Exam 4 Flashcards
Indicator Trait
A trait that may or may not be of importance itself, but is used as a means of selecting a genetically correlated trait
Economic Weight
The change in return expected from a one unit change in performance of a trait
Breeding Objective
The weighted combination of traits defining aggregate breeding value for use in a selection index
Marker Assisted Selection
A type of indirect selection in which selection is on specific DNA sequences
Single Trait Selection
Selection for one trait
Tandem Selection
Selection for first one trait, then another
Multiple Trait Selection
selection for more than one trait
Selection Target
A level of breeding value that is optimal in either the absolute or practical sense
Environmental Correlation
A measure of the strength of the relationship between environmental effects on two traits
Phenotypic Correlation
A measure of the strength of the relationship between performance values for two traits
Genetic Correlation
A measure of the strength of the relationship between breeding values for two traits
Effective Proportion Saved
Reflects correct selection intensity when selecting for multiple, uncorrelated traits
Aggregate Breeding Value
The breeding value of an individual for a combination of traits
Pleiotropy
Same gene(s) affecting more than one trait
Economic Selection Index
A combination of weighing factors and genetic information on multiple traits
Independent Culling Levels
Minimum standards for traits under selection with individuals failing to meet a single criterion not selected regardless of merit in other traits
Direct Response
Genetic change in a trait resulting from selection for that trait
Correlated Response
Genetic change in one or more traits resulting from selection for another trait
QTL
Quantitative Trait Locus
Categories of traits for which marker assisted selection is expected to be useful
Carcass Traits
Sex-limited Traits
Two causes of genetic correlations? Specify permanent or temporary cause
Pleiotropy- same gene affects both traits (permanent)
Linkage- genes affects different traits, but are close on chromosome (temporary)
Briefly define each direct response to selection and correlated response to selection.
Direct Response: changing 1 trait by selecting on that trait (selecting x to change x).
Correlated Response: changing 1 trait by selecting on a different trait (selecting x to change y)
Briefly define each direct response to selection and correlated response to selection.
Direct Response: changing 1 trait by selecting on that trait (selecting x to change x).
Correlated Response: changing 1 trait by selecting on a different trait (selecting x to change y)
Provide one example of when correlated response would be considered beneficial. Provide another example where correlated response would be a concern. Use specific traits for examples.
Beneficial: increasing scrotal size in bulls decreases puberty age in daughters
Concern: increasing yearling and weaning weights would increase birth weights which could cause calving difficulties
List the three methods of multiple trait selection discussed in class. For each method, provide one advantage and one disadvantage associated with the method.
Tandem Selection:
- Pro: easiest to implement
- Con: least effective
Independent Culling Level:
- Pro: select at any time
- Con: an animal may have 1 bad trait, but excell in other traits yet still get culled for the 1 bad trait
Economic Index:
- Pro: most effective
- Con: Hard to calculate
Describe the causes of genetic correlations and correlated response to selection
Linkage and pleiotropy cause genetic correlations and correlated response to selection. Linkage refers to genes affecting each trait which are close together on a chromosome, and pleiotropy refers to the same gene or genes affecting both traits.
Are positive correlations between traits always favorable? Explain.
Positive correlations are not always favorable. It is not always desired to have the two traits positively related, as one may desire a positive change in one trait and a negative change in the other trait.
- Positive example: Carcass weight and Mature Size; Fat thickness and Yield Grade; Milk Production and Total Energy Intake
- Negative Example: Birth Weight and Direct Calving Ease; Average Daily Gain and Feed to Grain Ratio
Factors affecting Correlated Response to Selection
- genetic correlation between traits X and Y
- Accuracy of selection for trait X
- Selection intensity for trait X
- genetic variation for trait Y
- Generation Interval
Why would a breeder choose to practice indirect selection for a trait rather than direct selection?
- A trait may be too expensive to measure
- A trait may be too dificult to measure
- Accuracy of selection may be greater for the correlated trait
- the correlated trait may occur much later in life
- the correlated trait may be sex-limited
Describe some situation in which indirect selection might be preferable to direct selection?
- Direct Selection- scrotal circumference
- Correlated Response- decreased age at puberty of daughters
- Direct Selection- days to 230
- Correlated Response- backfat thickness
- Direct Selection- Weaning Weight
- Correlated Response- Yearling Weight or Slaughter Weight
What are potential benefits of genetic correlations? Give examples.
- Genetic correlations can be beneficial if the two traits are favorably, genetically correlated. In this case, it may be possible to use direct selection for a trait that is difficult to measure (carcass traits), or a measure not until later in life (longevity)
- Alternatively, genetic correlations may be a draw if unfavorable. For example, selection in weaning weight may lead to increases in birth weight, which could be unfavorable.
Pros and Cons of Tandem Selection
- Pros:
- Easy
- Cons:
- Inefficient
- Effectiveness depends on genetic correlations
How do genetic correlations influence the effectiveness of tandem selection?
- Unfavorable genetic correlations can be particularly damaging for tandem selection as selection for one trait can result in unfavorable change in the other trait (example of egg wt. vs egg number). However, favorable genetic correlations can be beneficial for tandem selection, as selection for one of the traits also improves the other trait.
List the pros and cons of selection using independent cullint levels
- Pros:
- allows simultaneous selection for more than one trait
- convenient when selection occurs at different stages of an animals life
- Cons:
- superiority in one trait does not compensate for inferiority in another
- more difficult than tandem selection
- difficulty in selecting truncation points
- intensity decreases with each addition trait
In the development of an economic selection index, what criteria are used to determine the traits in the breeding objective?
- traits included in the breeding objective should be those that are economically important
What criteria are used to determine the traits in the index?
- Traits included in the index should be those for which performance records can be easily collected and cheaply and that contribute to or are related to the traits in the breeding objective
List the pros and cons of selection using an economic selection index
- Pros:
- most effective method
- allows flexible objectives
- Cons:
- complex
- must have data for all traits before selections are made
Why is there a loss of selection intensity in indivudual traits when selection is applied to more than one trait at a time?
- Selection intensity decreases for each trait as more traits become involved. Breeders cannot be as “choosy” with respect to a certain trait if animals are required to to excel in other traits as well.
Explain how marker assisted selection works.
- Marker assisted selection is selection for specific alleles by using genetic markers that can be recognized by biochemical methods. These genetic markers are linked closely enough to loci of interest, QTL or loci for simply inherited characters, that they can be used to identify alleles at the loci of interest. Thus if a marker allele M is linked on a chromosome to a QTL allele Q, and a marker allele m is linked to a QTL allele q, selection for M will result for selection for Q.
Describe a scenario showing how marker assisted selection can be used to help select for a simply-inherited trait.
Marker assisted selection could be used for a simply-inherited trait such as coat color in cattle. For the black/red coat color under complete dominant gene action, it is unknown if an animal with a black coat color is BB or Bb. Marker assisted selection could be used to determine which animals are homozygous black and which are heterozygous black, which may be useful for mating decisions.
Why is it important that a genetic marker be as closely linked to the gene of interest as possible?
The more closely linked the genetic marker and the gene of interst, the less likely that cross-over events will separate the two.
Under what conditons is marker assisted selection and or marker assisted mating for a polygenic traits likely to be the most effective?
- Marker assisted selection for polygenic traits is likely to be most effective for traits that have not received much previous selection pressure; traits such as disease resistance, meat quality, and lowly heritable traits.
- It may also be used in combination wither superovulation and other embryo production procedures to allow accurate selection of embryos to be transferrred to recipients.
Equation for Ratio of Response
General Formula for Correlated Response to Selection
Other formula for Correlated Response to selection
Solving for Pe
Direct Response Equation
rBV,BV hat
accuracy of selection for x
OBV Y or X
standard deviation of BV of y or x
