Lec 6

• Gene Action


  Interaction of alleles at a single locus
• Dominance


  The presence of an allele which masks the expression of another allele at the same locus
• Dominance relationships


  • Complete Dominance
  • Co-dominance
  • Incomplete Dominance
  • Over-dominance
• Complete dominance


  • A dominant, a recessive
• Co-dominance


  • Both alleles are expressed
• Incomplete dominance


  • Neither allele is fully dominant
• Over-dominance


  • Heterozygote is superior to both homozygotes
• Complete dominance - coat colour in Holstein dairy cattle


  • Black (B) dominant to red (b)
• Co-dominance - coat colour in Shorthorn cattle


  • Red (RR)
  • Roan (Rr)
  • White (rr)
• Epistasis


  Inter loci allelic interaction or interaction of genes at different loci
• Types of epistasis


  • Masking effect - gene at one locus masks the expression of gene at a second locus
  • Modifying effect - modifies an expression rather than masking the expression
• Simple autosomal dominance situation

  1. AaBb x AaBb

  2. 9 A_B_

  3. 3 A_bb

  4. 3 aaB_

  5. 1 aabb
• Epistasis - masking effect in coat colour of Labrador dogs


  • B_E_ - black
  • bbE_ - brown
  • __ee - yellow
  • B_ee - yellow with black nose
  • bbee - yellow with pale nose
• Epistasis - modifying effect in coat colour of Dobermans


  • B_D_ - black
  • bb D_ - red
  • B_dd - blue (faded black)
  • bbdd - fawn (faded red)
• Multiple Alleles


  More than two possible alleles at a locus in a population, but only 2 alleles ever occur in any one individual
• Multiple alleles - ABO blood groups in humans


  • A
  • B
  • AB
  • O
• Alleles


  Different forms of genes for the same characteristic in corresponding loci on homologous chromosomes
• Homozygous genotype

  Alleles are the same (e.g. TT)
• Heterozygous genotype


  Alleles are different (e.g. Tt)
• Penetrance


  Percentage with which a dominant or homozygous recessive gene expresses itself in the phenotype
• Incomplete Penetrance


  Not all individuals with a particular genotype express the expected phenotype
• Incomplete Penetrance - fused tail in mice

  • Fufu - extremely fused and abnormal tails

  Fufu x fufu - 87 fused, 129 non-fused, 21 non-fused were Fufu genotype

  Penetrance = 87/(87+21) = 80.6%
• Incomplete Penetrance - ear tufts in Araucana fowl


  • EtEt - lethal

  Etet - trait

  EtEt male x etet female - 49 tufted, 8 non-tufted

  Penetrance = 49/(49+8) = 86%
• Incomplete Penetrance - stripe marking on forehead of foxes

  • SS - full stripe

  Ss - half stripe

  ss - no stripe

  SS x ss - 18% show no stripe, 82% penetrance
• Incomplete Penetrance - malignant hyperthermia syndrome in pigs

  • Pigs with syndrome show increased body temperature, muscle rigidity, respiratory complications when exposed to mild stress or halothane vapour
• Expressivity


  Intensity of expression of a given phenotype among those individuals expressing that phenotype
• Expressivity - polydactyly in humans


  • Extra digit may occur on one or more appendages, digit can be full size or just a stub
• Expressivity - white spotting in mice and cats


  • Area of pigmentation loss can vary greatly
• Expressivity - mule-foot in cattle and pigs

  • Normal having cloven hooves, affected having 1-4 feet affected
• Lethal Genes


  Genes that lead to death of the individual
• Lethal Genes - parrot jaw in cattle

  • P = normal, p = parrot jaw, P dominant over p
• Lethal Genes - hairless condition in dogs


  • H = hairless, h = hair, H dominant over h

  HH - dies, Hh - hairless, hh - hair
• Aneuploidy


  Condition of having less or more than the normal diploid number of chromosomes
• Monosomy


  Lack of one of a pair of chromosomes
• Trisomy


  Having three chromosomes
• Cytoplasmic or Mitochondrial Inheritance

  DNA found in mitochondria, inherited from the dam
• DNA is found in chloroplasts in plants