Your boss? What's your job?
"It is recommended that your evaluators be current/former professors that can speak to your academic ability in the sciences."-TMDSAS
Extra Letters
TMDSAS will forward
one letter
in addition to the required letters. You will indicate in the application if you are submitting an extra letter and the name of the evaluator writing the extra letter.
If you indicate that you are submitting an extra letter, many schools will not consider your file complete until
both the required letters
and extra letter are received.
Other additional letters may be sent directly to the schools, but applicants will need to contact each school to verify if additional letters will be accepted.
https://www.tmdsas.com/dental/application-instructions.html#eval_letters