"Also extremely illegal! My grandfather is an exception but in general stars are really racist."

"Can I pretty please look at your genome sometime?"

"Knock yourself out." 

"What are 'genes.'"

"--Oh shit, you're way pre-Mendel. Uh, I will explain later because when I start explaining I will be difficult to stop until I'm done." 

He nods in acceptance. 

"I'd like to know what genes are too."

"The extremely short version is that they are the way your body stores hereditary information. The stuff that makes you look like your parents. Bloodlines."

"Huh. I wonder if I'm genetically a prime, that runs in families."

"I would probably have to look at several people's genes from your world to determine that. Most genes aren't labeled in big block letters with what they do, you have to figure it out by comparing with other people's genes. Like, if you look at a dozen people's genes, and seven of them have brown hair and five of them are blonde, and all the brown-haired people have the same gene and the blond people have the same different thing in that same spot, you can guess that that gene-spot probably codes for hair color. I mean, it is way, way more complicated than that, on a wide variety of levels, but that's the basic principle." 

"Huh. I will be interested to hear the longer version." Her breakfast is gone. She consults the nearest clock to see how long they have till class.

About fifteen minutes. 

"The reason Edie expects to be able to find anything interesting looking at Lucy's genes is because a human being shouldn't be able to have children with a giant star-crab-mountain person. Like--do you know enough about agriculture to know that mules are sterile?"

"Yes. When I was a kid I was very confused both by the intentional breeding of mules and by gelding in general."

"Mules exist and are sterile because horses and donkeys are closely related enough to have children, but not closely related enough to have fertile children. I don't recall exactly why sterility is what happens in the gap between 'fully interfertile'--like dogs with wolves and coyotes--and 'not interfertile at all,' but it is; tiger-lion hybrids exist and are sterile, as another example. Stars and giant space crabs being closely related enough to have fertile offspring I'll buy; the morphological variations within domestic dogs get pretty absurd. But neither one ought to be close enough to humans to have children at all, so probably something weird is going on." 

"--Oh, yeah, I actually know the weird thing that's going on." 

"You do?"

"Yeah, Judgments and Messengers aren't, uh, the word that gets used might translate as 'animals' but as always with the Correspondence it's much more complicated than that--they didn't evolve like terrestrial life and terrestrial people did." 

"That...raises...more questions..." 

"Such as: what does 'evolve' mean." 

"Oh wow you don't want to get me started on that one either but the short version is that another term for it is natural selection; it's like artificial selection also known as selective breeding, except that instead of human breeders selecting for the highest milk output or meat quantity or wool quality it's nature selecting for who lives long enough to have enough babies." 

--He parses the implications of this. "And this happened to all life on Earth?"

"Yep. When I say different species of animals are more closely related I don't just mean they're more similar, I mean they're literally more closely related. You and I are cousins millions of degrees removed with monkeys, cats, and even mushrooms and flowers." 

(Wei Wuxian is fascinatedly scribbling notes.)

"That is both fascinating and not about mind control at all, which outperforms the academics I was prepared to absorb today!"

Giggle. "Not the most practical thing, but even being paranoid we don't have to be practical every minute of the day."