Extra Chromosomal Inheritance

The genetic substance in our bodies, DNA, is organised on chromosomes. Through replication, transcription, and translation, it stores, transfers, and expresses genetic information. All phenotypes are governed by nuclear DNA, which is found in the nucleus of a cell and is passed down in a precise manner from parents to children. There are various well-known inheritance patterns, including autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive. Genes on autosomes inherited by autosomal inheritance, whereas genes on the X chromosome inherit via X-linked inheritance. Mendelian inheritance is defined as the transfer of genotypes from parents to children through reproduction. Several genes in the cytoplasm, however, do not obey this rule. Cytoplasmic inheritance, non-Mendelian inheritance, and extrachromosomal inheritance are all terms for the same thing.

What is Extrachromosomal Inheritance?

Boris Ephrussi discovered extrachromosomal inheritance, also known as cytoplasmic inheritance or non-mendelian heredity, in yeast in 1949. Organelles such as chloroplasts and mitochondria, as previously stated, have their own genome. Scientists are still baffled as to how these organelles formed their own DNA. It was a symbiotic connection, according to one view. Mitochondria were formerly thought to be free-living microorganisms.

It has developed a symbiotic connection with eukaryotic cells and has established itself in the cytoplasm over time. Later on, it developed into an organelle. Similarly, the chloroplast in green plants is derived from free-living algae, which developed a symbiotic connection with eukaryotic plant cells and eventually settled into the cytoplast of green plants.

Extrachromosomal Inheritance criteria:

• Extrachromosomal DNA has a non-Mendelian inheritance pattern- Organelle genes, unlike nuclear DNA, do not inherit in the Mendelian fashion. Because it is a circular genome with no centromere, it cannot segregate like genomic DNA.

• They have their own protein production mechanism- These sub-genomes synthesis their own DNA and produce their own protein since they have their own replication, transcription, and translation machinery.

• Maternal inheritance- Extrachromosomal DNA is passed down from the mother.

Examples:

Non-mendelian inheritance in Mirabilis Jalapa plastid DNA was originally reported by Carl Correns in 1908. M.M. Rhoades described another extrachromosomal inheritance in 1933. He proposed that male sterility in maize is regulated by maternal inheritance, and this became one of science’s most important findings.

Another feature that distinguishes extrachromosomal DNA is maternal inheritance. It is passed on from mother to kids. This demonstrates that all females in the population are capable of transmitting cytoplasmic DNA.

There is no scientific evidence to explain why and how this happens, but one idea says that the female reproductive cell (ovum) is larger than the sperm cell, has more cytoplasm, and has more organelles than the male reproductive cell. Researchers believe this has an impact on non-mendelian or maternal inheritance.

One of the popular examples of maternal inheritance is ,

Cytoplasmic Male Sterility in Maize

In maize, nuclear genes play no role in sterility; instead, sterility is passed down through the egg cytoplasm to children. All F1 plants remain sterile when a male sterile plant is crossed with a normal fertile plant. The sterility endures in the progeny when all F1 sterile plants are backcrossed with a normal fertile plant until all chromosomes from the male sterile line are switched to male fertile.

Male-sterile lines are commonly abbreviated as tcs, T (Texas), C (Cytoplasmic), and S (Synthetic) (Sterility). T (Texas) cytoplasm is thought to be linked to disease susceptibility in maize, such as leaf blight and yellow blight. Male sterility is not affected by chromosomal/nuclear DNA, according to the findings (particularly in maize). Furthermore, the majority of the cytoplasm, which contains organelles, is inherited from the mother. The sterility is transmitted from the cytoplasm, according to the current understanding.

The current discoveries mark a watershed moment in plant science and agricultural enhancement. In hybrid sterile maize plants, maize corn can be consistently generated. Maternal inheritance is a natural wonder, since it explains how certain genotypes on the maternal side impact a variety of critical characteristics.

However, nuclear/chromosomal inheritance can also be inherited from the mother. Mother inheritance is complicated; studies demonstrate that numerous maternal genotypes impact phenotypes.

The Maternal Effect in Snail:

Coiling behaviour in snails is determined by maternal inheritance. The snail Limnaea peregra has two shell coiling phenotypes: one is dextral, which coils for the right side, and the other is sinistral, which coils for the left side. The development of coiling style is solely due to the mother’s genetics (not a phenotype) in this case. Assume that the D+ genotype indicates dextral (right side) coiling and the D genotype indicates sinistral coiling.

When a D+D+ female crosses with a DD man, all F1 and F2 children become dextral since the mother is D+D+ dextral. The DD recessive trait is not manifested here, and there is no Mendelian 3:1 ratio (all four are dextral). When a DD sinistral female is bred with a D+D+ dextral man, the F1 offspring become sinistral with genotype D+D. Mentioning genotype is key since genotype, not phenotype, governs inheritance.

All F2 progeny become dextral and coil for the right side when this F1 progeny is inbred (D+D x D+D). Because all F1 progeny are sinistral, all F2 children become dextral, these findings revealed that the phenotype of parents has no effect on the phenotype of descendants. 

A thorough analysis reveals that the orientation of coiling is determined by the spindle generated during the second metaphase division.

 The spindles of the dextral snail are tipped to the right, whereas those of the sinistral snail are tilted to the left. Intriguingly, maternal genes regulate spindle organisation in metaphase. So, in snails, the true phenotype of “type of coiling” is determined by maternal genes and not by the phenotype of any of the parents.

Inheritance of kappa particles in paramecium:

Paramecium is a chemical discovered in some killer paramecium strains that kills sensitive paramecium strains. The kappa particles in the paramecium’s cytoplasm control the formation of paramecium.

The KK gene, which is dominant over the kk gene, is responsible for the synthesis of kappa particles. When it comes to kappa particle inheritance, cytoplasmic exchange during conjugation is critical.

When KK killer strains are conjugated with kk strains, all of the offspring are heterozygous for genotype Kk, however the paramecium phenotype is dependent on the presence or absence of kappa particles and is regulated by conjugation duration.

Conclusion

The cytoplasm is an essential part of the cell, not just for transporting organelles but also for character inheritance. Some illness conditions in humans are caused by cytoplasmic inheritance and maternal inheritance. Any difficulty with extrachromosomal gene inheritance leads to major physical, mental, and metabolic issues.