Jace Alexander died July 5, 2018, after finding a 9mm Sig Sauer handgun in his home, Fresno Police Chief Jerry Dyer said.
"This was a death that was completely avoidable," the chief said. "It's a responsibility to our children to store (firearms) in a safe and secure manner."
Dyer said Ramos was booked on a felony charge alleging he failed to store his handgun properly. If convicted, he could be sentenced to three years in prison.
Ramos, who lived in the home with the boy's father, had a gun safe in his bedroom, Dyer said. But the chief added that the handgun had been left out.
Jace's death was the third fatal gun accident in Fresno in the past few years, records show.
In 2011, a 2-year-old boy shot his 6-year-old sister to death after finding their stepfather's handgun. In 2015, a 10-year-old girl killed her 8-year-old sister with a handgun belonging to their father, a lieutenant with the Madera County Sheriff's Department.
In the first case, a 34-year-old man received three years probation after pleading no contest to felony storage of a handgun. In the second case, the district attorney's office declined to prosecute.