Cytoplasmic Inheritance by Sidd Sir – Male sterility in maize, variegated leaves in Mirabilis jalapa.

/ NCERT LINE BY LINE, PRINCIPLES OF INHERITANCE AND VARIATION / By Prof. Siddharth Sanghvi

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Comparison of Cytoplasmic and Nuclear Inheritance

Feature	Cytoplasmic Inheritance	Nuclear Inheritance
Definition	Transmission of traits via genes located in organelles like mitochondria or chloroplasts.	Transmission of traits via genes in the nuclear chromosomes.
Location of Genes	Found in mitochondrial or chloroplast DNA (extranuclear).	Found in chromosomal DNA inside the nucleus.
Inheritance Pattern	Maternal (zygote inherits cytoplasm from the egg).	Biparental (genes inherited from both parents).
Type of DNA	Circular DNA in organelles.	Linear DNA organized into chromosomes.
Segregation	Non-Mendelian; no segregation during meiosis.	Mendelian; follows laws of segregation and independent assortment.
Examples	Male sterility in maize, variegated leaves in Mirabilis jalapa.	Mendel’s pea traits, human eye color.

Types of Cytoplasmic Inheritance

Type	Explanation	Example	Key Features
Chloroplastic Inheritance	Traits determined by genes in chloroplast DNA.	Variegated leaves in Mirabilis jalapa	– Mutation in chloroplast DNA. – Results in green (normal chloroplasts), white (mutated chloroplasts), or variegated leaves (mixture of normal and mutated chloroplasts).
Mitochondrial Inheritance	Traits determined by genes in mitochondrial DNA.	Male sterility in maize	– Mutations in mitochondrial DNA disrupt energy production for pollen development. – Leads to failure in anther or pollen formation.

Cytoplasmic Inheritance: Key Examples

Example	Details
Variegated Leaves in Mirabilis jalapa	– Phenomenon: Leaves exhibit green (normal chloroplasts), white (mutated chloroplasts), or variegated (mix of both). – Cause: Mutation in chloroplast DNA affecting chlorophyll production. – Inheritance: Maternal (zygote inherits chloroplasts from the egg). – Significance: Demonstrates non-Mendelian inheritance via organelle DNA.
Male Sterility in Maize	– Phenomenon: Male plants fail to produce functional pollen. – Cause: Mutations in mitochondrial DNA disrupt energy production during pollen development. – Rare Pollen: Occasionally, male-sterile plants produce rare viable pollen. – Inheritance: Maternal (mitochondria inherited from the egg). – Application: Used in agriculture for hybrid seed production. Eliminates manual emasculation.

Variegated Leaves in Mirabilis jalapa

Aspect	Details
Phenomenon	Leaves show patches of green, white, or a mixture (variegation).
Cause	– Mutation in chloroplast DNA: – Green patches: Normal chloroplasts. – White patches: Non-functional chloroplasts (no chlorophyll).
Inheritance Pattern	Maternal: Zygote inherits cytoplasm (and chloroplasts) from the egg. – Normal egg: Offspring are green. – Mutated egg: Offspring are white. – Heteroplasmic egg: Heteroplasmy (mixture of normal and mutated chloroplasts) results in variegation.
Significance	Non-Mendelian inheritance. Demonstrates the role of organelle DNA in traits.

Male Sterility in Maize

Aspect	Details
Phenomenon	Male-sterile plants fail to produce functional pollen, leading to male sterility.
Cause	Mutations in mitochondrial DNA impair energy production required for pollen and anther development.
Inheritance Pattern	Maternal: Mitochondrial DNA is inherited from the egg. Sperm does not contribute mitochondria.
Rare Pollen Formation	Male-sterile plants occasionally produce rare, viable pollen capable of fertilizing female plants.
Fertilization Process	– Rare viable pollen from male-sterile plants fertilizes eggs (either male-fertile or male-sterile plants). – Offspring inherit maternal cytoplasm, thus retaining male sterility.
Backcrossing	– Backcrossing involves repeated fertilization of male-fertile plants using rare pollen from male-sterile plants. – Male sterility persists due to the maternal inheritance of mitochondrial DNA.
Significance	Male sterility persists even after many backcrosses, allowing for the continuous use of sterile lines in agriculture.
Applications	– Simplifies hybrid seed production. – Eliminates the need for labor-intensive manual emasculation (removal of male parts).

Cytoplasmic Inheritance: Key Examples (Summary)

Example	Details
Variegated Leaves in Mirabilis jalapa	– Phenomenon: Leaves exhibit green (normal chloroplasts), white (mutated chloroplasts), or variegated (mix of both). – Cause: Mutation in chloroplast DNA affecting chlorophyll production. – Inheritance: Maternal (zygote inherits chloroplasts from the egg). – Significance: Demonstrates non-Mendelian inheritance via organelle DNA.
Male Sterility in Maize	– Phenomenon: Male plants fail to produce functional pollen. – Cause: Mutations in mitochondrial DNA disrupt energy production during pollen development. – Rare Pollen: Occasionally, male-sterile plants produce rare viable pollen. – Inheritance: Maternal (mitochondria inherited from the egg). – Application: Used in agriculture for hybrid seed production. Eliminates manual emasculation.