Delayed Chromosomal and Extrachromosal Inheritance

• Characteristics showing delayed inheritance still conforms with the principles of chromosomal genetics but sidetracked by the ties of a parent.
• 2 important features in the egg resulting to maternal inheritance:

  (1) the orientation of the mitotic spindle axis

  (2) the high cytoplasmic continuity between the egg and the zygote with very little or no contribution from the sperm
• Allele A is involved in the production of Kynurenin which is involved in pigment synthesis.
• Maternal Effect Gene


  Phenotype of offspring is determined by genotype of mother.
• Some genes are in the cytoplasm.
• True


  T/F: Nucleus contains genes and they are inherited. But even if nucleus is removed, genes are still inherited.
• Extranuclear inheritance occurs when genetic information from mitochondria or chloroplast organelles is passed from parent to offspring.
• Homoplasmy: the presence of a uniform collection of mtDNA; either completely normal mtDNA or completely mutant mtDNA.
• Heteroplasmy: the presence of some mitochondria that have a mutation in the mtDNA and some that do not.
• Heteroplasmy: a rare condition and is often associated with mitochondrial DNA disease.
• The rate of heteroplasmy formation is a product of random segregation between wild-type and mutant DNA occurring during early embryogenesis.
• Mutation in mitochondrial genome in Saccharomyces cerevisiae produces PETITE strains which can grow on media with only non-fermentable carbon sources (ex: glycerol and ethanol).
• Mutation in mitochondrial genome in Saccharomyces cerevisiae when grown in the presence of fermentable carbon sources (ex: glucose) they can form small anaerobic colonies.
• The mitochondrial genome in petites cannot code for cytochrome oxidase.
• In Neurospora crassa, POKY strain has low activity of respiratory enzymes due to defective cytochrome complement in mitochondria.
• Mitochondrial disorders are clinically heterogeneous group of disorders that arise as a result of dysfunction of mitochondrial respiratory chain.
• Leber’s hereditary optic neuropathy (LHON)


  (1) Mutation of the electron transport chain proteins, cytochrome oxidase and ATPase (2) Results to partial or complete blindness from optic nerve degradation
• Kearns-Sayre Syndrome (KSS)


  (1) Neuromuscular defects due to large deletions in mtDNA (2) Accumulation of pigments in retina
• Mitochondrial Inheritance Pedigree

  • inheritance only thru maternal lines
  • affected males do not pass on the genes
• Possible factors that may cause this high mitochondrial mutation rate:

  • insufficient DNA repair system
  • lack of DNA proteins such as histones
  • spontaneous error arising during DNA replication
• Plasmids are capable of self-perpetuation and independent transmission and may therefore be considered as genetic units fully equal to those in the chromosomes.
• CRITERIA for extrachromosomal inheritance

  1. Maternal Inheritance
  2. Non-Segregation
  3. Non-Mendelian Segregation
  4. Indifference to Nuclear Substitution
  5. Infection-like transmission
• In reciprocal crosses , progenies show characteristics of their female parent . If chromosomal differences can be ruled out, maternal inheritance can usually imply transmission through the cytoplasm simply because the female gamete provides more, if not all, of the cytoplasm to the zygote.
• Failure to show segregation under appropriate circumstances may indicate extrachromosomal heredity.
• When segregation occurs, but in a manner that is inconsistent with chromosome segregation, non-chromosomal factors might have accounted for the phenotypic variations.
• When a heritable characteristic persists in the presence of nuclei known to have been associated with alternative characteristics, the control of the nuclear genetic material over the characteristic may be ruled out
• When a heritable phenotype is transmitted without nuclear transmission, it seems unlikely that chromosomes control that phenotype.
• (Mammals) AGA, AGG

  nuclear dna : arginine

  mtdna : stop
• (Invertebrates) AGA, AGG

  nuclear dna : arginine

  mtdna : serine
• (Fungi) CUA

  nuclear dna : leucine

  mtdna : threonine
• (all of the above) AUA

  nuclear dna : isoleucine

  mtdna : methionine
• (all of the above) UGA

  nuclear dna : stop

  mtdna : tryptophan
• mode of inheritance:

  • nuclear genome - mendelian
  • mitochondrial genome - maternal