Extranuclear Inheritance

Introduction to Extranuclear Inheritance

Extranuclear inheritance refers to the transmission of genetic information to offspring through the cytoplasm rather than the nucleus. This mode of inheritance often does not follow Mendelian principles and is now recognized as a significant aspect of genetics. The main varieties of extranuclear inheritance include organelle heredity, infectious heredity, and the maternal effect.

• Organelle heredity: Genetic information is contained in mitochondria or chloroplasts and determines certain phenotypic characteristics of offspring.

• Infectious heredity: Phenotype is affected by symbiotic or parasitic microorganisms present in the host’s cytoplasm.

• Maternal effect: Nuclear gene products are stored in the egg and influence the phenotype of the offspring through the ooplasm.

Organelle Heredity

DNA in Chloroplasts and Mitochondria

Organelle heredity involves the inheritance of traits determined by DNA in chloroplasts or mitochondria. These organelles contain their own genetic material, which is typically inherited from one parent, most often the mother. The phenotype of the offspring is frequently determined by the source of the ovule (egg cell).

Chloroplast Mutations

Chloroplast mutations can lead to observable phenotypic changes in plants. For example, in Mirabilis jalapa (the four o’clock plant), mutations in chloroplast DNA can result in white, green, or variegated leaves due to the loss of chlorophyll production.

• Key Point: The phenotype of progeny is determined by the genotype of the branch (ovule source), not the pollen donor.

• Example: A variegated branch can produce white, green, or variegated offspring, regardless of pollen source.

Chlamydomonas as a Model System

Chlamydomonas, a unicellular green alga, is used to study plastid inheritance. It contains a single large chloroplast with multiple copies of circular double-stranded DNA. Ruth Sager’s studies on streptomycin resistance in Chlamydomonas demonstrated cytoplasmic inheritance, as the trait was passed through the parent contributing the cytoplasm.

Mitochondrial Mutations in Fungi

Studies in Neurospora crassa (bread mold) and Saccharomyces cerevisiae (yeast) revealed that mitochondria also contain genetic systems. Mutations in mitochondrial DNA can affect cellular respiration and energy production.

• Neurospora crassa: The 'poky' mutant grows slowly due to impaired mitochondrial function, specifically the absence of cytochrome proteins needed for electron transport.

• Saccharomyces cerevisiae: 'Petite' mutants form small colonies due to deficiencies in cellular respiration. These mutations can be nuclear (segregational), cytoplasmic (neutral), or suppressive.

Knowledge of Mitochondrial and Chloroplast DNA

Structure and Function of Organelle DNA

DNA in mitochondria (mtDNA) and chloroplasts (cpDNA) is distinct from nuclear DNA. Both are typically circular and double-stranded, but mtDNA is generally smaller than cpDNA. Chloroplast genes encode products involved in photosynthesis and translation, while most mitochondrial proteins are encoded by nuclear genes and imported into the organelle.

• Ribosomal variation: Cytoplasmic ribosomes are 80S, mitochondrial ribosomes range from 55S to 80S, and chloroplasts have genes for 16S and 23S rRNA.

Endosymbiotic Theory

The endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living bacteria that were engulfed by ancestral eukaryotic cells. This symbiotic relationship allowed eukaryotes to acquire oxidative respiration and photosynthesis capabilities.

• Bacteria were engulfed by larger cells and became organelles.

• Organelles lost autonomy but provided essential metabolic functions.

Mutations in Mitochondrial DNA and Human Disorders

Human mtDNA and Disease

Human mitochondrial DNA contains 16,569 base pairs and encodes 13 proteins essential for aerobic respiration. Mutations in mtDNA can lead to severe disorders, as mtDNA is more susceptible to mutations due to lack of histone protection, limited repair mechanisms, and exposure to reactive oxygen species (ROS).

• Heteroplasmy: The presence of both normal and mutated mtDNA within a cell. The proportion of mutated mtDNA can influence the severity of mitochondrial diseases.

• Examples of mtDNA Disorders:

  • MERRF (Myoclonic Epilepsy and Ragged-Red Fiber Disease): Maternal inheritance, lack of muscular coordination, ragged-red muscle fibers.

  • LHON (Leber’s Hereditary Optic Neuropathy): Sudden loss of vision in young adults.

  • KSS (Kearns–Sayre Syndrome): Progressive external ophthalmoplegia, pigmentary retinopathy.

Prevention and Therapy

Genetic testing can identify individuals at risk for mtDNA-based diseases. Experimental therapies, such as mitochondrial replacement therapy (MRT), can prevent transmission of mtDNA mutations to offspring.

Maternal Effect

Definition and Mechanism

The maternal effect occurs when the phenotype of the offspring is determined by nuclear gene products present in the egg. These gene products are transcribed in the oocyte and distributed to the embryo, influencing early developmental patterns.

Examples of Maternal Effect

• Snail Shell Coiling (Lymnaea peregra): The direction of shell coiling is determined by the genotype of the mother, not the offspring’s own genotype.

• Drosophila Embryonic Development: Maternal gene products establish molecular gradients that organize the body plan of the embryo.

Additional info: The maternal effect is distinct from maternal inheritance, as it involves nuclear genes whose products act in the cytoplasm of the egg, rather than direct transmission of organelle DNA.