I used a genetic mutator on my pet, Storm.

I'm a little new so I might be confused, but I believed this was meant to add a gene or a mutation (either visible or carried) on one of the pet's empty slots. Yet nothing on her appears to have changed. Did it only have a CHANCE to add a mutation/gene? Did I misunderstand something?

It looks like the lesser genetic mutator gave your pet a carried trait. Carried traits can only be seen if the pet is genetically tested, which your pet has not been yet. You can find many people willing to test your pet for free in the Breeding Discussion forum ^.^ Once she has been tested, you will be able to see what trait the mutator gave her.

Oh!

I guess I thought that if a pet's genetics were already 10/10 (100%) there was really no need to genetically test them. Or that this would somehow come included.

Thank you.

The 10/10 on the genetics has nothing to do with genetics actually XD It has to do with whether the pet is fed/healthy. If you don't feed your pet for a day, it will go down to 9/10, and so on. If it gets too low you won't be able to fight with it or breed it until you feed it.

Genetic testing, on the other hand, lets you see carried and hidden traits, as well as letting you see the true maturity of a pet when you look at their image url. It also lets you see a pet's mutations and mutation colours when it is still a hatchling, or a pet's stats while still an eggling ^.^