Introduction:
The genes of nuclear chromosome have different roles in inheritance, cellular metabolism, development and mutation of the organism in which they occur. The gens of nuclear chromosomes have a significant and key role in the inheritance of almost all characters from one generation to other generation. However, they alone cannot be considered as the sole vehicles of inheritance because certain experimental evidence suggests transmission of some genetic information form one generation to the next generation also through the cytoplasm which is known as extranuclear or cytoplasmic inheritance. The role of cytoplasm in hereditary is determined form the results of reciprocal crosses in which the source of male and female gametes is reversed. In chromosomal inheritance, it makes no difference in transmission of a gene whether it comes from which aren’t and identical phenotype are obtained in reciprocal crosses.
Extrachromosomal inheritance is aloes known as extranuclear inheritance, non-mendelial inheritance, uniparental inheritance, maternal inheritance or cytoplasmic inheritance.
Eukaryotes:
Many genetics have studied various cases of extrachromosomal inheritance in different eukaryotes. Some of the most importance examples of extranuclear inheritance in eukaryotes are as follows:
Plastid inheritance
The inheritance pattern due to plasma gene located in plastids is known as plastid inheritance. Plastid inheritance was frost discovered by Correns and Baur in 1908. All the knowledge about the plastid inheritance in higher plants is based on studies on chlorophyll variation in leaves. Variegated refers to the presence of white or yellow spot of variable size on the green background of leaves.
- Correns and Baur found the four-o’clock plant, Mirabilis jalapa, in contrast to the other plants contains three types of leaves and parts as follow
- Full green leaves or branches having chloroplast
- White leaves or branches having no chloroplast
Variegated or mixed leaves or branches with green and white patches. These leaves have leucoplast in white are and chloroplast in green area. As the leucoplast are incapable of producing food by photosynthesis the white branches of this plant survive by receiving nourishment form green plants.
| Leaf phenotype of the brance used as the female parent | Leaf phenotype of the brance used as the male parent | Leaf phenotype of the progeny |
| Green | Green White Variegated | Green Green Green |
| White | Green White Variegated | White White White |
| Variegated | Green White Variegated | Green, White and Variegated Green, White and Variegated Green, White and Variegated |
Fig: Cholorplast inheritance in four o’clock plants
Correns reported that the flowers on green branches produced only green offspring regardless of the genotype and phenotype of pollen parent. Likewise, flowers form the white branches produced only white offspring regardless of genotype and phenotype of pollen parents. However, these offspring die soon because they lack chlorophyll and cannot carry on photosynthesis. The flowers from the variegated branches yielded with various ratios in mixed progeny of green, white and variegated plants. The above experiment clearly shows that the phenotype of progeny is the same as that of the female parent. Except, when the female parent is variegated, all three combination occurs.
Mitochondrial Inheritance
In fungi Neurospora crassa a number of mutation of mitochondria are inherited via the female parents. But the poky strain of Neurosporacrass was the best studied mutant, which was first isolated by Mitchell and Mitchel in 1952. Poky strain grows very slow, forms small colony and exhibit poorly differentiated mitochondria. The mitochondria which are required for the respiratory electron transport chain generally lack membrane bound cytochrome a and b. It also possesses a greatly reduced number of small ribosomal units.
The traits seem to be inherited in certain strains in non-mendelial fashion, indication ist extrachromosomal nature. In cross between normal female and poky male the resultant ascospores all produces normal clones. And, in reciprocal cross between poky females and normal male parents, all the resultant colonies are poky. This indicates that the poky trait is found to be inherited in a fashion of maternal inheritance.
Maternal effect
Where the development of some characters in several organisms is either governed or influenced by the genotype of the female parent, it is known as maternal effect. Such phenotypic expression of maternal genes in the offspring may be short lived or may persist throughout the life span of the individual. The substances which produce the maternal effects in the progeny are found to be transcriptional products of maternal nuclear genes like mRNA, tRNA, rRNA which have been produced during oogenesis and which exist in the ooplasm of fertile eggs in the form of inactive rRNA and tRNA. These maternal genes transcriptional products produce their phenotypic effects during early cleavage and blastulation when there is very little or no transcription because maternal and paternal genes of zygote remain engaged in mitotic replication or duplication of DNA. There may be other reasons of maternal effect which are still little understood.