In order to understand Non-Mendelian genetics you first should know a little about Mendelian genetics. Mendelian genetics is about determining the probability of traits surfacing by simple dominant-recessive relationships. Confusing? Here is a good example. Let's say there are two frogs, one is green and one is light green. Green is represented by a G, light green by a g. Also it is known that green is dominate to light green. Therefore if the progeny of a green frog (Gg) and a light-green frog (gg) are Gg, the frog is green, but if it is gg it is light-green. That is Mendelian genetics. Non-Mendelian genetics, however, occurs much more often and includes all the exceptions to mendelian inheritance. It is everything Mendelian genetics is not. The reason it occurs more often is that it is not quite as straight forward and accounts for mutations and strange types of inheritance.