page_title: Non-Mendelian inheritance
section_title: Extranuclear inheritance
hierarchical_section_title: Non-Mendelian inheritance / Types / Extranuclear inheritance
caption_attribution_description: English: Pedigree for a maternally inherited genetic trait
context_page_description: Non-Mendelian inheritance is 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.
  Non-Mendelian inheritance plays a role in several disease processes.
context_section_description: Extranuclear inheritance (also known as cytoplasmic inheritance) is a form of non-Mendelian inheritance also first discovered by Carl Correns in 1908. While working with Mirabilis jalapa, Correns observed that leaf colour 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 mould Neurospora crassa begun by Mary and Hershel Mitchell ultimately led to the discovery of genetic material in the mitochondria, the mitochondrial DNA.
  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.
  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.
  In humans, mitochondrial diseases are a class of diseases, many of which affect the muscles and the eye.
caption: Example of a pedigree for a genetic trait inherited by mitochondrial DNA in animals and humans. Offspring of the males with the trait don't inherit the trait. Offspring of the females with the trait always inherit the trait (independently from their own sex).
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