Extranuclear Inheritance - Chapter 7

Extranuclear inheritance
	mutations that violate Mendel's rules
	phenotypes not controlled by genes in nucleus
	three types:
		maternal effects
			higher eukaryotes produce complex eggs or seeds
				contain cytoplasmic gene products 
				 from mother
				mutations in these maternal genes		
		 	 	 cause mutant phenotypes in offspring
				results in unique pattern of inheritance:
					phenotype is determined by 
					 MOTHER'S genotype
			ex: shell coiling in Limnaea peregra (see Figure 7.1)
				right-handed vs. left-handed spiral
					one gene, two alleles
						D- gives dextral
						dd gives sinistral
					determined by MOTHER'S genotype
					detected by reciprocal cross
		maternal (uniparental) inheritance
			some organelles have chromosomes:
				mitochondria (mt)
				chloroplast (cp)
			these organelles have symbiotic origins
				single circular chromosomes
					similar to bacteria
					present in many copies per 
					 organelle
			genes on organellar chromosomes
				inherited differently from nuclear genes
			inheritance pattern of mutations depends on:
				inheritance of organelle (maternal or 
				 paternal)
					animal mt usually inherited from mother 
						male and female children get 
						 mother's mt, but only females can 
						 pass it on
				copies of organelle chromosome in cell
					homoplasmic vs. heteroplasmic
			leakage possible
				rarely, male parent can pass on mt
			examples of uniparental inheritance:
				mitochondria
					MERRF disorder in humans
				chloroplast
					plant variegation
						inheritance (Fig. 7.9)
			organellar genes can interact with nuclear genes
		infectious inheritance
			some parasitic agents reproduce in cytoplasm
				presence alters phenotype
			horizontal & vertical inheritance possible
			4 types:
				bacteria
					to be discussed in following chapters
				viruses
					ex: HIV in humans 
						shows maternal inheritance 
						 pattern
							but not due to maternal 
							 inheritance
							due to exposure of fetus to 
							 virus in mother's blood
				viroids
					naked RNA molecules in cytoplasm
					show maternal inheritance
						can also infect damaged cells
							horizontal transfer
					problem in plant agriculture
						potato spindle tuber viroid
							replication appears to 
							 cause phenotype
								spindly tubers
					no known treatment
				prions
					proteinaceous particles
						no nucleic acids
						wide host range
					cause neural degeneration disease
						transmissible spongiform 
						 encephalopathies (TSE)
						mad cow disease
					prion protein (PrP)
						normal form (PrPc)
							found in all adult 
							 mammals
						second form
							structural variant
							acts as seed
								to change normal 
								 PrPs
							aggregates
							disrupts cellular function
							cell dies
							variants spread 
					inheritance
						infectious (horizontal)
						Mendelian autosomal dominant