Exactly what is says on the tin.

GMOs occur in what...one event out of the whole year? They're expensive (the tigon situation not withstanding), both in event and normal currency, so it's my opinion that a GMO should not be able to produce any low tier mutations. The cost/reward is WAY skewed. No user should use a GMO and get a boring Achromia, Foldie, Bobtail, Tailless, or any nonsense like that. Such an expensive item should yield mane-mutations at the very least.

Getting a common mutie from such an expensive item is just insulting.

ADDENDUM: PLEASE READ

Instead of outright blocking common mutations, the use of a GMO makes them rather uncommon. Occuring in maybe 10-20% of cubs successfully affected by a GMO Cow. The rest will be mid and high tier Mutations.