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Terms in this set (69)
What makes simply-inherited traits straight forward?
how many loci are involved
how many genes are involved at each locus
how many alleles are expressed
genotypes or probable genotypes of potential parents
mating designed to reveal the genotype of an individual for a locus or a small number of loci
If two heterozygous black cattle are bread together, what are their offspring coat colors going to be?
75% hetterozygous black and 25% red
The probability of any one ____________ carrying a particular gene to the frequency of that particular gene in the population
initial gene frequency
The fastest change in gene frequency occurs at intermediate gene freqauencies.
at high frequencies of the less desirable allele, there are relatively few of the more desirable genes
the ability of the individual and its corresponding phenotype and genotype to contribute offspring
refers not just to an individual's ability to be selected, but to the number of offspring it produces.
degree of dominance at the locus with respect to fitness
When dominance is complete, homozygous dominant individuals and heterozygous individuals are equally fit, they contribute an average same number of offspring
With no dominance heterozygote is midway between homozygous dominant and homozygous recessive loci
identify those individuals of desired genotype and select
genotypes are unkown and there is no way to identify which genes and which loci are invloved
genetic prediction for
rate of change for quantitative traits
represents the genetic and environmental contributions to a single performance record on one animal
for every performance record there exist values for P,G,E
values are trait specific
G and E are expressed as deviations from a mean, the average of genotypic values and the average environmental effects across the population is zero.
G and E are considered independent.
In selecting for polygenic traits, breeders try to choose as parents those individuals with the best sets of genes.
Breeding value is parental value (genetic parent)
Breeding Value is not in the genetic model
Genotypic value illustrates the overall effect of an individual's genes
Breeding Value represents parents
independent breed effects
They reflect the value of each gene independently
An animals breeding Value is simply the sum of the independent effects.
A prediction of breeding value
A prediction of progeny difference
half an individual's breeding value- the expected difference between the mean performance of the individual's progeny and the mean performances of all progeny (assuming randomly chosen mates)
a prediction of progeny difference
Sum of independent gene effects (additive gene effects)
genotypic value that is due to independent gene effects and therefore transmittable gene effects
gene combinatino effects (non additive gene effects) that are part of an individual's genotypic value that is due to the effects of gene combination (dominance and epistasis) and cannot, therefore, be transmitted from parent to offspring.
genetic model for quantitative traits
Breeding value (BV) is the part of an individual genotypic value that is due to independent gene effects that can be transmitted from parent to offspring
Estimated breeding value (EBV) is a prediction of a breeding value
Progeny difference (PD) half the parents breeding value for a trait is the expectation of what is inherited from the parent
PD= 1/2 BV
Progeny differences are not directly measurable, but can be predicted for performance data. Such predictions are called expected progeny differences (EPDs).
What to know about the genetic model for quantitative traits
That an individual animal (sire/dam) does not transmit its progeny difference (exactly half its breeding value) to every offspring
A parent always passes on half its genes, but the genes transmitted constitutes a random sample of the parents genes
Additive Proportion of Breeding Values
BV offspring = 1/2 BVsire + 1/2 BVdam
An offspring's BV for a trait will be, on average, the average of its parents breeding value for the trait
A prediction of an animal's next record
What is different from and EPD
Not expressed on a continuous scale
genetic model summary
The difference between permanent and temporary environmental effects
How the genetic model relates to threshold traits
Difference between independent gene effects and gene combination effects.
measurable in numbers and has units
Body weight (lbs, kg)
Height (ft, m)
Days to 250lbs (days)
The values for: P, G & E
Future performance (repeatability)
Offspring performance (heritability)
- the group of interest
subset of the group you are interested in
A sample should represent their population. Do these samples represent their population?
the TRUE statistics of a population
Usually impossible to know
Estimated using random sampling
randomly selected subset of the population
All individuals in the population have an equal opportunity
True randomness necessary for unbiased prediction
statistic of the sample
Used to estimate the truth
differences among individuals in a population
Breeders care about variation because:
Variation is vital for genetic change
Balancing genetic variation with predictable product
Measures of variation include:
Standard Deviation 𝛔
cov(x,y) measures of how two values vary together
Provides the average direction of change:
(+) traits move in same direction
(-) values move in opposite directions
Reported in units2
Body weight(x) and height(y) have positive covariance
The traits move in the same direction
As weight increases (+), height increase (+)
As weight decreases(-), height decreases (-)
In cattle, body weight(black) and meat tenderness(white) have negative covariance
The traits move in opposite directions
As weight increases (+), tenderness decreases(-)
As weight decreases(-), tenderness increases (+)
In cattle, marbling(black) and meat tenderness(white) have no covariance
The values are independent.
A measure of the strength of the relationship between two variables.
Provides direction of change (+) or (-)
Value between -1 and 1
1 = perfect correlation
0 = no correlation
Calculated using cov(x,y) and 𝝈x and 𝝈𝐲
Value between -1 and 1
Why are covariance and correlation important to breeders?
Breeders can select for change in two traits of interest simultaneously
Positive Covariance: breeding for an increase in one will result in an increase in the other
Negative covariance: breeding for an increase in one (+) will result in a decreases in the other (-)
Regression Coefficient (by∙x )
the expected or average change in one variable (Y) per one unit change in another (X).
Calculated using cov(x,y) and 𝝈x2
Does NOT have to be a number between -1 and 1
Basis for prediction equations
Question: Is bY,X is not the same as bX,Y?
Regression helps us improve?
Example: I want to predict a (randomly chosen) student's shoes size.
If I only know the class average, what is my best guess?
Can I collect other information that will improve my estimate?
Phenotypic Example: I want to predict a calf's weaning weight based on its birth weight.
b(Pww , Pbw )
Genotypic Example: I want to estimate an individuals BV based on..
It's own phenotype
The phenotype of its parents
All an individuals relatives
What is the relationship between regression, correlation and covariance?
Covariance: Describes the dirrection of shared variation between two traits.
Difficult to interprate
Correlation: Provides the strength and direction of relationship (consistency) between two traits
Easy to interprate (Standardized covariance)
Regression: used for prediction and most important for breeding
Gives expected units change in Y for every one unit change in x (bY.X
Heritability (h2- Narrow)
Ranges form 0 to 1 (0% heritable to 100% heritable)
Definition A: A measure of the strength of relationship between P (performance/phenotype) and BV (additive effects) for a trait in a population
A squared correlation
Remember R2 interpretation from statistics?
Definition B: proportion of differences in performance for a trait that are attributable to differences in breading value for the trait
A ratio of of variances
When heritability is high:
An individuals P is a good indicator of the P offspring
Related individuals are more similar to unrelated individuals
Genes have a larger effect on phenotype than the environment
When heritability is low:
An individuals P is poor indicator of the performance of its offspring
Related individuals appear no ore similar than animals in the same environment
The environment has a larger effect on the phenotype than genes
When repeatability is high:
A single record for an individual is a good indicator of future performance
An individual's PA has a larger effect on P than ET
When repeatability is low:
A single record for an individual is a poor indicator of future performance
ET has a larger effect on P than an individual's PA
Ranges form 0 to 1 (0% heritable to 100% heritable)
Definition A: A measure of the strength of relationship between repeated P (performances) for a trait in the population
Definition B: measure of the strength of the relationship between single performance record and producing ability for a trait of interest in a population
To what extent the differences we observe in animal performance are due to inheritance
The degree to which offspring resemble their parents in performance for a trait
Heritability is the?
portion of the phenotypic differences among animals that is due to inheritance
*Heritability is a measure of the strength of the relationship between performance (phenotypic values) for a trait and breeding values for a trait
Broad sense heritability measures
the total influence of genetics on the expression of a trait because it includes the contribution of both breeding value and gene combination value
Gene combination values?
cannot be inherited. Broad sense heritability does not reflect the relationship between the performance of animals and their breeding values
From a selection stand point broad sense heritability is not very useful
Factors affecting the rate of genetic chane
Maximize rate of genetic change
Could be accomplished if we could always pick those animal with the best breeding value to be parents
Problem - we do not know true breeding value - use predictions of breeding value (may or may not be very informative)
Genetic change is directly proportional to three factors
Accuracy of selection
(Inversely proportional to generation interval)
Accuracy of selection (accuracy of breeding value prediction)
Relationship between true breeding value and their predictions for a trait (sounds like another correlation doesn't it)
Accuracy is never perfect, but the higher the accuracy the better
Accuracy of selection depends on?
Measures the strength of the relationship between performance and breeding value
Higher the h2 the better each piece of performance information is as a predictor of breeding value
Take all steps to increase heritability
Manage animal uniformly
Adjusting for known environmental effects
Using contemporary groups
Measures how "choosy" breeders are in selection
Select intensity - mean you choose top individuals on what ever trait you are selecting for
High intensity of selection increases the possibility that we chose the animals with best breeding value - want to always select those better than average
Is the variability of breeding values in the population for a trait under selection
Greater variation the more progress in selection
Time required to replace one generation with another
Time required for a cow's calf to have a calf
Shorter the generation interval the more genetic change
Can you have high intensity in selection without high accuracy?
You can intently select independent of accuracy.
Yes - but the results may be disappointing
You can still select what appears to be the best animals
You need high intensity and high accuracy to make rapid genetic change
Accuracy vs. generation interval
Decrease in generation interval usually causes decrease in accuracy of selection
Why? Fewer records, especially progeny records, are available for use in prediction
Accuracy versus intensity
Increase accuracy is often accompanied by a decrease in selection intensity and vice versa
Do I test more males and have less records on each?
Do I test fewer males and have more records on each?
Intensity limited with single birth species
Should one: Fewer males
Increasing selection intensity
But increasing generation interval
Or keep more females
Decreasing selection intensity
Decreasing generation interval
Selection intensity versus risk
Selection risk - the true breeding value of replacement will be poorer than expected
Intensity can be increased by using only the best bulls (sires) and very few of them with the use of A.I.
By using more sires selection intensity is reduced but so is selection risk
Male vs. female selection
Selection of males is much more important than selection of females
Males contribute much more to mean breeding value than do females
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