Chapter 9: Extranuclear Inheritance

Extranuclear Inheritance mode of transmission challenges the core principles of Mendelian genetics. Extranuclear inheritance is categorized into three main types: organelle heredity, involving DNA housed in chloroplasts (cpDNA) or mitochondria (mtDNA); infectious heredity, caused by inherited symbiotic microorganisms; and maternal effect, where the phenotype is determined by the mother's nuclear genotype, whose products are stored in the egg cytoplasm. Analyzing organelle-based traits is difficult because functional gene products are derived from both nuclear and organelle DNA, and because individual cells frequently exhibit heteroplasmy, carrying a mixed population of both normal and mutant organelles. Key examples of organelle heredity include the leaf variegation in four o’clock plants (Mirabilis jalapa), where coloration depends strictly on the ovule source, and streptomycin resistance (strR) in Chlamydomonas, which is transmitted only through the mt+ parent. Studies of mitochondrial mutations feature the slow-growing poky strain (mi-1) in Neurospora crassa, which displays maternal inheritance and impaired respiration, and the petite mutations in yeast (Saccharomyces cerevisiae), which may be segregational (nuclear), neutral (mtDNA deletion, surviving due to biparental mitochondrial inheritance) or suppressive (mutant mtDNA dominating wild-type function). The structural similarities between organelle DNA (circular, lacking histones) and bacterial DNA support the endosymbiotic theory, which posits that mitochondria and chloroplasts originated from primitive free-living bacteria engulfed by eukaryotic cells billions of years ago. Human mtDNA is highly susceptible to mutation due to limited repair mechanisms and exposure to reactive oxygen species (ROS), resulting in a mutation rate at least tenfold higher than nuclear DNA, contributing to aging and numerous disorders. Genetic diseases, such as MERRF, LHON, and KSS, are classified as mitochondrial disorders if they exhibit maternal inheritance, reflect a deficiency in bioenergetic function, and involve an mtDNA mutation. To prevent the transmission of these serious disorders, mitochondrial replacement therapy (MRT), or "three-parent in vitro fertilization," involves transferring the nucleus from an affected egg into a donor egg with healthy mitochondria. Finally, the maternal effect is demonstrated by shell coiling in the snail Lymnaea peregra, where the direction (dextral or sinistral) is fixed by the mother’s nuclear genotype, and by the bicoid gene in Drosophila, where the maternally supplied protein gradient establishes the embryo's anterior-posterior axis.