7: Breeding Value and Genotypic Value Flashcards
What is genetics
Study of heredity of all kind of traits
Evaluates how differences btw individuals are transmitted from one gen to the next
Frequency of diff alleles change due to…
migration, selection and genetic drift
Major source of variation between populations
Difference in allele freq
When can allele freqs be used to predict results of mating
Mainly for single locus traits
What are polygenic traits
Traits that are influenced by many genes and environmental factors
Numerous possible phenotypes
Continuous or categorical variation
What is quantitative genetics
The analysis of traits whose variation is determined simultaneously by many genes and environmental factors
In polygenic traits, phenotype does not tell us about…
the entire underlying genotype
Quantitative genetics focuses on…
relationship between parents and offspring (connection between animals)
What is measured in quantitative genetics? What is determined?
Performance (phenotype) of individuals of known genetic relationship (pedigree) are measured
Genetic and environmental sources of variation are determined statistically
Why do quantitative traits show a normal distribution in the population
- underlying genetic distribution (genetic variation)
- underlying environmental distribution (environmental variation)
Most economically important traits are..
polygenic
When does it become difficult to observe and separate the effects of individual genes
When more genes affect that trait
Equation for number of phenotypic classes
(n gene pairs * 2) +1
+1 for the het
What is the infinitesimal model for polygenic traits
Increasing the # of genes affecting the trait will increase the # of phenotypic classes and lead to a normal distribution
Slide 14**
Example of pre- and post-birth, pre-weaning traits
Example of post-weaning and pre-maturity traits
Animals own genetics + maternal effects
Animals own genetics + nutritional and current environmental influence
Example of post-maturity and reproductive traits
Example of meat quality and carcass traits
Strong influences of non-additive genetic effects and environments
Pre-slaughter stresses and post-mortem conditions
Examples of polygenic traits in beef? Dairy?
Calving interval (days), weaning weight
Days open, milk yield
What is unknown when it comes to polygenic traits
Alleles or even loci affecting the trait
What is the mean, variance and standard deviation
Mean: sum of all measurements divided by # of measurements
Variance: average squared deviation of the observations from the mean
SD: square root of variance
Slide 17 to see equations**
Parameters to measure qualitative vs quantitative traits
quali: p and q
quanti: mean, variance, SD
slide 18 for full comparison
What are the main requirements for genetic improvement
Phenotypic variability and heritability
What is the breeders equation
deltaG = h^2 * sd * a * (i/t)
genetic progress = heritability * variation * accuracy * intensity of selection
What part of the breeders equation has the largest effect
heritability
How do we separate genetic influence from environmental influence for polygenic traits?
P = G + E
phenotypic value = genotypic value + environmental deviation
Individual performance is based on…
genes and environmental effect
What is phenotypic value? population mean?
How do you measure
PV = individual performance record
u = average phenotypic value for all animals in pop
these are directly measurable
What is genotypic value? Environmental effect?
GV = effect of individuals genes on its performance (singly or combination)
EV = all nongenetic factor influencing individual’s performance (environmental deviation)
When looking at a graph, phenotypic value is…
compared to the average of the population
P - u
e.g. if the mean is 500lb weaning weight, then a cow weighing 550lb will be at +50 on the graph
Three rules of genetic model for quantitative traits
- expressed as deviation from population mean
- considered independent
- average G = average E = 0
How are G and E independent?
Changing genes at birth does not affect the environment, changing environment does not change genes
But they do interact!
We can split genotypic value into three separate components…
A = additive genetic effect or independent gene effect
non-additive effects:
D = dominance effect
I = epistasis effect
Which of A, D and I are transmitted to offspring? Why?
A is transmitted to offspring
D and I are not because they are due to gene combination effect
Because A is transmittable, it is considered…
as a Breeding Value (BV) i.e. value as a genetic parent
The complete equation of the genetic model for quantitative traits is…
P -= u + BV + GCV + E
Interactions between alleles at one locus is…
Interaction between genes at different loci is…
Dominance (complete, partial, no, over)
Epistasis
The best animals to chose as parents for the next gen are…
The ones with the highest breeding value (BV) = best sets of genes
Slides 29, 30
Look
Why is GCV for the next generation not predictable?
Combination of genes will be lost as parents only transmit half of their gene alleles to their offspring
Breeding value is the sum of…
the independent effect of all that animal’s genes on a trait
Genotypic value vs breeding value
GV = value of genes to SELF (effect of individuals genes on its own phenotypic performance)
BV = value of genes to PROGENY (that part of the genotypic value that can be transmitted from parent to offspring)
Gene combination value is…
the part of the genotypic value that is not transmitted from parents to offspring
The base for phenotypic resemblance between relatives is…
breeding value
Genotype and environment are independent, but…
they interact
Genotypes respond differently across a range of environments
Diff in performance btw two (or more) genotypes changes from env to env
Classic example of environment and genotype interacting
Animals that are genetically adapted to temperate locations versus animals genetically adapted to tropical areas
Performance of animals used to temperate will suffer in tropical
To test interaction between genotype and environment you must have…
at least two genotypes and at least two environments
What traits are most susceptible to G x E interaction
Traits that are less influenced by the animal genotype (low heritability) like fertility and survivability
