Selection for extremes of physical type in some breeds has damaged their intelligence and temperament. Inbreeding has a deleterious effect on temperament. Studies show that children get bitten by dominant purebred male dogs. Male dogs can make good children's pets if they are desexed. Puppies should be wormed and vaccinated prior to purchase.
Many breeds have particular genetic problems that occur commonly in that breed.
Make yourself familiar with these diseases and be sure that the breeder you chose screens their dogs for these problems. Rare breeds of dog come from a small gene pool so that they are more likely to be inbred. Inbreeding increases the likelihood of dogs having genetic diseases, reduced fertility, neurotic behaviour and a suppressed immune system. Most dog breeders disapprove of inbreeding but say that they practice linebreeding for type which is different. Linebreeding is inbreeding - the distinction is only a matter of degree.