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From the trivia section of his IMDb page:

>Dean is of three eighths Japanese descent; Dean's paternal grandfather, John Megumi Tanaka, was of Japanese background, and Dean's paternal grandmother, Grace E. Miyoko Sugiura, was of half Japanese and half European descent. Other than Japanese, Dean's ancestry includes English, French-Canadian, Irish, and Welsh.

Curiously enough, he looks more Japanese than his father did, at least judging by this photo.