14.2 Inheritance Patterns of Organelle Genes

Maternal inheritance is a unique genetic phenomenon where offspring inherit DNA from their mother's organelles. This process primarily involves mitochondria and chloroplasts, which are passed down through the egg cell during fertilization. Unlike nuclear DNA, organelle genes follow distinct inheritance patterns.

The transmission of organelle genes has significant implications for genetic diversity and health. Mitochondrial DNA is exclusively inherited from mothers, while chloroplast DNA follows similar patterns in most plants. This inheritance mode can lead to specific diseases and is used to study evolutionary relationships.

Maternal Inheritance and Organelle Genetics

Concept of maternal inheritance

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• Transmission of genetic material from mother to offspring through organelles (mitochondria, chloroplasts)
• Organelle DNA separate from nuclear DNA follows different inheritance patterns
• During fertilization, zygote inherits organelles primarily from egg cell maternal contribution
  • Sperm cells typically contribute little to no cytoplasmic content including organelles
• Offspring inherit organelle genes almost exclusively from mother
  • Paternal organelle DNA rarely transmitted to offspring (rare exceptions)

Transmission of organelle genes

• Mitochondrial DNA (mtDNA) passed down from mother to all offspring
  • Mitochondria present in egg cell cytoplasm inherited by zygote during fertilization
  • All offspring, regardless of sex, inherit mtDNA from their mother (consistent maternal lineage)
• Chloroplast DNA (cpDNA) also maternally inherited in most plant species
  • Chloroplasts present in egg cell transmitted to zygote during fertilization
  • In rare cases, paternal inheritance or biparental inheritance of chloroplasts observed (some gymnosperms, angiosperms)
• Organelle genes do not undergo recombination during sexual reproduction
  • mtDNA and cpDNA passed down as intact, non-recombining units across generations (clonal inheritance)

Implications and Examples of Organelle Gene Inheritance

Implications for genetic diversity

• Maternal inheritance of organelle genes reduces genetic diversity compared to nuclear genes
  • Lack of recombination and paternal contribution limits mixing of organelle DNA (less variation)
• Lower genetic diversity in organelle genomes can impact evolutionary processes
  • Reduced variation may limit ability of organelle genomes to adapt to changing environments (selection pressure)
• Organelle genes used as molecular markers for studying evolutionary relationships and population genetics
  • mtDNA and cpDNA used to trace maternal lineages and investigate phylogenetic relationships (species divergence, migration patterns)

Diseases from organelle mutations

• Mitochondrial disorders caused by mutations in mtDNA or nuclear genes affecting mitochondrial function
  • Examples: Leigh syndrome, MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis, Stroke-like episodes), LHON (Leber's Hereditary Optic Neuropathy)
  • Symptoms vary depending on affected tissues and severity of mutation (neurological, muscular, metabolic)
• Chloroplast gene mutations can affect photosynthesis and plant growth
  • Examples: variegated leaf patterns (chimeric tissue), altered chlorophyll production (pale or yellow leaves)
  • Some chloroplast mutations may be advantageous, such as those conferring herbicide resistance in agricultural crops (glyphosate tolerance)