Non-Mendelian inheritance

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 is applicable in co-dominance and incomplete dominance. 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 is applicable in co-dominance and incomplete dominance. Non-Mendelian inheritance plays a role in several disease processes. 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. 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. Gene conversion can be one of the major forms of non-Mendelian inheritance. Gene conversion arises during DNA repair via DNA recombination, by which a piece of DNA sequence information is transferred from one DNA helix (which remains unchanged) to another DNA helix, whose sequence is altered. This may occur as a mismatch repair between the strands of DNA which are derived from different parents. Thus the mismatch repair can convert one allele into the other. This phenomenon can be detected through the offspring non-Mendelian ratios, and is frequently observed, e.g., in fungal crosses. Another form of non-Mendelian inheritance is known as infectious heredity. Infectious particles such as viruses may infect host cells and continue to reside in the cytoplasm of these cells. If the presence of these particles results in an altered phenotype, then this phenotype may be subsequently transmitted to progeny. Because this phenotype is dependent only on the presence of the invader in the host cell’s cytoplasm, inheritance will be determined only by the infected status of the maternal parent. This will result in a uniparental transmission of the trait, just as in extranuclear inheritance. One of the most well studied examples of infectious heredity is the killer phenomenon exhibited in yeast. Two double-stranded RNA viruses, designated L and M, are responsible for this phenotype. The L virus codes for the capsid proteins of both viruses, as well as an RNA polymerase. Thus the M virus can only infect cells already harboring L virus particles. The M viral RNA encodes a toxin which is secreted from the host cell. It kills susceptible cells growing in close proximity to the host. The M viral RNA also renders the host cell immune to the lethal effects of the toxin. For a cell to be susceptible it must therefore be either uninfected, or harbor only the L virus.