According to Mendel's law phenotypical characteristics would be verified by pair of factors (alleles) that separate independently in gametes. What are the main types of inheritances that are exceptions to Mendel's rules?
There are many parts of inheritance that do not follow the mendelian pattern. Notable between them are: multiple alleles, gene interactions (complementary genes, epistasis and quantitative, or polygenic, inheritance), linkage with or without crossing over and sex-linked inheritance.
Pleiotropy, lacking of dominance and lethal genes do not fit as variations of inheritance as genes can have these behaviors and at the similar time obey mendelian laws.
Mutations and aneuploidies are abnormalities that also change the mendelian pattern of inheritance as well as mitochondrial inheritance (passage of mitochondrial DNA from the mother by the cytoplasm of the egg cell to the offspring).