"The country of chargeability is the country of the applicants birth. Visas for the dependents will be first charged against the dependents country of birth. If unavailable, the visa may be charged against the country of birth of the principal applicant. If the visa is unavailable for the principal applicants country of birth, the visa may be charged to the country of birth of the principal applicants spouse (but not the children). If the visa is unavailable for the principal applicants spouses country of birth, the visa may be charged to the country of birth of the principal applicants parents. Note that a parent may not charge to the country of birth of his/her child. See section 202 of the INA for rules of chargeability." - I-485 SOP (P.48 of 269)
http://blog.wenxuecity.com/blogview.php?date=200708&postID=19650#chargeability