Genes are one of the basic units of living world. Genes are responsible for everything, our appearance, our characters, internal structures, diseases, our strength and other factors.
Genes are located on chromosomes but not all of them. Certain genes are located in cytoplasmic organelles like mitochondria and plastids (absent in animal cells).
These organelles are self-replicating and they transmit their genes to daughter organelles. These genes are also well known as the cytoplasmic genes or plasmagenes. As we know all the chromosomal genes responsible for the formation of genomes, similarly plasmagenes are responsible for the formation of plasmone. Like chromosomal genes, plasmagenes don’t show Mendelian inheritance. The plasmagenes are transmitted from one generation to other by cytoplasm only and its inheritance is called cytoplasmic inheritance or extranuclear or extra- chromosomal or non- chromosomal inheritance as these genes don’t lie in the chromosome or in the nucleus. As the zygote mostly constitutes of mother’s cytoplasm so this inheritance is also called as maternal cytoplasmic inheritance.
Correns (1909) first noted cytoplasmic inheritance. The cytoplasmic inheritance differ from nuclear inheritance, so let’s discuss how these inheritance differ –
1. Nuclear inheritance is usually bi-parental (from both the parents) whereas the cytoplasmic inheritance is maternal (single parent).
2. Nuclear genes control the nuclear inheritance while cytoplasmic inheritance is controlled by plasmagenes.
3. In nuclear inheritance, the result of reciprocal crosses are usually same while in cytoplasmic inheritance is usually different and depend on maternal cytoplasm.
4. The number of nuclear genes is same in all the cells of an individual in case of nuclear inheritance while in cytoplasmic inheritance the number of plasmagenes differs.
5. The number of nuclear genes is same for both in male and female while plasmagenes are only present in female gamete, very few or absent in male gamete.