WHAT IS CYTOPLASMIC INHERITANCE? - BIOLOGY - 2025

The cytoplasmic inheritance is the transfer of genes in the cytoplasm cell that are not linked to the chromosomes of the core. This type of inheritance is also called foreign inheritance and is part of the different inherited patterns known as non-Mendelian.

It was discovered by the German botanist and geneticist Carl Erich Correns at the beginning of the 20th century (1908). While Correns was working with the plant known as Maravilla del Perú or Clavellina (Mirabilis jalapa), he observed that the inheritance of the coloration of the leaves of this plant seemed to be independent of the paternal phenotype.

Mendelian and non-Mendelian inheritance patterns. Taken and edited from: Ghadisu.

The inheritance of this trait, which did not comply with the laws of Mendelian genetics, seemed to depend exclusively on the genotype of the mother; As a result, she proposed the hypothesis that these traits came from organelles or agents present in the cytoplasm of the ovule.

After more than 100 years of this discovery, and despite the development of molecular genetics, the knowledge about the how and why of the mechanisms of extracellular inheritance are partly uncertain and the studies to clarify them relatively scarce.

Cytoplasmic inheritance vs Mendelian inheritance

Mendelian inheritance

This is the best known form among the different hereditary processes. It was proposed by Gregor Mendel, a monk and scientist born in Heinzendorf, the former Austrian empire, currently known as Hynčice (Czech Republic), in the mid-19th century (1865-1866) and rediscovered in the early 20th century.

His hypotheses about heredity and his theories were tested and served as the basis for many other theories. His discoveries are the basis of what is known today as classical genetics.

Mendelian inheritance indicates that each parent provides one of two possible alleles for a trait to be expressed; These alleles are found in the nucleus of reproductive cells (genetic material), indicating that Mendelian inheritance is biparental.

When the genetic makeup of both parents (genotype) is known, Mendelian laws are used to predict (not always apply) the proportion and distribution of observable traits (phenotypes). Mendelian inheritance applies to most organisms that reproduce sexually.

Cytoplasmic or extracellular inheritance

This type of inheritance was discovered in 1906 by botanist Carl Correns. It is considered non-Mendelian because the transmission of genes does not involve the nucleus, which is the organelle considered in classical genetics to be responsible for containing all hereditary genetic material.

In this case, heredity occurs due to certain organelles, such as mitochondria and chloroplasts, which contain their own genetic material and which can reproduce within the cell.

In the case of mitochondria, which can be present in numbers close to 10,000 per female cells or ovules (with multiple copies of their genome), they can replicate independently of cell division.

This type of replication allows mitochondria to have higher mutation rates than nuclear DNA, evolving faster than nuclear DNA.

During the reproductive process, specifically in fertilization, the mitochondria present in the male reproductive cells are excluded from the zygote (they only have a few hundred of these), while those of the ovule are maintained.

In this way, the mitochondrial genetic material is only inherited through the mother (cytoplasmic inheritance). By this, it is understood that the extracellular or cytoplasmic inheritance is uniparental.

As a result of this, a phenotypic expression difficult to explain from the Mendelian point of view is obtained, mutations that do not have phenotypic expression, as well as different pathologies.

Organelles

Mitochondria

Mitochondria are the most obvious and notable organelles in the cellular cytoplasm of eukaryotic cells. They have the function of producing energy for the cell. An interesting characteristic of these organelles is the one already mentioned of their maternal origin. While another peculiar characteristic is that they present their own DNA.

Parts of a mitochondrion. Taken and edited from Kelvinsong; modified by Sowlos.

Chloroplasts

Chloroplast. By Gmsotavio, from Wikimedia Commons

Chloroplasts are characteristic organelles of eukaryotic cells and organisms that contain chlorophyll. Its main function is to carry out photosynthesis, to produce sugars.

Like mitochondria, they have their own DNA and can multiply within the cell without the help of cell division. Likewise, its inheritance is through the mother, that is, during reproduction, only the oocell provides the chloroplasts.

Evolution

The theory proposed in 1967 by the American biologist Lynn Margulis on endosymbiosis, points to the origin and evolution of eukaryotic cells, based on the long-term endosymbiotic relationship between prokaryotic and ancestral eukaryotic organisms.

According to Margulis, organelles such as chloroplasts and mitochondria are of prokaryotic origin (cyanobacteria and proteobacteria respectively). Other organisms incorporated, phagocytosed, or engulfed chloroplasts and mitochondria.

After incorporating them, the eukaryotic precursors did not digest or process these prokaryotes (chloroplasts and mitochondria), which remained in the host cell and after millions of years of evolution, they became the organelles of the eukaryotic cell.

Among the facts that give weight to this theory are the aforementioned peculiarities that these organelles have their own DNA, and that they can replicate independently within the cell and without its help.

It is worth mentioning that the researchers maintain that endosymbiosis, the presence of DNA in these organs, the high rate of replication and mutation of chloroplasts and mitochondria, as well as cytoplasmic inheritance, are the precursors and responsible for the great leap in complexity. and evolution of life.

Other forms of non-Mendelian inheritance

Gene conversion

It is common to observe during the crossing between mushrooms. It occurs when a gene sequence replaces another homologous sequence. During meiotic division, when there is homologous recombination of heterozygous sites, a mismatch occurs between the bases.

In trying to correct this mismatch, the cell causes one allele to replace the other causing non-Mendelian inheritance called gene conversion.

Infectious inheritance

Viruses participate in this type of inheritance. These infectious agents infect the host cell and remain in the cytoplasm, inserting their genome into the host genome.

Genomic imprinting

This type of non-Mendelian inheritance occurs when alkyne compounds derived from methane and histones are involved in the DNA molecule by methylation, all this without any type of modification in the genetic sequence.

This incorporation will remain in the male and female reproductive cells of the progenitors and will be maintained through mitotic cell divisions in the body cells of the descendant organisms.

Other processes of non-Mendelian inheritance are mosaicism and trinucleotide repeat disorder.

References

1. Extranuclear Inheritance - Non-mendelian Inheritance of Organelle Genes. Recovered from medicine.jrank.org.
2. Non-Mendelian inheritance. Wikipedia. Recovered from en.wikipedia.org.
3. Mitochondrial Inheritance. Encyclopedia.com. Recovered from encyclopedia.com.
4. GH Beale (1966). The role of the cytoplasm in heredity. Proceedings of the Royal Society B.
5. Extranuclear inheritance. Wikipedia. Recovered from en.wikipedia.org.
6. Gene conversion. Recovered from en.wikipedia.org.
7. Genomic imprinting. Recovered from en.wikipedia.org.