Extrachromosomal Inheritance - 5th semester (CBCS)

 Extrachromosomal Inheritance

The genes of nuclear chromosome have a significant & key role in the inheritance of almost all traits from generation to generation , but they cannot be consider as the soul behicles of inheritance , because certain  experimental evidences suggest the  occurrence of certain extra nuclear genes or DNA molecules in the cytoplasm of many prokaryotes & eukaryotes . This extra chromosomal gene half significant impacts in the individual.

In cases of extra chromosomal inheritance, generally the character of only one of the two parents is transmitted to the progeny. Such inheritance is also referred to as Extra Nuclear Inheritance , Maternal inheritance  & more commonly as Cytoplasmic Inheritance .

Genes governing the trait throwing Cytoplasmic inheritance are located outside the nucleus in the cytoplasm & are referred to as  Cytoplasmic genes , cyto genes , extra nuclear genes or extra chromosomal gene or plasma genes.

Extra chromosomal Inheritance is defined as non Mandelian inheritance, which occurs when phenotype results from the genetic influence other than the biparental transmission of gene located on the chromosome on nucleus. Generally only one parent contributes organelle heredity.

☑Organelle that contain chromosome are chloroplast & Mitochondria

☑Infectious heredity – Infectious heredity involves a symbiotic or parasitic association with a micro organism.

Maternal Effect: - When the expression of a character is influence by the genotype of female parent.

Criteria for extra chromosomal inheritance :-

The extra chromosomal DNA follows a non- Mendelian pattern of inheritance: Unlike the nuclear DNA genes present on organelle don’t follow the Mendelian pattern of inheritance. It is a circular genome locking centromere & thus cannot segregate like genomic DNA. Own machinery for protein synthesis.

Maternal inheritance : Extrachromsomal DNA is inherited from the maternal site.

Characteristic of Cytoplasmic inheritance :-

1) Reciprocal difference :- Reciprocal crosses show marked differences for the characters governed by plasmagenes . In most cases plasma genes form only one parent , generally the female parent are transmitted , this phenomenon is known as uniparental inheritance .

2) Lack of segregation: - In general F2 F3 and the subsequent generation don’t show segregation for a Cytoplasmic inherited trait, because F1 individual generally receive plasmagenes from one parent only.

3) Irregular segregation in biparental inheritance – In some cases plasma genes from both the parents are transmitted to the progeny, this is known as biparental inheritance ,  which gives rise to irregular segregation ratios in the F1 generation of higher plants. But in Chlamydomonas often zoospores show 1:1 ratio for the alleles of a plasmagene , but only after they  have undergone several mitotic divisions.

4) Somatic segregation: Plasma genes generally show segregation during mitosis a feature of rare occurrence in case of nuclear gene

5) Association with organelle DNA :- The demonstration of association of a plasma gene with organelle DNA   is definite evidence for the trast produced by the former to be cytoplasmically inherited.

6) Nuclear Transplantation: - In nuclear transplantation reveals a trait tp be governed by the genotype of cytoplasm and not by that of nucleus , Cytoplasmic inheritance of the trait is strongly indicated.




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