Definitions of Cytoplasmic Inheritance:
Extra-chromosomal inheritance, extra-nuclear inheritance, somal inheritance and maternal inheritance are all synonyms. All these terms can be defined as the inheritance of characteristics of only one of the two parents, usually the female parent to the progeny. The reciprocal crosses show consistent differences as well as there is a lack of segregation in F2 and subsequent generations.
The genes controlling cytoplasmic inheritance are present outside the nucleus and, in the cytoplasm, they are known as plasma genes, cytoplasmic genes, cytogeneses, extra nuclear genes or extra chromosomal genes.
The sum total of the genes present in cytoplasm of a cell is known as Plasmon. All the genes present in a plastid are known as plastoms. Similarly, all the genes present in a mitochondrion are known as chondrioms. The genes present in plastid and in mitochondrion are located in their own DNAs and are known as cp DNA and mt DNA, respectively. These DNAs are collectively termed organelle DNA.
The important characteristic features of cytoplasmic inheritance are briefly described below:
1. Reciprocal Differences:
Characters which are governed by cytoplasmic inheritance invariably exhibit marked differences in reciprocal crosses in F1, whereas in case of nuclear inheritance such differences are not observed except in case of sex linked genes.
2. Maternal Effects:
In case of cytoplasmic inheritance, distinct maternal effects are observed. This is mainly due to more contribution of cytoplasm to the zygote by female parent than male parent. Generally ovum contributes more cytoplasm to the zygote than sperm.
Nuclear genes can be easily mapped on chromosomes, but it is very difficult to map cytoplasmic genes or prepare linkage map for such genes. Now chloroplast genes in Chlamydomonas and maize, and mitochondrial genes in human and yeast have been mapped.
4. Non-Mendelian Segregation:
The mendelian inheritance exhibits typical segregation pattern. Such typical segregation is not observed in case of cytoplasmic inheritance. The segregation when occurs, is different from mendelian segregation.
5. Somatic Segregation:
Characters which are governed by cytoplasmic genes usually exhibit segregation in somatic tissues such as leaf variegation. Such segregation is very rare for nuclear genes.
6. Infection-Like Transmission:
Cytoplasmic traits in some organisms exhibit infections like transmission. They are associated with parasites, symbionts or viruses present in the cytoplasm. Such cases do not come under true cytoplasmic inheritance.
7. Governed by Plasma Genes:
The true cases of cytoplasmic inheritance are governed by chloroplast or mitochondrial DNA. In other words, plasma genes are made of cp-DNA or mt-DNA.