The Grand Gala apple gets its bloated size from cells that get bigger and bigger rather than dividing.
A mutant Gala apple is extra big, extra sweet, and extra crispy.
Scientists have probed the apple's genes to figure out what makes it that way.
The findings could help growers breed new and better varieties of apples.
When a mutant apple tree in Tennessee produced unusually enormous and extra crispy fruit, scientists took note.
"To find a mutant in an orchard is not uncommon, but these were 40 percent bigger," said Peter Hirst, a horticulturalist at Purdue University in West Lafayette, Ind. "From 50 feet away, you could tell they were much larger."
Now, Hirst and colleagues have figured out what gives the apples -- called Grand Galas -- their extra-large personalities: A set of mutations that make their cells grow larger than normal. The finding might eventually help growers produce crispy, delicious and giant apples that consumers would choose over other varieties.
"When I take this fruit home, that's the only one my family asks for," Hirst said. "This is another step in trying to understand how apples grow, so we can grow better apples and grow them more profitably."