DNA and RNA is the genetic material that carries information from one generation to another. Apart from this genetic material in the nucleus, the cytoplasm also contributes to the inheritance of some characters. Such characters are cytoplasmic inherited characters and this phenomenon is called as cytoplasmic inheritance. It is also called extra nuclear inheritance, because in this type of inheritance factors lies outside the nucleus of the cell.
During sexual reproduction, the zygote is formed by the fusion of male gamete and female gamete. The male gamete which is the sperm, carries very little or no cytoplasm at all while female gamete carries large amount of cytoplasm. Hence in cytoplasmic inheritance, the male parent doesn’t contribute while female parent alone contribute s cytoplasmic characters.
The cytoplasm contains various cell organelles including mitochondria, chloroplast are regarded as semi-autonomous as they contain their own genetic material-DNA. Hence cytoplasmic inheritance involves inheritance of mitochondrial DNA and/or chloroplast DNA.
One such example is plastid inheritance in Mirabilis jalapa. In this plant, three types of branches are seen on the same plant; green, white and valigated (patches of green and white). This green colour is due to chloroplasts and white colour is due to leucoplasts. When a flower on green branch is considered as female and is pollinated by pollen grains of all 3 varieties. The resultant plant produce only plants with green branches. Therefore in cytoplasmic inheritance, the male parent doesn’t contribute and the inheritance factors totally come from female cytoplasm.
In Paramecium, small granular particles are present in the cytoplasm called as kappa particles. Kappa particles produce a protein called paramecin that kills the sensitive strain without kappa particles. The strain with the kappa particles is called as killer/resistant strain and the other strain is called as sensitive strain. Gene responsible for producing kappa particles is present in homozygous or heterozygous dominant condition in killer strain while it is present in recessive form in sensitive strain. Paramecium reproduces both by sexual and asexual mode. During asexual reproduction, when conditions favour the cells undergo cell division while the kappa particles cannot divide at the same rate. Therefore the daughter cells receive less number of kappa particles and after few generations there can be complete loss of kappa particles. In this situation, strains with dominant alleles remain without any kappa particles. This proves the gene for kappa particles can only help in multiplication of existing kappa particles. During sexual reproduction, when two opposite strains conjugate, the killer strain and the sensitive strain remain as the period of conjugation is less. When conjugation period increases, the kappa particles move from killer strain to sensitive strain through conjugation tube converting sensitive strain to killer strain.
The normal strain of Drosophila can withstand long periods of Carbon dioxide treatments and revert back to normal life. A pure breeding Carbon dioxide sensitive strain is later identified in Drosophila. On long exposure, the flies would become unconscious and the legs will be paralyzed. When a cross between a Carbon dioxide sensitive female and a normal male fly was made, all the flies in the progeny showed Carbon dioxide sensitivity indicating that Carbon dioxide sensitivity is due to cytoplasmic inheritance. Carbon dioxide sensitive flies possess small virus like granular particles called sigma factor in the cytoplasm, measuring in the size of 0.07 micrometers in diameter. Carbon dioxide sensitive flies were crossed with normal flies for many generations until all chromosomes of Carbon dioxide sensitive strain was replaced by normal strain; it still retains Carbon dioxide sensitivity proving that nuclear genes have no role to play in determining this character.
Cytoplasmic male sterility in maize is another example of alternative inheritance patterns. In this condition plants cannot produce fertile pollen grains. This is regulated by three mechanisms including genetic male sterility, cytoplasmic male sterility and cytoplasmic genetic male sterility. In genetic male sterility, the recessive gene is responsible which follows Mendelian inheritance. In cytoplasmic male sterility, the condition is caused by the cytoplasm alone. If a male sterile female is selected for a breeding experiment, all plants in the progeny will be male sterile. In cytoplasmic genetic male sterility, both nuclear genes and cytoplasmic factors are involved.