A dog's reproductive system can be mature as young as 6 months old. It is better to wait until after the dog is two years of age to breed it so that the dog is physically mature. A responsible dog breeder will then take the dog to the vet to have it screened for possible poor genetic traits that could be passed to the offspring, this can include having the eyes, heart, elbows, hips, and knees examined and/or x-rayed to find any defects. If there are no defects, or the defects are very minor, the dog is a good breeding prospect. The dog's personality should also be taken into consideration, dogs with good personalities or more likely to have puppies with good personalities.