6: Selection for Simply Inherited Traits Flashcards
Define simply-inherited vs polygenic traits and give e.g.
Simply: affected by one (or a few) genes, affected very little by environment, usually know genes/alleles involved
e.g. coat colour, horns
Poly: affected by many genes, affected by environment, might know a few of the genes/alleles
e.g. birth weight, litter size
What is a recessive lethal condition
Two copies of a certain allele =death
How could a trait come in both polygenic and simply-inherited varieties? E.g.?
Can be polygenic but with single genes that have large effects
e.g. height in cattle
e.g. muscle mass is polygenic but myostatin has large effect
Qualitative vs quantitative? E.g.?
Quali: phenotypes chosen from few possible values / categories
e.g. coat colour
Quanti: phenotypes are scored numerically and are usually continuous
e.g. birth weight
Polygenic traits are typically…
quantitative traits
Example of a qualitative polygenic trait
Calving ease (assisted or unassisted)
How do you go about selection for polygenic traits
- infer the genetic merit of individuals for the trait based on phenotypes observed in the individual and/or relatives (calculate breeding values)
- choose parents based on breeding values
Why isn’t genotyping effective for polygenic traits
Usually poorly understood which genes affect the trait, and each has a small role, can’t just genotype a few loci to determine which animals carry desirable alleles
What is breeding value
Value of an individual as a contributor of alleles to the next generation
What is genotyping
determining which alleles are carried by an individual at one or more loci by examining the DNA in a biological sample
How do we go about selection for simply-inherited traits
- infer genotypes of selection candidates through phenotyping, genotyping or test crosses (test matings)
- select animals based on those genotypes
slide 8,9
look
How effective is using phenotype for selection on traits with complete dominance
If desirable allele is recessive, we can impose strong selection using phenotype
If desirable allele is dominant, can only impose moderate selection bc of heterozygotes
How effective is using phenotype for selection on traits with partial or no dominance
Very (each genotype has a unique phenotype)
Example of selection based on phenotype
HYPP (hyperkalemic periodic paralysis) in horses (partial dominance): do not breed horses with twitching phenotype (H/H or n/H)
Gaitedness in horses is controlled by which gene? How do we select for it
DMRT3
Phenotype hard to assess due to subtle effects on performance
Gene and alleles are known, so can genotype animals
Blood disorder in Irish setters… How do we select against it
Canine leukocyte adhesion deficiency Type 1
Disorder is autosomal recessive, so disease absence can still be carriers
Genes and alleles are known, genotype dogs to select parents or make mating decisions
Symptoms of CLAD Type 1
Abnormal blot clotting, recurrent infections, lameness
How do we test mate (e.g.)
Wanna know if a black bull is BB or Bb, mate bull to red cow (bb) and if you see red calf you know its Bb
See only black calves, confidence in bull being BB increases
How do you calculate ‘probability of detection’ (e.g. that the bull is BB)
1-0.5^n
n= number of test matings
How are the genes behind simply-inherited traits identified
Genotype many individuals from each phenotypic group at numerous loci in the genome, compare genotypes (look for differences/similarities)
Verify through additional experiments
Seven steps of genotyping
Decide on tests-> find lab->collect samples->(extract DNA->run genotyping assay-> report results)->make selection decisions
( ) lab
What is the VGL
Veterinary Genetics Laboratory at University of California Davis
Offers tests for simply-inherited traits in variety of species
In TaqMan SNP genotyping assay, how many probes? Bound by what? What do they do?
Two probes, one for each allele of target gene
- bound by diff fluorescent molecules (VIC or FAM)
- Quencher attached
- Probe binds locus with matching allele
- Forward primer anneals, DNA polymerase extends it and cleaves the probe when it reaches it, releasing fluorescent molecule
- fluorescent model drifts away from quencher, and emits detectable light
How does the intensity of selection influence effectiveness
More intense selection leads to faster progress (e.g. no bb animals breed instead of 50% breed)
Slides 29-32
Intensity of selection
Why is it difficult to eradicate an unfavorable recessive allele
Can avoid using the recessive phenotypes in breeding (e.g. bb) but there will still be many heterozygotes (Bb) who are passing down their b’s
Why not genotype all selection candidates to identify carriers? (4)
- costs (large populations)
- valuable animal may lose value if identified as carrier
- different levels of genomic technologies in dispersed populations (e.g. many breeders control small # dogs)
- genes involved may still not be known
