Sometimes my children are born with the normal vanilla portraits and sometimes they've got the Gaelic ones (I'm Scotland). Is this normal?
Check your parents and grandparents and see if they are Gaelic or have Gaelic portrait. Portrait type is inherited.
Always thought it had to do with DNA and what was the most prevalent culture type.
Graphical culture is inherited randomly I believe from father or mother. What parts are which is decided by DNA. Graphical culture and character's culture have nothing to do with each other. One is identity, the other is ethnicity.
You're overthinking it a bit i think. When a black character marries a White Character the graphical culture outcome for their children are just one of these three. Black, Mixed (uses the Arab portraits) or White.