Question

I don't get this, body altering spells are said to not be able to be done. Yet science states that one and life can mutate which is an altering of DNA or the physical being. So how is it not possible to cast altering spells.

Mutations are natural occurrences undertaken when DNA is replicated incorrectly, whether that is done through natural process or forced through radiation, they still occur naturally and gradually.

The spells that claim you can become a different gender, creature, grow wings, change your hair colour, eye colour, etc aren't ever going to happen, as they require something naturally impossible to occur. The best mutated cells can do, is alter small aspects of your cells, increase cell growth, etc, this is done gradually, they can't allow you to grow gills, or become a different species instantly which is what most of these spells claim to be able to do.