EXCLUSIVE: John Stamos discovers heartbreaking family history on 'Who Do You Think You Are?'


John Stamos takes a turn in digging deeper into his family history.

On Sunday's episode of Who Do You Think You Are?, the 53-year-old actor learns the heartbreaking truth about his grandfather: He was an orphan by the time he was 13 years old.

In ET's exclusive sneak peek, Stamos -- who is of Greek heritage -- is understandably shocked and saddened by the news. "Really? Orphan?" he asks. "That's sad, isn't it? How'd his parents die?"

RELATED: John Stamos and Bob Saget Get Emotional Talking About Don Rickles

19 PHOTOS
John Stamos out and about
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John Stamos out and about
HOLLYWOOD, CA - FEBRUARY 22: John Stamos arrives at the 87th Annual Academy Awards at Hollywood & Highland Center on February 22, 2015 in Hollywood, California. (Photo by Steve Granitz/WireImage)
LOS ANGELES, CA - NOVEMBER 22: Actor John Stamos poses in the press room at the 2005 American Music Awards held at the Shrine Auditorium on November 22, 2005 in Los Angeles, California. (Photo by Vince Bucci/Getty Images)
UNIVERSAL CITY, CA - JANUARY 23: Actor John Stamos of the television show 'Jake in Progress' listens to a question during the ABC 2005 Television Critics Winter Press Tour at the Hilton Universal Hotel on January 23, 2005 in Universal City, California. (Photo by Frederick M. Brown/Getty Images)
BEVERLY HILLS, CA - MARCH 31: Actor John Stamos, arrives at 'A Stellar Night' Gala Honoring actress Dakota Fanning by the Starlight Starbright Children's Foundation held at the Beverly Hilton Hotel, on March 31, 2006 in Beverly Hills, California. (Photo by Frazer Harrison/Getty Images)
NEW YORK - APRIL 18: Actor John Stamos arrives at the A&E Network's 'Up-Fronts' event to unvail its Fall 2006 season line-up at the Time Warner Center's Fredrick P.Rose Hall April 18, 2006 in New York City. (Photo by Clarence Elie-Rivera/Getty Images)
BEVERLY HILLS, CA - JANUARY 15: Actor John Stamos arrives at the NBC/Universal Golden Globe After Party held at the Beverly Hilton on January 15, 2007 in Beverly Hills, California. (Photo by Frazer Harrison/Getty Images)
NEW YORK - FEBRUARY 13: (EXCLUSIVE ACCESS) Actor John Stamos meets with television personality Rachael Ray on the 'Rachael Ray' show on February 13, 2007 in New York City. (Photo by Bryan Bedder/Getty Images)
BURBANK, CA - AUGUST 03: Actor John Stamos arrives to 'Comedy Central Roast Of Bob Saget' at the Warners Brothers Studio Lot on August 3, 2008 in Burbank, California. (Photo by Alberto E. Rodriguez/Getty Images for Comedy Central)
404347 27: Actor John Stamos arrives at the Roundabout Theater Company's 2002 Spring Gala April 22, 2002 in New York City. Plummer received the inaugural Jason Robards Award for contributions to the theater. (Photo by Lawrence Lucier/Getty Images)
NEW YORK - OCTOBER 7: Actor John Stamos joins the cast of 'Nine' with Eartha Kitt at the Eugene O'Neill Theatre October 7, 2003 in New York City. (Photo by Scott Gries/Getty Images)
UNIVERSAL CITY, CA - JANUARY 23: Actor John Stamos arrives at the ABC's Winter Press Tour Party on Wisteria Lane on January 23, 2004 in Universal City, California. (Photo by Vince Bucci/Getty Images)
CULVER CITY, CA - OCTOBER 27: ActorJohn Stamos front row at the Shakti Spring 2005 show at the Mercedes-Benz Fashion Week at Smashbox Studios October 27, 2004 Culver City, California. (Photo by Carlo Allegri/Getty Images)
377853 18: Actress Rebecca Romijn-Stamos kisses her husband John Stamos as they arrive at the 2000 Emmy Awards September 10, 2000 in Los Angeles, CA. (Photo by David McNew/Newsmakers)
HOLLYWOOD, CA - OCTOBER 11: Actor John Stamos attends the Los Angeles Premiere of 'Team America: World Police' at the Grauman's Chinese Theater October 11, 2004 in Hollywood, California. (Photo by Kevin Winter/Getty Images)
FULL HOUSE - Cast Gallery - July 1, 1987. (Photo by ABC Photo Archives/ABC via Getty Images)JODIE SWEETIN;BOB SAGET;JOHN STAMOS;DAVE COULIER;CANDACE CAMERON
File Photo of actor John Stamos taken in Los Angeles, Calif. on 7/27/84. (Photo by Bob Riha Jr/WireImage)
FULL HOUSE - Cast Gallery - June 26, 1987. (Photo by ABC Photo Archives/ABC via Getty Images)JOHN STAMOS
FULL HOUSE - 'Knock Yourself Out' - Airdate: October 30, 1987. (Photo by ABC Photo Archives/ABC via Getty Images)JOHN STAMOS
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Though the answer to that question remains unrevealed, Stamos, who lost his mother Loretta in 2014 and his father Bill in 2001, reflected on what life must have been like for his grandfather without his parents.

"I couldn't imagine growing up without both parents. Right when I saw that, I was like oh, my heart, you know?" he said. "My grandfather was a very successful man, a very successful businessman, a very proud father and grandfather, too. I guess he had to make up for lost family."

Who Do You Think You Are? airs Sundays at 10 p.m. ET/PT on TLC.

For more, watch the video above.

RELATED: John Stamos Posts Sweet Birthday Message to His Late Mother

23 PHOTOS
'Full House' while on set!
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'Full House' while on set!
FULL HOUSE - Cast Gallery - August 8, 1989. (Photo by ABC Photo Archives/ABC via Getty Images)DAVE COULIER;JODIE SWEETIN;MARY-KATE/ASHLEY OLSEN;BOB SAGET;CANDACE CAMERON;JOHN STAMOS
FULL HOUSE - 'The Perfect Couple' - Airdate: December 14, 1993. (Photo by ABC Photo Archives/ABC via Getty Images)BOB SAGET;DAVE COULIER;LORI LOUGHLIN;JOHN STAMOS
FULL HOUSE - 'The Perfect Couple' - Airdate: December 14, 1993. (Photo by ABC Photo Archives/ABC via Getty Images)BOB SAGET;DAVE COULIER
FULL HOUSE - 'I've Got A Secret' - October 18, 1994. (Photo by ABC Photo Archives/ABC via Getty Images)DAVE COULIER;DOT JONES
FULL HOUSE - 'Air Jesse' - February 7, 1995. (Photo by ABC Photo Archives/ABC via Getty Images)BOB SAGET;DAVE COULIER;BUDDY THE WONDERDOG;DYLAN/BLAKE TUOMY-WILHOIT;JOHN STAMOS
FULL HOUSE - 'Air Jesse' - February 7, 1995. (Photo by ABC Photo Archives/ABC via Getty Images)DAVE COULIER;BOB SAGET;JOHN STAMOS
FULL HOUSE - 'Michelle Rides Again - Part I' - May 23, 1995. (Photo by ABC Photo Archives/ABC via Getty Images)JOHN STAMOS;BOB SAGET;DAVE COULIER;BIFF MANARD
FULL HOUSE - 'Arrest Ye Merry Gentlemen' - December 13, 1994. (Photo by ABC Photo Archives/ABC via Getty Images)DAVE COULIER;MARY-KATE/ASHLEY OLSEN;JOHN STAMOS;DYLAN/BLAKE TUOMY-WILHOIT;BOB SAGET;MICKEY ROONEY;JODIE SWEETIN;MARY-KATE/ASHLEY OLSEN;CANDACE CAMERON;LORI LOUGHLIN
FULL HOUSE - 'On The Road Again' - November 8, 1994. (Photo by ABC Photo Archives/ABC via Getty Images)BOB SAGET;DAVE COULIER
FULL HOUSE - Cast Gallery - August 22, 1994. (Photo by ABC Photo Archives/ABC via Getty Images)DAVE COULIER;BOB SAGET;JODIE SWEETIN;MARY-KATE OLSEN;ANDREA BARBER;BLAKE TUOMY-WILHOIT;LORI LOUGHLIN;JOHN STAMOS;DYLAN TUOMY-WILHOIT
FULL HOUSE - 'Breaking Away' - October 4, 1994. (Photo by ABC Photo Archives/ABC via Getty Images)DYLAN/BLAKE TUOMY-WILHOIT;DAVE COULIER;JOHN STAMOS
FULL HOUSE - 'I've Got A Secret' - October 18, 1994. (Photo by ABC Photo Archives/ABC via Getty Images)BOB SAGET;JOHN STAMOS;DAVE COULIER
UNITED STATES - SEPTEMBER 27: FULL HOUSE - On location in San Francisco - Season Eight - 9/27/94, Pictured, from left: Dave Coulier (Joey), Bob Saget (Danny), Jodie Sweetin (Stephanie), Mary Kate Olsen (Michelle), Candace Cameron (D.J.), Andrea Barber (Kimmy), Blake Tuomy-Wilhoit (Nicky), Lori Loughlin (Rebecca), Dylan Tuomy-Wilhoit (Alex), John Stamos (Jesse). , (Photo by Craig Sjodin/ABC via Getty Images)
UNITED STATES - SEPTEMBER 27: FULL HOUSE - Cast gallery - Season Eight - 9/27/94, Pictured, from left: Andrea Barber (Kimmy), Dave Coulier (Joey), Ashley Olsen (Michelle), Candace Cameron (D.J.), Bob Saget (Danny), Blake/Dylan Tuomy-Wilhoit (Nicky/Alex), Jodie Sweetin (Stephanie), Lori Loughlin (Rebecca), John Stamos (Jesse), (Photo by Craig Sjodin/ABC via Getty Images)
FULL HOUSE - 'Another Opening, Another No Show' - Airdate: November 2, 1993. (Photo by ABC Photo Archives/ABC via Getty Images)CANDACE CAMERON;LORI LOUGHLIN;DAVE COULIER
FULL HOUSE - 'The Prying Game' - Airdate: November 16, 1993. (Photo by ABC Photo Archives/ABC via Getty Images)JOHN STAMOS;DAVE COULIER
FULL HOUSE - 'The Prying Game' - Airdate: November 16, 1993. (Photo by ABC Photo Archives/ABC via Getty Images)DAVE COULIER;DYLAN/BLAKE TUOMY-WILHOIT
FULL HOUSE - 'Fast Friends' - Airdate: October 12, 1993. (Photo by ABC Photo Archives/ABC via Getty Images)DAVE COULIER;JOHN STAMOS
FULL HOUSE - 'Wrong-Way Tanner' - Airdate: September 28, 1993. (Photo by ABC Photo Archives/ABC via Getty Images)DAVE COULIER;MARY-KATE OLSEN
FULL HOUSE - 'The Long Goodbye' - Airdate: September 29, 1992. (Photo by ABC Photo Archives/ABC via Getty Images)DAVE COULIER;BOB SAGET
FULL HOUSE - 'Come Fly With Me' - Airdate: September 22, 1992. (Photo by ABC Photo Archives/ABC via Getty Images)CYD STRITTMATTER;JODIE SWEETIN;MARY-KATE/ASHLEY OLSEN;BOB SAGET;DAVE COULIER;JOHN STAMOS
THE VIEW - Actor and comedian Jay Mohr guest co-hosts; Guests include Full House actors John Stamos, Bob Saget and Dave Coulier reunite; Anthony Bourdain (ABC's 'The Taste'); La La Anthony (author, The Love Playbook) today, Wednesday, January 29, 2014 on ABC's 'The View.' 'The View' airs Monday-Friday (11:00 am-12:00 pm, ET) on the ABC Television Network. (Photo by Heidi Gutman/ABC via Getty Images) JOHN STAMOS, BOB SAGET
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