Is it possible to be intelligent and not know how to spell?

I always wonder about adults who can't spell correctly. I'm referring to simple words that all literate people should know. Sometimes the misspellings are not even phonetic. Such as, "disaplan" for "disciplined". I try not to be judgemental, but obviously these people don't read if they can't differentiate between "your" and "you're", etc. I feel so bad for their children, who helps them with their homework? Can one be intelligent and have the vocabulary of a 3rd grader?