The D-Rex’s big arms are the mutated arms right?

I’m not going to pretend to have a grasp on the franchise’s mutations/hybridization lore, but if the D Rex is a mutated T Rex, wouldn’t that mean that it’s the larger, weight bearing arms that are the mutated arms? Which is odd given what some have said about mutated limbs likely being a cause of pain/discomfort. Is it possible that instead of frog dna being used to fill gaps in the genome, they tried some type of ape dna?