About half of the cats respond to catnip and the other half are completely unaffected. What is it?

The cat’s sensitive genes for catnip (perennial herb) are inherited, primarily the genetic response to the chemical substance of nepetalactone.
The scent of catnip binds to the receptors in the cat’s nose, stimulating the sensory nerves in the cat’s brain, and then it shows a pleasing and enjoyable look.