In Attack on Titan, what differs royal blooded people from Subjects of Ymir? Subjects of Ymir are all descendants of Ymir. What differs the so called royal bloodline from the rest?
That's actually a really good question. I suppose you are talking about how only the royal bloodline can use the full potential of the Founding Titan. My theories as a somewhat mediocre fan to solve this apparent plot hole are as follows:



 It actually doesn't matter from what we have seen in the manga. Eren was able to seize control of the Founding Titan by convincing Ymir, the First Titan, to make her own choice as she was a slave to the will of the royal bloodline since the days of the founding king, Fritz. Hence the royal bloodline wasn't really special. They just had the advantage of manipulating a nigh-omnipotent girl to do their bidding.


The first theory can be shot down when you consider the fact that Zeke's spinal fluid can be used to turn Eldians into pure Titans. Thus perhaps the royal bloodline is special.
I'm no geneticist, just a hyperactive high schooler, so what I'm about to propose may be scientifically preposterous. I think because since the royal family is directly descended from Maria, Rose, and Sina, the first humans to eat Titan blood raw I might add, it's possible that there's a higher concentration of the original power in the royal bloodline hence their abilities. How you might ask? They may have taken a page from the Targaryens.


I hope this answer is ok. If I'm wrong somewhere, please correct me.