Caterina Scorsone in "Private Practice."

Physically speaking, Toronto's Caterina Scorsone really could be Patrick Dempsey's sister.

"I had never thought of the resemblance, but once people mentioned it to me, I could totally see it," Scorsone said. "Even the colouring, everything."

Well, Scorsone has become a regular on Private Practice, the Grey's Anatomy spinoff series, playing Dr. Amelia Shepherd, the little sister of Grey's staple Dr. Derek Shepherd (Dempsey).

And on Thursday, Oct. 7, Scorsone's Amelia will cross over and appear in a new episode of Grey's Anatomy. Private Practice airs Thursdays on ABC and A, while Grey's Anatomy airs on ABC and CTV.

Amelia and Derek - aka "McDreamy" - are estranged, so her arrival in the world of Grey's Anatomy is both unexpected and unwelcome, as far as Derek is concerned. And it wouldn't be Grey's Anatomy if raw emotional issues between the siblings - both past and present - didn't bubble to the surface.

"My character has some really interesting stuff with Derek," Scorsone said.

As a kid, Scorsone - now 28 - appeared on the classic Canadian TV series Mr. Dressup. She kept busy in recent years with some film and episodic TV work, but her acting profile took a big leap forward when she was cast as the lead in the high-profile mini-series Alice, which aired in December 2009.

So how did Scorsone wind up as McDreamy's little sister?

"At the end of last season on Private Practice, Eric Stoltz was directing one of the episodes, and he had worked with me on a film in Toronto (My Horrible Year!) about a decade ago," Scorsone recalled. "He had cast me in that film.

"So when this role came up, he kind of talked to Shonda (Rhimes, creator of Private Practice and Grey's Anatomy) about me. And there was the resemblance. So Shonda was like, 'We should bring her in.'

"It was the regular audition process and it went well. So I did five episodes of Private Practice at the end of last season and they decided to bring me back as a regular character this season."

Scorsone has been on quite a "role roll" over the past year and a half, and she attributes much of it merely to being ready when opportunity comes knocking.

"I've been very blessed, and I have fantastic people around me personally, which allows me to be open and available when people call," Scorsone said. "A friend of mine once said to me, 'It's sad to be ready and not be called on, but it's tragic to be called on and not be ready.' "

Scorsone was a fan of Grey's Anatomy before any of this happened, so the crossover episode is especially cool.

"One of my sisters in particular is a big fan of the show, so yeah, my family is really happy," Scorsone said. "They're feeling like they're part of the extended family.

"My sister can say, 'I'm sort of weirdly related to Derek Shepherd. She's MY sister, and she's HIS sister!"

The Scorsones? Naw, just call them the McDreamys.

bill.harris@sunmedia.ca