Iowa is looking for people who are close to you that can account for your overall character and academic ability. The admission person at Iowa told me that they are one of the most liberal schools when it comes to what they are looking for in a letter. They don't limit who can write you a letter because they know that if they do that they will get a lot of bad letters.
Here would be a perfect combination:
2 professors that know you and you have gotten to know. (in classes you did well in)
1 Dentist that you have shadowed/know that would endorse you as a dentist
If you have a boss or someone like that who think highly of you that could also be good.
Help?