Section: Writing and Language

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How would you fix the following portion of the sentence in paragraph 4?

is recessive, what is one reason the pattern is so rare.

1. NO CHANGE
2. is recessive which is one reason the pattern is so rare.
   
3. is recessive what is one reason the pattern is so rare.
4. is recessive, which is one reason the pattern is so rare.

Answer 4: The correct pronoun for the appositive is "which," and since it's an appositive, it should begin here with a comma.