George Joestar I is Jonathan's father. He lost his wife in a carriage accident where he met Dario Brando. After Dario's death, Dio went to live with George and Jonathan at the Joestar Estate (because George thinks Dario saved his life so takes in Dio as thanks. When in actuality George knew all along but still took Dio in). Eight years later George had caught a disease. After Jonathan returned from Ogre Street, Dio attacked Jonathan with a knife (which George blocked) and got stabbed in the back. His blood landed on Dio which turned Dio into a vampire when he died.

(Source: Wikipedia)