How long have you been out of unergrad? I think that makes a huge difference. If you've been out of school a few years your supervisor would be best able to speak to who you are and what kind of candidate you are now. Also, I would think if all of your other recommenders are in the field 1 outside would be ok as long as the outside one was really strong. You could discuss with your letter writer that it is important to stress the areas where there is a connection. For instance in IT you probably had to do a lot or problem solving and critical thinking which could connect to the biology degree. Also, I could be wrong, but I would think biology would involve a fair amount of data analysis and having someone talk about your strength in that area could be really helpful to your application.