So... I have a cockatice which has Acid gland and spitting muscle and still can't breath acid :S what's the reason of that?¿

Here's the Pet 

In order for a gene to have an active effect on its carrier, it must be dominant. o: Your cockatrice only has recessive acid-spitting traits, which means he can pass them down to offspring, but not benefit from them himself.

The green genes are active, the greys are recessive so they're carried & can be passed along but it doesn't actually have them in a usable form. Try finding & breed it to a pet with the same genes you want.