Non-Mendelian inheritance is a general term that refers to any pattern of inheritance in which traits do not segregate in accordance with Mendel’s laws. These laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. In Mendelian inheritance, each parent contributes one of two possible alleles for a trait. If the genotypes of both parents in a genetic cross are known, Mendel’s laws can be used to determine the distribution of phenotypes expected for the population of offspring. However, there are many abnormalities which show familial recurrence and have a clear genetic component, but do not show regular Mendelian segregation patterns. Defining the causative gene for non-Mendelian diseases is more difficult, and even when the underlying gene is known, there is uncertainty for prenatal prediction. However, detailed examination of the different mechanisms that underlie non-Mendelian segregation provides insight into the types of interaction that regulate more complex disease genetics.