“A non-mendelian pattern of inheritance governed by the DNA present in the cytoplasm is known as extrachromosomal inheritance or cytoplasmic inheritance.”
The DNA is the genetic material of us and arranged on chromosomes. It helps to store and transfer information (called traits) through the process of replication, transcription and translation.
Nuclear DNA is the basis for inheritance of almost all type of phenotype of ours. It inherited in a particular fashion from parents to their offspring.
Though all genes are inherited in Mendelian style, some genes present in the cytoplasm of the cell, inherited in a non-mendelian pattern. This type of inheritance is called as extrachromosomal inheritance or cytoplasmic inheritance.
In the present article, we will discuss one of the amazing topic- “extrachromosomal inheritance.”
What is extrachromosomal inheritance?
The extrachromosomal inheritance also is known as cytoplasmic inheritance or non-mendelian inheritance was first reported by Boris Ephrussi in yeast during 1949.
Cytoplasmic DNA or extrachromosomal DNA is present significantly in some important organelles like chloroplast and mitochondria. It is a big mystery that how actually these organelles created their own genome.
One theory which stated that it was a symbiotic relationship. It is believed that mitochondria were once free-living bacteria. Over a period of time, it created a symbiotic relationship with eukaryotic cells and established themselves into the cytoplasm and ultimately evolved as an organelle in living eukaryotic cell.
Similarly, the chloroplast in green plants comes from the free-living algae and established a symbiotic relationship with eukaryotic plant cells and settled into cytoplast of green plants.
Read more on chloroplast DNA
Both types of sub-genome have well-developed DNA machinery which is equipped with all the component required for central dogma. Additionally, chloroplast has antibiotic resistance genes indicate that it was derived from bacteria, previously.
The genome is made up of few genes and several thousand base pairs, still it has their own rRNA, tRNA and DNA for replication, translation and transcription.
“The extrachromosomal DNA present in the cytoplasm and not on chromosomes which follows the non-Mendelian pattern of inheritance is known as extrachromosomal inheritance.”
Criteria for extrachromosomal inheritance:
The extrachromosomal DNA follows a non-mendelian pattern of inheritance
Unlike the common Mendelian segregation pattern is not observed in the extrachromosomal DNA because it does not have the centromere it can not segregate, unlike the normal nuclear DNA.
Their own machinery for protein synthesis:
Unlike nuclear DNA, the organelle DNA or the extrachromosomal DNA has its own replication and transcription machinery. It synthesised their won DNA.
The extrachromosomal DNA inherited from the maternal side.
The segregation is observed in somatic cells rather than germ cells, unlike nuclear inheritance.
Carl Correns in 1908, first reported non-mendelian inheritance in Mirabilis Jalapa plastid DNA. Another extrachromosomal inheritance was reported by M M. Rhoades in 1933. He postulated that inheritance of male sterility in maize is governed by maternal inheritance and it becomes one of the greatest discoveries in science.
Another important point that makes extrachromosomal DNA even unique is maternal inheritance. It inherits from mother to their offspring which means that only female individual from the entire population can inherit cytoplasmic DNA.
One theory suggests that female reproductive cell (ovum) is bigger, contain more cytoplasm and more organelles than male reproductive cells. This would be expected to influence non-mendelian inheritance or maternal inheritance.
One of the classical examples of maternal inheritance is :
Cytoplasmic male sterility in maize.
Here nuclear genes do not play any significant role rather, the sterility is inherited through egg cytoplasm from generation to generation.
When a male sterile plant is crossed with a normal fertile plant, all the F1 plants remain sterile. When all F1 sterile plants are backcross with a normal fertile plant, until all chromosomes from the male sterile line are exchanged to male fertile, the sterility persists in the progeny.
Generally, male-sterile lines are denoted as tcs, T (Texas), C (Cytoplasmic), S (Sterility). It was believed that T (Texas) cytoplasm is associated with susceptibility against several types of disease like leaf blight disease and yellow blight disease in maize.
This result indicates that chromosomal nuclear DNA does not have any significant role in male sterility (particularly in maize). Furthermore, most of the cytoplasm and organelles are inherited from the maternal side. From the scientific findings, it is confirmed that the sterility is inherited from the cytoplasm.
This discovery becomes a crucial milestone in crop improvement. Hybrid sterile maize plant becomes more popular as the corn of maize developed uniformly. The hybrid seed becomes more popular for mass production of maize.
Though maternal inheritance may be extrachromosomal or chromosomal, it is one of the miracle events in nature. Here genetic compositions of maternal side influence several phenotypes of offspring.
In some organism, not only maternal inheritance rather the genotype of the maternal side has great influence on the phenotype of offspring. Here phenotype of mother does not have any role in the development of phenotype in offspring.
Read some of the interesting articles:
The maternal-effect in snail:
The character of coiling in snail is governed by maternal inheritance. Snail, Limnaea peregra, has two types of shell coiling phenotypes: one is dextral shells which coil for the right side and another is a sinistral shell which coils for the left side.
Here, the mother’s genotype (not a phenotype) is exclusively responsible for the development of coiling style. Assume that D+ genotype codes for dextral (right side) coiling and D is codes for sinistral coiling. The reciprocal cross of D+ and D is shown in the figure:
Crossing between D+D+ female and DD male, all the F1, as well as F2 progeny, become dextral as the mother is D+D+ dextral, here the DD recessive phenotype is not expressed and typical Mendelian 3:1 ratio is not obtained (all four are dextral).
In another condition when DD sinistral female is crossed with D+D+ dextral male, F1 offspring become sinistral with genotype D+D, here mentioning genotype is important because the inheritance is governed by genotype not by phenotype.
When this F1 progeny is inbred (D+D * D+D) all the F2 progeny become dextral and coil for the right side. This results indicated that phenotype of parents do not have any influence on the phenotype of progeny because although all of the F1 progeny are sinistral, all F2 offspring becomes dextral.
Detailed investigation shows that spindle formed during the second metaphase division decides the direction of coiling. The spindles of dextral snail are tipped to right and vice verse for sinistral. Interestingly, spindle arrangement in metaphase in controlled by maternal genes.
So the actual phenotype of “type of coiling” in snail is governed by maternal genes and it does not depend on the phenotype of any of parent.
In some of the organism, the amount of exchange of cytoplasm plays a crucial role in the inheritance of phenotype.
Inheritance of kappa particles in paramecium:
Paramecin is a substance found in some of the killer strain of paramecium which kills the sensitive strains. Paramecin production is governed by the kappa particles present in the cytoplasm of the paramecium.
Here KK gene is responsible for the production of kappa particle which is dominant over kk gene. In case of inheritance of kappa particle, cytoplasmic exchange during conjugation plays a crucial role.
When KK killer strains are crossed with kk strains by conjugation, all the progeny obtained are heterozygous with genotype Kk but the phenotype of paramecium depends on presence or absence of kappa particles and it will be influenced by time of conjugation.
If both are conjugates for a shorter period of time, in F1 generation killer strains remain killer and non-killer remain non-killer in the heterozygous condition. Here only nuclear genes are transferred but the cytoplasm is not exchanged between both strains.
In another condition, if killer and non-killer strains are conjugated for a longer period of time, due to the exchange of kappa particles, sensitive strain receives kappa particles through cytoplasmic exchange and sensitive strains become killer in F1 generation.
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The cytoplasm is an important component of the cell, not only for transferring organelles but also for the inheritance of characters. Cytoplasmic inheritance and maternal inheritance are responsible for some of the disease condition in the human. Any defect in the inheritance of extrachromosomal genes results in serious physical, mental and biochemical problems.
In the next classes of extrachromosomal inheritance, we will discuss mitochondrial DNA, chloroplast DNA and inheritance of extrachromosomal disorders.
Article written by- Tasneem Gandhi
The article reviewed by- Tushar Chauhan
This article is written by our dearest friend Tasneem Gandhi on special request of team genetic education. We are heartily thankful to Tasneem.