Actress Christina Applegate had to laugh when she discovered her great-grandfather killed his brother while researching her family tree on U.S. Tv show Who Do You Think You Are? - because the rest of her ancestral history was so dark and miserable.

The star and her father Robert Applegate signed up to trace their roots for the genealogy show in July (13) - but neither expected the family story to be so sad and desperate.

Researching her grandmother's side of the family, the Applegates learned all about the abuse ancestors suffered and they were both quite relieved when they discovered there had been a murder in the family.

Promoting the show in a new interview with Jay Leno, she says, "We were both really scared because we knew it wasn't going to be good information. We knew it was going to be really sad because she (her grandmother) died really young. It was one of those weeks where it was like every single day it was one like one more awful, horrible, sad story, to the point where some of the things we found out didn't air.

"And one of the things was that my great-grandfather killed his brother, and my dad and I literally started laughing about that one. We're like, 'That's hysterical, what a wild man.' Because we had heard so much drama, that was the funny part of the week. We were like, 'Oh my God, grandpa burned his brother, that's crazy'."