Triceps lagging in size and strength? It's not "bad genetics." It probably because your primary triceps exercise is something like rope pulldowns. Try a couple of these exercises instead and watch your triceps explode.
Use this underrated accessory movement to build your triceps.
Lay on your back on the floor and do a tricep extension. At the bottom of the movement place the dumbbells on the ground near your head.
Use the same movement you'd use with standing tricep extension, but laying on the floor. Allow the plates to touch the ground behind your head.
You can use a different bar (like a football bar) to get a neutral grip, or add accommodating resistance with bands.
Attach a band to the rig, keep your elbows tight to the body and spread it apart at the bottom. Try for 100 reps.
Do these pulldowns with palms facing up to hit the medial head of the triceps.