after millions of mutants were killer in the Genosha incident, Charles enters Polaris mind to try to figure out why her occasionally violent behavior, and she start telling that she wasnt satisfy with the results of the paternity thing and she went to Genosha to prove it by herself.
She new that it was supposedly proven that he wasnt his father, but the one who proved it wasnt trustworthy... she was wondering if the proof was a lie. so she won magneto trust, and got a little genetic material. The test results determined that magneto was his father, and almost right them the sentinels went and killed everyone