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. Polygenic traits Many traits are produced by the interaction of several genes. Traits controlled by two or more genes are said to be polygenic traits. Polygenic means "many genes" are necessary for the organism to develop the trait. For example, at least three genes are involved in making the reddish-brown pigment in the eyes of fruit flies. Polygenic traits often show a wide range of phenotypes. The broad variety of skin colour in humans comes about partly because at least four different genes probably control this trait. Non-random segregation Non-random segregation of chromosomes is a deviation from the usual distribution of chromosomes during meiosis and in some cases of mitosis. Gene conversion 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. Infectious heredity 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