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.
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.