Entertainment Tonight ET Online ETOnline.com

Sarah Hyland -- who plays spunky big sister Haley Dunphy on Modern Family -- is opening up about a secret disease that led her to receive a kidney transplant last month.

VIDEO: Modern Family Unleashed on Disneyland

The 21-year-old actress reveals in the new issue of Seventeen magazine that after fighting a lifelong battle with kidney dysplasia, she underwent the transplant on April 13, with her father as the donor. She calls the new kidney "a second chance at life" that "not a lot of people get."

RELATED: Modern Family Stars Tackle the Slopes For Operation Smile

She tells the magazine -- on newsstands May 22 -- that the experience emphasized the importance of her extended family. "You know that family is always going to be there for you -- no matter what. My dad gave me a freakin' kidney!" she says. "But it's also the families that you create outside of your family. And you really find out what kind of people you're friends with. It was just amazing, and it really opened my eyes to see who's there for me and who's not."

PICS: Modern Family Saddles Up For Third Season

Sarah also praised her boyfriend, actor Matt Prokop, for helping her through the ordeal. "He helps me with my medication and takes me to doctor appointments and all the stuff that I need to do to take care of myself. Any other 21-year-old guy would not be there, I guarantee it. I am one of the luckiest people in the world to have him be there for me."