Answer: Genetics
Explanation: This may sound a little strange, but in order to have animals with certain traits, they must be born with or have in the blood line to have said traits. Even then the traits exhibited may not be hereditary. There is a 50% chance that the genetics that you want the offspring to have, may not occur. When a child is made, this means that the parents donate chromosomes from both sides to give the child some of their looks. This could mean disease, eye color, hair thickness.