Genetics

• Delayed chromosomal inheritance


  Characteristics showing delayed inheritance still conform with the principles of chromosomal genetics but are sidetracked by the ties of the parent
• Delayed chromosomal inheritance


  • The ties are usually between the maternal parent and the offspring

  Results from 2 important features of the egg but not the sperm:

  • The orientation of the mitotic spindle axis
  • The high cytoplasmic continuity between the egg and the oocyte with very little or no contribution from the sperm
• Delayed chromosomal inheritance


  • Caspari's example of maternal influence in the flour moth Esphestia kuhnielle
• Kynurenin


  A diffusible, hormone-like substance involved in pigment synthesis
• In some cases, maternal influence does not diminish during development but lasts throughout adult life
• Inheritance of the direction of coiling of the snail Limnea peregra's shell


  • Dextral (right) or sinistral (left) coiling
• Inheritance of the direction of coiling of the snail Limnea peregra's shell


  F1 shows the same direction of coiling as the maternal parent

  F2 are all dextrally coiled

  F3 segregates 3 dextral: 1 sinistral
• Dextral-sinistral alternative


  Depends on a gene pair in which the allele for dextral is dominant
• The developmental consequences of delayed gene action may also be detrimental, e.g. grandchildless mutation in Drosphila
• Extrachromosomal inheritance


  Cytoplasmic factors capable of self-perpetuation and independent transmission, considered genetic units fully equal to those in the chromosome
• Extrachromosomal inheritance


  • Tends to be maternal because most of the zygote's cytoplasm is derived from the egg

  Reciprocal crosses give different results, like delayed chromosomal inheritance

  Unusual phenotypic ratios do not disappear after one generation
• Plasmid inheritance


  Perpetuation through DNA replication, a second system for the transmission of traits
• Cytoplasmic inheritance in Chlamydomonas

  • Streptomycin resistance (sr) or sensitivity (ss) inherited in a regular Mendelian fashion
• Cytoplasmic particles in Paramecium aurelia

  • Killer vs. sensitive trait determined by the gene K and the cytoplasmic particulate material called kappa
• Chloroplasts


  • Responsible for photosynthesis

  Structure, pigments, and enzyme systems can be affected by mutations

  Capable of self-replication

  Contain DNA, have their own genetic machinery
• Mitochondria


  • Have their own DNA

  Divide and reproduce by themselves

  Explain the cytoplasmic continuity of neutral and suppressive petites in yeast
• The human mitochondrial genome includes only 37 genes, 13 of which encode proteins important in cellular respiration
• Genetic disorders associated with mitochondrial DNA in humans

  • Progressive external opthalmoplegia

  Kearn-Sayre Syndrome

  Pearson Syndrome

  Leber Hereditary/Optic Neuropathy
• Cytoplasmic-nuclear male sterility


  Male sterility due to cytoplasmic factors (e.g. genes in the mitochondrial genome) that can interact with nuclear genes (e.g. fertility restorer genes) to restore male fertility
• Cytoplasmic-nuclear male sterility is widely used in the production of F1 hybrid seeds of many crop species
• Cytoplasmic - Nuclear Male Sterility

  Male sterility due to cytoplasmic factors that can interact with nuclear genes to restore fertility
• Cytoplasmic male sterility (CMS) has been observed in many plant species
• Male sterility is due to some cytoplasmic factors (e.g., genes in the mitochondrial genome)
• These mitochondrial genes can interact with genes in the nuclear genome (e.g., fertility restorer genes) and restore male fertility despite sterile cytoplasm
• This phenomenon is widely used in the production of F1 hybrid seeds of many crop species, e.g., rice, corn, etc.
• The F1 hybrids are more vigorous and productive than the pure lines or open pollinated varieties because of hybrid vigor or heterosis
• Dominance hypothesis


  There is accumulation of favorable parental dominant genes in the hybrid
• Overdominance hypothesis


  Heterozygotes are more vigorous, productive, and perform well over a range of environments than either homozygotes
• Without CMS, hybrid seed production requires emasculation/disabling of pollen before anther dehiscence and stigma receptivity
• Using CMS lines as the female parent restorer line (R line) as the male parent, the tedious process of emasculation/ detasseling is bypassed in the F1 hybrid seed production
• Plasmids of extracellular origin

  • They are not normal or indispensable components of the normal cell
  • They can be transmitted from infected to uninfected individuals by cell extracts from the former
  • They are not capable of moving in and out of the chromosomal DNA of the host and can exist in the nucleus of the cytoplasm of the host
• Infective heredity


  Heritable traits transmitted through virus-like particles
• Infective heredity


  • Mammary cancer susceptibility in mice
  • CO2-sensitivity in Drosophila
• Episomes


  Genetic elements that may exist either as an integral part of a chromosomal DNA molecule of the host or as an independently replicating DNA molecule (plasmid), free of the host chromosome
• The pattern of transmission for a phenotype based on any of the various potential extranuclear systems cannot be predicted with certainty
• The most feasible procedure is to examine whether there are instances where transmission is such that the chromosomal basis is either ruled out or at least unlikely
• Criteria for extrachromosomal inheritance

  • Difference in reciprocal cross results
  • Maternal inheritance
  • Non-mappability
  • Non-Mendelian segregation
  • Indifference to nuclear substitution
• When one follows the transmission of characteristics based on chromosomal heredity, the reciprocal crosses are ordinarily identical, except in cases of sex-linked genes
• Maternal inheritance


  A characteristic form of difference in the results of reciprocal crosses where the progenies show the characteristics of their female parent
• If the chromosomal differences can be ruled out, maternal inheritance usually implies transmission through the cytoplasm