How are the subjects of ymir descendants of her when her descendants are royal. How did some of her descendants inherit less royal blood than others? I understand how quickly ymirs blood can make its way into a population in the span of 500 years, but wouldn’t all the subjects inherit “royal” blood? If the answer is incest for why the royal family has more of ymirs blood, then shouldn’t someone like zeke not inherit royal blood since he is a “half breed”?

Royal blood in real life doesn’t actually mean anything given that most of the population can trace their ancestry back to some type of ruler in some time in history. Fact check me on this, but at least half of europe can trace back to Charlemagne? Royal blood is a status symbol, given that many people can trace back to be the “lost descendant” of royalty or cut from the family.

Attack on Titan is a story more about the human condition. It has a lot of symbolism and references to real life. So I do understand that it clearly isn’t scientifically accurate. The subjects of ymir is simply something that wasn’t explained well within the story.