Two of my PFS characters are married to each other and I'm working on building a new character as their child, but I figure thant normally the child would be a mix of the parents' ethnicities, but the rules don't have anything for that and I'm clueless about genetics and don't know which would come out more dominant.
So would the child of a Garundi man and a Chelaxian woman be more Garundi or Chelaxian?
Most people are already a mix of ethnicities anyways. Chelaxians are mix of Azlanti and Ulfen to begin with. But considering that Cheleax was once a colony of Taldor there is probably also a lot a Talddan blood in their as well. I don’t have the information on the Garundi right of hand but they probably have a similar mix of ethnicities as well.
I'd say the ethnicity is almost as much about where you were born/raised as who your parents were. So, whichever country your character grew up in, make that their ethnicity.
If you're trying to get a idea of the cosmetics, try Googling "half-Egyption half-Irish." (While this is oversimplifying, the Garundi are basically north Africans, while the Chelish are basically brunette Irish (with the occasional redhead).)
Culturally, it'll pretty much depend on where your other PCs settled, with the nation being vastly more important than the actual ethnicity. Rahadoum, Thuvia, Osirian, Katapesh, and Nex are all Garundi nations that are radically different from each other. While Cheliax, Isger, Andoran, Molthune, and Nirmathas are all Chelish nations that are generally radically different from each other. (Even Isger's experienced very different problems from what Cheliax has.)
It's worth keeping in mind that people of mixed ethnicities can have a wide range of appearances. Some people look much more like one than the other, some seem to have a blend of both. Sometimes, even siblings with parents of different ethnicities look completely different from each other. Long-winded way of saying that you can pretty much do what you like.
In general, your Chelish/Garundi character would probably have at least slightly darker skin than someone of solely Chelish descent, but there's a lot of variation.
And their more cultural traits will depend on where they were raised, and whether the parents tried to teach them about both cultures, only one, or a third entirely different culture (if they settled somewhere besides Garund or Cheliax).
The ethnic languages can be a clue that ethnicities are more cultural than physical anyway. A character of pure Mwangi ancestry who never learned Polyglot should probably be counted as being of the ancestry of the people who raised him, for example.
Hm, rechecking the mother's character sheet (been quite a while since I played her), she's listed as Chelaxian and her homeland is Brevoy. The father is Garundi from Rahadoum.
I currently have the child's traits as Draconic Destiny to go with her and her father being a bloodrager & sorcerer respectively, and Deathtouched since Xane died 5-6 times by level 12 and Ionna has 4 @ level 9...
But I like the Child of two Peoples idea, so I may go with that instead.
As for likely homelands for the child include Absalom, Jalmeray, Taldor, Andoran, Osirion, and Varisia.
I would probably go with Garundi as the child's ethnicity for the extra language (Osiriani) if you have other ideas than Child of Two Peoples to spend your limited trait slots on. I would only go for Chelaxian if you want to pick up a trait or other feature that requires the character to be Chelaxian.