<P> Non-Mendeian 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> <P> According to the endosymbiont theory, mitochondria and chloroplasts were once free living organisms that were each taken up by a eukaryotic cell . Over time, mitochondria and chloroplasts formed a symbiotic relationship with their eukaryotic hosts . Although the transfer of a number of genes from these organelles to the nucleus prevents them from living independently, each still possesses genetic material in the form of double stranded DNA . </P> <P> It is the transmission of this organellar DNA that is responsible for the phenomenon of extranuclear inheritance . Both chloroplasts and mitochondria are present in the cytoplasm of maternal gametes only . Paternal gametes (sperm for example) do not have cytoplasmic mitochondria . Thus, the phenotype of traits linked to genes found in either chloroplasts or mitochondria are determined exclusively by the maternal parent . </P>

When was the calculation of mendelian inheritance discovered