User blog comment:NickFusi0n/A Theory on The Coordinate and The Wall Titans/@comment-3074300-20140524172812/@comment-93.209.16.74-20140525065851

"My theory is that the serum that Grisha injected into his son actually turned him into a normal titan." - That's how I understood it, it does not pass down through a family by birth, they have to eat a shifter. I believe Annie, Reiner and Berthold were all forced to eat their dads. In her flashbacks, Annie's father cried a lot and said he was sorry abuot what Annie had to do but "daddy will always be together with you".

If the serum was not the Coordinate, it probably did to Eren what the ape did to that village, transforming him into a Titan. He then ate Grisha, becoming a shifter. Whether Grisha had the Coordinate and Eren acquired it together with the shifting abilities, I don't know, it seems to be related to the mysterious Eren-like woman who can alter memories. Both is a form of mind control. Maybe Eren had the Coordinate before the shifter ability, and Grisha deliberately and indeed desperately turned him into a shifter, knowing it.