Lec 4

• Traits

  Character, attribute, parameter
• Traits

  • Observable - colour of feathers, coat colour, presence of horns
  • Measurable - body weight, height, hormone level, feed intake, litter size
• Types of traits


  • Qualitative - single genes, categorical, observable, discrete classes
  • Quantitative - many genes, measurable, metric, continuous
• Qualitative traits


  • Coat colour in cattle
  • Polledness (absence of horns) - polled gene P is dominant to horned gene p
  • Double muscling in cattle
  • Milk protein types (k-casein and b-lactoglobulin) affecting cow's milk protein
  • Spider syndrome in sheep
  • Susceptibility to scrapie in sheep
  • Wool fibre diameter in Romney sheep
  • Fecundity/Booroola gene in Merino sheep
  • Fecundity/Inverdale gene in Romney sheep
• Quantitative traits

  • Phenotypic expression of an individual which can be measured, quantified, counted
  • Normally under the influence of many genes (polygenes), each having small effect - could also be due to genes with major effects (quantitative trait loci - QTL)
• Quantitative traits of economic importance

  • Dairy cattle - milk yield, % milk fat, % milk protein, calving interval, no. services per conception
  • Beef cattle - weaning weight, yearling weight, average daily gain (ADG), dressing %, % lean in carcass
  • Meat goats - weaning weight, yearling weight, ADG, age at puberty, prolificacy rate, dressing %, % lean in carcass
  • Poultry - layers: no. of eggs, egg weight, feed intake
  • Poultry - broilers: feed intake, feed conversion ratio (FCR), weight for age
  • Swine/pigs - weight for age, ADG, FCR, carcass weight, % lean, back fat thickness
• Gene action


  How a gene behaves by itself and in association with other genes
• Gene action types

  • Single genes - dominance, codominance, incomplete dominance, overdominance, allelic series, pleiotropy, lethals
  • Multiple genes - epistasis, polygenic traits
  • Genes & the environment - sex-influenced traits, environment-dependent gene expression, incomplete penetrance
• Dominance


  A locus with 2 alleles where one expresses itself in the phenotype of the heterozygote as dominant and the unexpressed allele as recessive
• Mendel's experiments with garden peas observed qualitative traits and concluded that an allele may be expressed to the total exclusion of the other allele (dominant vs recessive)
• Dominance gene action

  • The ability of one allele to express its phenotype at the expense of an alternate allele
  • The dominant allele will make a gene product that the recessive allele cannot, so the dominant allele will express itself whenever it is present
  • The recessive allele is defined as an allele whose expression is suppressed in the presence of a dominant allele (as in a heterozygous condition)
• Pedigree analysis


  • Careful analysis will allow us to determine whether a trait is dominant or recessive
  • Mating between blood relatives often causes genetic disorders
  • Descendants of an individual who is a carrier for a specific disease have a much greater chance of inheriting the disease than the normal person
  • When close relatives mating increases the chance of recessive alleles (which might be rare in the general population) being passed on by both parents
  • Most humans carry 4 or 5 deleterious alleles in the heterozygous condition, and relatives often carry the same deleterious alleles in this condition (they share common ancestors), there is greater likelihood that these alleles will pair up and be passed along to offspring
• Autosomal dominant traits

  • Every affected individual has at least one affected parent
  • Affected individuals who mate with unaffected individuals have a 50% chance of transmitting the trait to each child
  • 2 affected individuals may have unaffected children
  • 2 unaffected parents only have unaffected offspring (only for hh parents)
  • The phenotype generally appears every generation
• Autosomal recessive traits

  • All affected are homozygotes
  • Unaffected outsiders are homozygous normal
  • Consanguineous matings are often (but not always) involved
• 1. linked dominant traits

  • Mothers pass their X's to both sons and daughters
  • Fathers pass their X to daughters only
  • Normal outsider rule for dominant pedigrees for females, but for sex-linked traits remember that males are hemizygous and express whichever gene is on their X
• 1. linked recessive traits

  • Males get their X from their mother
  • Fathers pass their X to daughters only
  • Females express it only if they get a copy from both parents
  • Expressed in males if present, recessive in females
  • Outsider rule for recessives (only affects females in sex-linked situations): normal outsiders are assumed to be homozygous
• 1. linked traits

  • Traits on the Y chromosome are only found in males
  • The father's traits are passed to all sons
  • Dominance is irrelevant: there is only 1 copy of each Y-linked gene (hemizygous)
• Dominance


  The major form of interaction between alleles within a locus, as opposed to inter-locus interaction of A alleles and B alleles at 2 different loci (Epistasis)
• Gene expression conditions


  • Homozygous Dominant - both genes present are the dominant ones (HH)
  • Homozygous Recessive - both genes present are the recessive ones (hh)
  • Heterozygous - both the dominant and recessive genes are present (Hh)
• Dominance relationships


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

  • Type of gene action said to be non-additive
  • Presence of a dominant gene completely masks the expression of the recessive gene
• Complete dominance

  • Coat colour in Angus and Holstein cattle - BB and Bb are black, bb is red
  • Polled (hornless) allele P is dominant to horned allele p
  • Wild type agouti/brown allele (B) is completely dominant to the black (b) allele in gerbils
• Dominance relationships

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

  • Type of gene action said to be non-additive
  • Presence of a dominant gene completely masks the expression of the recessive gene
• Coat colour in Angus and Holstein cattle

  • BB - black
  • Bb - black
  • bb - red
• Complete Dominance

  If two alleles have a complete dominance relationship, the heterozygote is indistinguishable from the homozygous dominant
• Complete Dominance

  • Polled (hornless) allele P dominant to horned allele p
• Complete Dominance in gerbils

  • BB gerbils are brown/agouti
  • bb gerbils are black
  • Bb gerbils are brown
• Genetic diseases in farm animals

  • Tibia hemimelia in Galloway cattle - homozygous recessives have shortened and twisted hind limbs and a large abdominal hernia
• Co-Dominance


  • The phenotype of the heterozygote will be mid-way or between the two homozygous phenotypes - homozygous dominant and homozygous recessive
  • The gene action is also said to be additive
• Fecundity gene/Booroola gene in Merino sheep

  • ++ - 1.40 ovulation rate
  • F+ - 2.82 ovulation rate
  • FF - 4.38 ovulation rate