CYTOPLASMIC INHERITANCE
Introduction
- Most of the traits of an individual are governed by nuclear genes; however, some of the characters may be governed by extra nuclear genes (via genes in cytoplasmic organelles). The inheritance of such traits is known as cytoplasmic inheritance.
- Since, such traits are governed by genes that occur outside the nucleus, hence also called extra-nuclear or extra-chromosomal inheritance.
- Plasmogenes-The factors governing the cytoplasmic inheritance are called plasmogenes or these are present in the chloroplasts or mitochondria.
- The plant characters which are inherited by plasmogenes are inherited in uniparental fashion by the female (egg) plant. Therefore, it shows reciprocal differences in the F1 generation.
- Cytoplasmic male sterility(CMS) is governed by plasmogenes located in the mt-DNA, which causes pollen abortion in higher plants. The CMS is useful in hybrid seed production of several important crop species like maize, sorghum, pearl millet, rice, wheat, pigeonpea where it eliminates the process of hand emasculation.
- Reciprocal cross- It can be used to detect sex-linked inheritance and maternal or cytoplasmic inheritance in animals. Sex linkage was reported by Thomas Morgan in 1910 in Drosophila.
Organelles
Mitochondria and chloroplasts
- Mitochondria are organelles which function to transform energy as a result of cellular respiration while Chloroplasts function to produce sugars via photosynthesis in plants and algae.
- The extranuclear genomes of mitochondria and chloroplasts replicate independently of cell division in response to a cell’s increasing energy needs. Since they replicate independently, genomic recombination of these genomes is rarely found in offspring; whereas, recombination is common in case of nuclear genome.
- Thus, mitochondrial DNA is inherited through the maternal lineage exclusively without any recombination of genetic material. Therefore, any trait coded for by mitochondrial genes will be inherited from mother to all of her offspring.
Types of Extranuclear Inheritance
- Vegetative segregation
It results from random replication and partitioning of chloroplasts and mitochondria during mitotic cell divisions. It results in daughter cells that contain a random sample of the parent cell’s organelles.
Example– vegetative segregation with mitochondria of asexually replicating yeast cells.
- Uniparental inheritance
It occurs in extranuclear genes when only one parent contributes organellar DNA to the offspring.
Example– The maternal inheritance of human mitochondria.
The mother’s mitochondria are transmitted to the offspring at fertilization via the egg. The father’s mitochondrial genes are not transmitted to the offspring via the sperm.
such type of inheritance differ from species to species and are quite complicated. For instance, chloroplasts have been found to exhibit maternal, paternal and biparental modes even within the same species.
- Biparental inheritance
It occurs when both parents contribute organellar DNA to the offspring. It is less common than uniparental extranuclear inheritance, and usually occurs in a permissible species only.
Frequently Asked Questions (FAQ)
What is cytoplasmic inheritance?
Cytoplasmic inheritance (extra-chromosomal) inheritance is governed via genes in cytoplasmic organelles. Examples of such organelles are viruses, mitochondria, and plastids.
What is Plasmogenes?
The factors governing the cytoplasmic inheritance are called plasmogenes or plasmon. These are present in the chloroplasts or mitochondria.