Cytoplasmic Inheritance

The term inheritance means passing on traits or characteristics of parents to their offspring. Genes play a crucial role in determining the character of an organism. But sometimes, the transmission of characteristics to offspring from parents is determined by cytoplasmic genes. It is known as cytoplasmic inheritance or maternal inheritance, or extranuclear inheritance. Cytoplasmic inheritance examples can be found in some common daily substances like maize, Mirabilis jalapa plant, cotton plant, etc. Cytoplasmic inheritance is common in eukaryotes. It is commonly known to occur in cytoplasmic organelles like mitochondria or unicellular parasites like bacteria.

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Cytoplasmic Inheritance Overview

Cytoplasmic inheritance meaning is the inheritance of organelle DNA in the offspring from the parent’s DNA. Cytoplasmic inheritance is different from nuclear genetics because it does not follow the laws of gene inheritance. Half of the genes will come from male and female parents according to gene inheritance. However, depending on the species, DNA found in the organelles can come exclusively from males, females, or an unequal mixture of both.

Cytoplasmic inheritance is governed by genes found in chloroplasts and mitochondria.

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Cytoplasmic Inheritance Types

They are of two primary types:

1. Plastid inheritance

• Chloroplasts are an essential type of plastids
• Applicable only on plants, as plastids are only found in plants

Some examples:

Mirabilis jalapa:

• The Mirabilis jalapa plant has three different types of leaves-green, white and variegated leaves
• In the case of Mirabilis jalapa, inheritance is administered by chloroplasts
• In Mirabilis jalapa, the green leaf has normal chloroplasts. The white leaf has mutant chloroplasts, whereas the variegated leaf has a combination of mutant and normal chloroplasts

Oenothera:

• In Oenothera, the whole chromosome set of pollen parent or pistillate of an individual is transferred to the gametes
• Gametes contain either all chromosomes of a male or a female parent
• Such inheritance of chromosomes is observed because of the series of reciprocal translocations

Iojap in Maize:

• Plastids govern the soap phenotype
• In the case of Iojap in maize, normal plastids are found in green leaves. Mutant plastids are found in white leaves, while a mixture of normal and mutant plastids is found in the striped leaves
• There are three different categories of plastids for a cross between the green male and the Iojap female- white, green, or both

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2. Mitochondrial inheritance

• Mitochondrial inheritance is common for both plants and animals

Some examples are:

Cytoplasmic male sterility in plants:

• There are three types of male sterility in crop plants: cytoplasmic, genetic, and cytoplasmic
• In maize, mitochondrial DNA determines cytoplasmic sterility
• Cytoplasmic sterility is detected in many other crop plants such as cotton, sorghum, pearl millet, etc.

Pokiness in Neurospora:

• Neurospora is a bread mould. It has two strains, the first is wild, and the second is poky
• The wild strain part of Neurospora has normal growth, whereas the mutant or poky part has very slow growth
• Suggesting the existence of cytoplasmic inheritance. It is due to the only distinction between the reciprocal crosses
• Females contribute the most to the cytoplasm in Neurospora

Petite in yeast:

• Petite yeast, also known as little yeast, is Ascomycetes
• Petite is very small in size
• Petite is a mutant, i.e., different from the normal conditions
• Cytoplasmic factors control this mutation

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Characteristics

1. Reciprocal Differences: Cytoplasmic inheritance governs the characters with notable distinctions in reciprocal crosses in F₁.
2. Maternal Effects: Distinct maternal effects are observed due to more contribution of cytoplasm to the zygote by a female parent than a male parent.
3. Mappability: Chloroplast genes and mitochondrial genes have been mapped, which were very difficult to map. Chloroplast genes in Chlamydomonas and maize and mitochondrial genes in humans and yeast are some of the examples.
4. Non-Mendelian Segregation: A typical segregation pattern not followed in cytoplasmic inheritance exhibits in the Mendelian inheritance.
5. Somatic Segregation: The characters, which are controlled by cytoplasmic genes, display segregation in somatic tissues. For example, segregation in somatic tissues like leaf variegation.
6. Governed by plasma genes: Chloroplast or mitochondria DNA governs or manages Cytoplasmic inheritance.

Significance

• Cytoplasmic inheritance helps determine the significance of cytoplasmic organelles in the passing on of characteristics in several organisms.
• Cytoplasmic inheritance helped map the chloroplast genome and mitochondrial genome in many species, such as humans, yeasts, maize, etc.
• Cytoplasmic male sterility has developed in crops like maize, cotton, pearl millet, etc. It has promoted the production of hybrid seeds.
• The role of mitochondria is gaining importance in heterosis, as heterosis results in vigorous growth of plants, which ultimately provides a higher yield of crops.
• The mutation leads to the generation of new variants of characteristics. In cytoplasmic inheritance, these mutations are observed in chloroplast and mitochondrial DNA.

Important features of Cytoplasmic DNA and Nuclear DNA

• The cytoplasmic DNA is of two types: the first is Mitochondrial DNA (mt-DNA) and the second one of Chloroplast DNA (cp-DNA)

• The cytoplasmic DNA is a circular molecule except for the ciliate protozoa, which is a linear molecule. In contrast, the nuclear DNA is linear in eukaryotes and circular in prokaryotes
• The synthesis of chloroplast DNA and mitochondrial DNA continues during the entire cell cycle, while the synthesis of nuclear DNA occurs during the interphase (S) of the cell cycle
• Cytoplasmic DNA replicates in chloroplasts and mitochondria in a semiconservative manner. However, nuclear DNA replicates in the chromosomes in a semiconservative manner
• Mutation of cytoplasmic DNA leads to change in plasma genes and mutation of nuclear DNA leads to change in nuclear genes
• Cytoplasmic DNA is capable of coding for RNA and protein synthesis like nuclear DNA
• In vitro synthesis of both cytoplasmic DNA and nuclear DNA is possible in the respective organelle
• The synthesis of cytoplasmic DNA and nuclear DNA is blocked when treated with ethidium bromide, acriflavine and actinomycin-D

Conclusion

We have learned the meaning of cytoplasmic inheritance. Cytoplasmic DNA is found in certain cell organelles and can be passed onto offspring strictly from the female, male, or a mixture of both parents. 

Cytoplasmic inheritance examples are Iojap in maize, pokiness in Neurospora, Oenothera, etc.

There are two types of cytoplasmic inheritance-Plastid and mitochondrial inheritance. Cytoplasmic inheritance is useful in many ways such as in explaining the role of various cytoplasmic organelles in the transmission of characters, facilitating the production of hybrid seeds, and many more.