Hitler's father, Alois Schiklgruber was born out of wedlock to Marie Anna Schicklgruber who later married Johann Georg Heidler. Heidler was also variously spelled Hitler. I don't recall if Johann Georg was Alois' father, but he gave him his family name, thus he (Hitler's father) became Alois Heidler or Hitler. Alois' second marriage, to Klara Poelzl, resulted in six children, of which Adolf was the fourth.

The British especially, ever so eager to make Der Führer appear ridiculous, traced his lineage and popularized the idea that his name should be Shicklgruber, a somewhat laughable name, even to the Germans.

Source(s):
Wm. Schirer, Rise and Fall of the Third Reich