[quote=@tipssyCalibrator]
<Snipped quote by Lmpkio>
Wait, Doom Slayer is a descendant of BJ Blazkowicz? I didn't know that

[/quote]

Apparently yes - according to John Romero and Tom Hall - which also means that one of his ancestors is also Commander Keen. 

Basically, B.J. is Keen's Grandfather, and Keen is Doom Guy's father or grandfather.