The historical tradition is that you "belong" to your father's clan, ie: the clan most closely associated with your surname.
What is the Scottish tradition when someone marries into a family with a non-clan-affiliated surname? Are the children relegated to a tartan-less existence?

I mean, surely there are modern born-and-raised Scottish Highlanders who carry names other than the traditional clan-affiliated names, but consider themselves Highlanders and wear an occasional kilt. Do they default to regional tartans?