I learned a lot from the purchase of the mean JRT. The breeder would not let me see any of her adult dogs, I suspect because they were too mean for me to handle. My JRT bit his first person when he (the dog) was 5 months old. I had to watch him closely and keep him away from people his entire life. I’ll never keep a dog like that again.
I’ll offer up never get a dog from a breeder that won’t allow you to inspect their facility. Thats at HUGE warning sign.