Final Exam Genetics

• Epistasis


  When the action of one gene pair interferes with or masks the expression of all allelic alternatives of another gene. This genetic interaction results in modified two gene ratios.
• Epistatic genetic (phenotypic) ratios


  • 9 : 3 : 4
  • 9 : 6 : 1
  • 9 : 7
  • 12 : 3 : 1
  • 13 : 3
  • 15 : 1
• All the epistatic genetic ratios still total 16, so independent assortment (9 : 3 : 3 : 1) still is occurring.
• Recessive epistasis


  When the homozygous recessive condition is required to mask the expression of a second gene.
• Recessive epistasis


  • Single recessive epistasis - 9 : 3 : 4 ratio
  • Double recessive epistasis - 9 : 6 : 1
  • Double recessive epistasis - 9 : 7
• Dominant epistasis


  When either the homozygous dominant or heterozygous condition at one gene will mask the expression at another gene locus.
• Dominant epistasis


  • Single dominant epistasis - 12 : 3 : 1 or 13 : 3
  • Double dominant epistasis - 15 : 1
• Biochemical explanation of epistasis

  1. Precursor

  2. Intermediary

  3. Product
• If either gene product is non-functional, the phenotype will be the same as the homozygous recessive condition.
• Chi square analysis


  Can be used to support a hypothesis on gene action controlling a trait
• To determine the observed ratio, use: (observed number/total number) x gene action value
• Gene action works off a base number of 4: single gene action = 4, two gene action = 16
• Example of chi square analysis


  • Mating true breeding chinchillas: black x spotted, F1 all black, F2 progeny: 224 black, 62 spotted, 24 white, total 310
• Epigenetics


  The study of the mechanisms by which chemical signals, outside the DNA sequence, are transmitted and effect specific traits
• Epigenetics


  • Imprinting
• John Cloud, 2010, Time magazine: 'Why Your DNA Isn't Your Destiny'
• Single winter of overeating as a youngster in Norrbotten Sweden (1800s) could lead one's grandchildren to die decades earlier (than their peers); difference in longevity varied from 6-32 years; occurred equally in males and females
• In most cases the maternal and paternal genes (alleles) in an individual turn on and off at the same time
• Exceptions have been found where imprinting occurs causing only one of the alleles, either the maternal or paternal to be expressed and the other silenced
• Big-bottomed sheep


  • An example of epigenetics
• What is happening with big-bottomed sheep

  1. Callipyge trait (plump rump) is due to a single mutation on chromosome 18

  2. Mutation affects the expression of several genes making them hyperactive (over-express)

  3. Increased protein synthesis - paternal expression

  4. Increased RNA - maternal expression

  
• If the mutation comes from the male you get big bottomed sheep, if the mutation comes from the female you get normal sheep, if you have an individual that is homozygous for the mutation the mutation is silenced resulting in svelte sheep

  
• Silencing could be due to the two genes interfering with each other, or the level of expression of the two is too high in the cells, triggering silencing
• This is a functional example of over-dominance where the heterozygote will show greater expression than either homozygote
• DNA methylation and DNA structural arrangement (ex. nucleosome position)


  Definitely controls gene expression within an organism, cell to cell transmission
• The process for imprinting


  1. Imprinting during gametogenesis with differences occurring between spermatogenesis and oogenesis

  2. Maintenance of imprinting during embryogenesis and in adult somatic cells

  3. Removal (erasure) and reestablishment of imprinting in the gametic cells

  
• Understanding of the process of imprinting is evolving and it is believed that it involves methylation of differentially metylated regions (DMR) near the imprinted gene
• Depending on the gene, methylation will occur in either the egg or sperm cell
• Epigenetics and imprinting may explain differences (discordance) observed for gene expression in identical twins
• Penetrance


  Percent of individuals with a given genotype that express the phenotype associated with that genotype in a population
• Expressivity


  The degree or extent that a given genotype is expressed phenotypically in an individual
• Puppies are hardwired to understand us
• Nearly 400 adorable puppies have helped researchers to show that dogs' ability to understand human pointing — a rarity in the animal kingdom — appears to be hardwired in doggy DNA

  
• Approximately 43% of the variation in performance of puppies responding to human cues was due to genetics
• The finding suggests people strongly selected for these abilities in the past, paving the way for dogs to become the human mind-readers they are today
• Environment


  The environment can influence the expression and level of expression of a gene. Factors such as temperature, light, and nutrition can reduce or prevent expression of a gene or genes
• Temperature response in fur color


  • Rabbits - Himalayan: As temperature decreases the fur on the extremities darkens
• How to measure environmental effects on plants


  Grow large populations of a single genotype in several environments then compare the differences in expression of a trait. Any differences observed have to be due to the different environments.
• How to measure environmental effects on animals


  Multiple matings to produce individuals with similar genotypes and study the F1 progeny in several environments
• How to measure environmental effects on humans


  1. Separate identical twins at birth and place them in separate environments

  2. Study difference between identical and fraternal twins for expression of various traits