This is the part where many cockatiel owners run away screaming. Genetics. For many, it is just hard mathematics and weird science, but the real aspect of the case is that one does not need to remember more than a few basic rules that can be applied in different circumstances then.
Cockatiel genetics and knowledge of color are very important issues, in particular to a person who is going to let the birds nest. This how the breeder can predict what kind of parents produce certain kind of chicks, or how likely is the emergence of a particular color to the clutch. The breeder will also understand what kind of birds create offspring with desired traits. On the other hand, he also learns to understand why even a relatively nice crested parents can produce a chick with week crest traits. Genes are not everything, of course. The environment affects greatly to the results: even a genetically good natured cockatiel can grow timid, fearful and aggressive, if it is treated badly. This section will guide you to the top criteria for familiarization in genetics. You will learn to identify colors of the cockatiel and to know how the colors are inherited. The section also explains why one should not breed certain color to another, why siblings should not be nested and what consequences may appear because of the botched reunification of birds.
A Guide to... Cockatiels and their mutations as pet & aviary birds (Tri. Terry Martin & Diana Andersen)