Okay, so, when Koma mentioned that male calico cats have a low fertility rate, he was technically incorrect. Male calico cats aren't technically male. So, the gene for the calico fur pattern requires two X chromosomes. Now, typically, a male has an X and a Y chromosome while a female has two X chromosomes (the exceptions are transgender people and some intersex people). For a male calico cat to be born, they actually have to have two X chromosomes and a Y chromosome. This results in them being sterile. If Kotarou is also sterile because of his connection with Koma, he's likely the same - two X chromosomes and a Y chromosome. In humans, this is called Klinefelter syndrome, and men with it typically have trouble growing chest and facial hair, have small testes, and have a more feminine body shape (narrow shoulders, wider hips, etc., though not always, of course, because human bodies are unique). Anyway, I thought this was a little fun fact to share that gave the series an interesting new lens. (=・ω・=)
I'm screaming and crying and tHOSE TWO BETTER BE OKAY.