- Stern-Ellis' favorite color is purple; Nappi's is gray -- and "we're really close, but there's little things that are really essential about our being that we don't know about each other," Stern-Ellis said.
As for hunting down this strapping, intelligent Colombian gent who fathered them, it's one area where they differ.
Nappi says she'd be curious to see what he looked like, but she doesn't have any real urge to meet him.
"I don't feel lacking in any way," she said.
But her sister has longed to find him since she was a little girl, though finding a sister instead will sate her for now.
"Since this happened, I'm definitely going to wait a while because Emily is more than enough."
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