In genetics, non-Mendelian inheritance is a general term for the inheritance of genetic information that does not obey Mendelian rules. Briefly stated, in Mendelian inheritance, the probability that a diploid parent will pass on to its progeny a given one of its two alleles for a particular locus is 50%, and each progeny receives one allele from each of its parents. Which allele comes from which parent is not important.
Inheritance can therefore be non-Mendelian in at least three ways:
First, if a genetic element has a greater-than-50% chance of being passed on to an offspring, such as transposition, gene conversion, unequal chromosomal crossover and segregation distorter then the inheritance is non-Mendelian.
Second, some genetic elements are inherited only from one parent, such as those in mitochondria and chloroplasts.
Third, a gene's behavior can depend on which parent the gene is inherited from, a phenomenon known as imprinting.