<P> 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 . There are several situations in which the proportions of phenotypes observed in the progeny do not match the predicted values . </P> <P> Non-Mendelian inheritance plays a role in several disease processes . </P> <P> Extranuclear inheritance (also known as cytoplasmic inheritance) is a form of non-Mendelian inheritance first discovered by Carl Correns in 1908 . While working with Mirabilis jalapa Correns observed that leaf color was dependent only on the genotype of the maternal parent . Based on these data, he determined that the trait was transmitted through a character present in the cytoplasm of the ovule . Later research by Ruth Sager and others identified DNA present in chloroplasts as being responsible for the unusual inheritance pattern observed . Work on the poky strain of the mold Neurospora crassa begun by Mary and Hershel Mitchell ultimately led to the discovery of genetic material in mitochondria as well . </P>

The first human trait shown to be inherited by mendelian rules was huntington's disease