ChiPolVee's comment on elricbrothersfan's answer:

Oh! I have a question on this too! Does that mean Reka and Toki's child will have this mark too? Even though Toki doesn't have one/doesn't have this one? And if two people with marks have a kid, will the kid have both?? Sorry, I wonder this stuff....

Reka: K-Kid?! Me and Toki?!?! o//o

Answer:

"Takeshi here for today's lecture on Gate customs*!

The child inherits the father's family mark. In the case where only one parent is from the Gates (which is extremely rare), whether the child receives a mark depends on where it is raised. If the child is raised in the Gates, it will be given a mark; if not, it most likely won't."
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Thanks for the question!!

Takeshi: It's a pleasure to meet you. ^^ I'm glad I could answer your question!

Reka: You kind of butted in on my topic, but...

Takeshi: You were too busy being flustered, so I stepped in.

*See "Eternal World" for info on the universe these characters come from.