Genetics- Non-additive gene action

• Trait


  Observable / measurable characteristic
• Phenotype


  Measured level of performance for a trait
• Traits and their phenotypes

  • Weaning weight (cattle): 190 - 240kg
  • Fleece weight (sheep): 3 - 4kg
  • Egg weight (chicks): 53 - 62g
  • Fiber diameter: 23 - 32µm
  • Polledness: Horned, polled
  • Coat colour (Angus): Red, Black
• Quantitative traits


  • Continuous variation
  • No distinct classes
  • Environment plays important role
  • Difficult to identify superior genotypes

  
• Qualitative traits


  • Discontinuous variation
  • Distinct classes
  • Environment plays "small" role
  • Easy to identify genotype
• Additive gene action


  • Many genes (~ 100-300)
  • Responsible for quantitative traits
  • Most economically important traits (e.g. growth, carcass traits, milk yield)
• Non-additive gene action


  • Only one or a few genes
  • Responsible for qualitative traits
  • Gene interaction, e.g. dominance
  • Traits like horns, coat colour

  
• Additive gene action


  • Members of gene pair (alleles) have equal ability to be expressed
  • Both alleles contribute to the phenotype
  • Many genes, no interaction, equal additive effect for each locus involved
  • Expression of trait = sum of the individual effects
  • Extreme phenotypes are rare: allele frequency equal or intermediate
• Non-additive gene action


  • Members of gene pairs (alleles) are not equally expressed
  • Gene interaction i.e. dominance and epistasis
  • Dominance affects phenotype not genotype
  • Phenotype rare at one extreme, abundant at the other
  • Mendelian inheritance

  
• Dominance


  • Expression of one allele (B) at locus dependent on the other allele (b) present
  • One allele has a greater effect than the other
  • In heterozygote the recessive (b) is not expressed / partly expressed

  
• Types of dominance


  • Complete dominance: Heterozygote phenotypically identical to homozygous dominant
  • Partial dominance: Heterozygote's phenotype is intermediate between homozygotes
  • No dominance: Expression of heterozygote exactly midway between homozygotes
  • Over dominance: Expression of heterozygote is outside the range defined by the homozygotes

  
• Epistasis


  • Genes at different loci interact
  • Expression of genes at one locus dependent on alleles present at other loci
• Epistasis examples


  • Coat colour of Labradors
  • Coat colour in cattle
• Sex-linked inheritance


  • Genes on sex-chromosomes (X, Y)
  • Tortoiseshell colour in cats
• Sex-limited inheritance


  • Phenotypic expression limited to one sex
  • Milk production (Additive gene action!)
  • Henny feathering in roosters
• Sex-influenced inheritance


  • Modes of gene expression differ between males and females
  • Dominant in one sex, recessive in other
  • Inheritance of scurs
• Inheritance of horns in cattle is breed-specific
• Colour inheritance in cattle


  • B loci: Basic black (B), red (b), black and brown (Bs), brindle (Br)
  • Dilution genes: Reduce colour intensity (D, Dc, Dj)
  • Spotting genes: Solid colour (S), white spotting (ss), Hereford pattern (Sh), Dutch belting (Sd), White on rump (Sc)