In normal conditions, the phenotype of the progeny depends on the genes present in the nucleus and is referred as Chromosomal Inheritance, where there is a simple connection between the genes located on the chromosomes and observed phenotypes in the progeny. The male gametes and female gametes equally contribute to the phenotype of progeny and there is no differences observed with phenotypes with reciprocal crosses. In some exceptional cases, the phenotype is not dependent on the nuclear genes and is controlled by the genetic content present in the cytoplasm. The control of phenotype of the progeny by the non nucleus component present in the cytoplasm is called extrachromosomal inheritance or organellar inheritance or cytoplasmic inheritance. Extrachromosomal inheritance is distinct from the Maternal effect, where the phenotype of progeny depends on the mother's genotype and on nuclear gene products (mRNA or protein) present in the cytoplasm of the egg. The genetic material present in the mitochondria or chloroplast (apart from nuclear chromosomal DNA) are responsible for the extrachromosomal inheritance and hence called organellar inheritance since these organs are responsible for phenotype of the progeny. In few cases apart from mitochondria or chloroplast, the phenotype of the progeny depends on the extra-chromosomal particles present in the cytoplasm, hence also referred as cytoplasmic inheritance. In extrachromosomal inheritance, the reciprocal crosses between male and females will give different results.
One of the classical examples of extrachromosomal inheritance is the variegated leaves found in the Mirabilis jalapa commonly known as four-o'clock plant. There are three kinds of phenotypes observed in leaves as white, green and variegated (patches of white and green on same leaf). The progeny derived from the seeds of the white and green plant gave rise to white and green respectively irrespective of the kind of pollens used from three phenotypes. The variegated plant gave rise to all three phenotype branches and leaves irrespective of the phenotype of the pollen plant. The phenotype of the seed primarily depends on the phenotype of the plant that produces the egg and the genes responsible for the phenotypes are present in chloroplast. The chloroplasts are transferred from the mother to offspring through cytoplasm of the female gamete and male gamete has no role in transmission of chloroplast DNA.
Carbon dioxide (CO2) sensitivity in Drosophila is also a good example for extrachromosomal inheritance. When certain strains of Drosophila exposed to CO2, become unconscious and are referred as CO2 sensitive strains. When crosses were made between CO2 sensitive strain and normal wild type strain, CO2 sensitive phenotype was always transferred from mother side to progeny. 100% of the progeny obtained from the CO2 sensitive mother are always CO2 sensitive irrespective of kind of father's phenotype. CO2 sensitivity phenotype of Drosophila was observed in files due to the presence of virus like particles called Sigma factors present in the cytoplasm of the sensitive Drosophila flies.
Kappa particles in Paramecium are other classical example for extrachromosomal inheritance, where the phenotype not only depends on the cytoplasm but also on the genetic material of the nucleus. Paramecin is the toxic substance produced by certain strains of Paramecium which kills the sensitive strains. The strain of Paramecium that produces paramecin is called Killer strain and hosts particles called Kappa particles which are responsible for paramecin production. Kappa particles require dominant allele K for its multiplication and genotypes KK and Kk can support Kappa particles, while genotype kk can't support Kappa particles. Different results are observed when Killer strain is allowed to conjugate with sensitive strain depending on the duration of conjugation. In short conjugation, where only genetic material is exchanged without the cytoplasmic exchange, the exconjugates will have 1:1 ratio of killer to sensitive strain and the further asexual division results in 1:3 of killer to sensitive strains. In prolonged conjugation, which involves exchange of both genetic and cytoplasmic content between conjugates, the exconjugates will have all killer strains and further asexual division results in 1:1 ratio of killer to sensitive strains.
Extrachromosomal inheritance has some practical advantages in agriculture and can also be used in prediction of diseases in humans for counseling. In plants Cytoplasmic Male Sterility trait can be used in plant breeding. A recent study involving congenital heart disease parents showed that the risk of getting congenital heart disease in the progeny was higher if the mother is affected rather than father. The advanced research on extrachromosomal inheritance associated with human diseases will go a long way in genetic counseling.
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