Selection Flashcards
Selection
Humans most effective means to influence rate of genetic change
Rate of genetic change with Selection
1) Heritability
2) Selection Differential
3) Population Size
4) Generation Interval
Heritability in Selection
Higher h2 = faster rate of change
Selection Differential in Selection
mean value of selected individuals - mean value of population from which they were selected
Population size in Selection
greater size = greater superiority
[more change for variation]
Basis for Selection
1) Individual performance
2) Pedigree (ancestors)
3) Collateral relatives (full sib vs half sibs)
4) Progeny (very powerful)
Accuracy of Selection
Measurement of the relationship between true breeding values and the predicted breeding values
Relative reliability of estimate
Statistically: Estimate of how accurately the genotype of the individual for a certain trait can be predicted from the phenotypic averages of it’s relatives
Will change over time (more information about the population as it changes will shift accuracy)
Selection Intensity
Measurement of how “choosy” breeders are in deciding what animals will become parents
[High selection intensity means to choose only best individuals ]
Generation Interval
Average age of the breeding herd when their selected offspring are born
Individual performance
Record from a single animal
Value highly related to h2
Disadvantages:
1) sex limited trait
2) traits requiring long tome
3) carcass traits
4) low h2 traits (=low accuracy)
Pedigree
Information based on ancestor performance
a) degree of relationship
b) h2 of trait
c) environmental correlation between ancestor and individual (must be high)
Pedigree Advantages
1) Cheap
2) Valuable for sex-limited traits
3) Valuable for traits expressed later in life
4) Low h2 traits
[Attention to records of ancestors increases accuracy of selection]
Pedigree disadvantages
1) Lack of accurate records/environmental influences of ancestors performance records
2) Provides no comparative basis for selection among individuals that are all descendants of the same ancestor
Collateral Relatives
Looks at sibling relationship
Value:
1) Sex-limited traits
2) Traits that require slaughter to measure
3) Accuracy tends to be lower than individual performance
Factors affecting accuracy of sib test [Collateral Relatives]
1) h2 of traits
2) Relationship of sib
3) Number of sibs tested (More # = more accurate)
4) Correlation between phenotypes of sibs
[Environmental correlation, including maternal environment]
Progeny Testing with Qualitative Traits
Objective: help determine likelihood that an animal is a carrier of detrimental recessive
Factors of “success” of test:
1) genetic makeup of test animal
2) number of offspring that can be produced
Progeny Testing
The evaluation of the genetic makeup of an individual by phenotypic of its progeny
Genetic worth of an animal= Qualitative and Quantitative traits
Progeny Testing with Quantitative Traits
Objective: to determine breeding value of parents
Basis for EPD and EBV
As h2 increases, more progeny required to equal accuracy of individual performance
Factors affecting accuracy of progeny test for quantitative traits
- Dams must be random
- Offspring tested in same environment
- Compare parents of similar age and environment
-Increase number of progeny = increased accuracy
Selection of more than one trait
Net value of animal dependent on several traits (May not be of equal economic value or independent of one another)
Three methods: (plus 1)
-Tandem
-Independent culling
-Selection index
[-BLUP]
Tandem
Selection for one trait at a time (start with one and build up)
Problems:
-Genetic relationship between traits
-more traits = slower progress (takes longer)
Selection Index
Def: A single number that predicts the breeding value of an individual for a weighted combination of traits
Place different degrees of emphasis on traits of interest
Separate determination of the value for each trait selected for, then the addition of these values gives a total score
Independent Culling Method
Minimal standard for each trait
If minimum isn’t reached for any trait trait, animal is culled
Problem: Potential to cull animals outstanding in one trait just because another didn’t meet the requirements
BLUP
Best Linear Unbiased Prediction
[Extension of Selection Index]
Method of genetic prediction (for selection) particularly appropriate when performance data come from genetically diverse contemporary groups
Keys for Selection Based on More than One Trait
1) Genetic relationship among traits
2) Number of traits selected for
3) h2 of traits
