22: Genetic Change Flashcards
What is genetic change
Change due to selection
Rate of change in the mean BV from one generation to another in a pop due to selection
Rate of genetic change aka
Response to selection
Four factors affecting the rate of genetic change
- Accuracy of selection
- Selection intensity
- Genetic variation
- Generation interval
What is accuracy of selection
Accuracy of BV prediction
Strength of the relationship between true BVs and their predictions (EBV) for a trait under selection
Accuracy of selection is the correlation between…
TBV and EBV (r BV, BV^)
EBV provides an estimate of…
the genetic potential of each animal
What is a major factor affecting accuracy of selection?
Heritability (h^2)
Strength of the relationship between performance and BVs
How to increase heritability
Managing animals uniformly, adjusting for environmental effects
Using more information (pedigree and own performance)
Range of accuracy
0 (not informative) to 1 (very accurate)
The lower the accuracy of an EBV, the higher the risk of…
ranking the animals in the wrong order and the larger the risk of a change in EBV when new information sources (e.g. offspring) become available
What is selection intensity (i)
How “choosy” breeders are in deciding which individuals are selected to be parents of the next generation
Animals selected based on selection criteria e.g. phenotypic values, BVs
What is high intensity vs no intensity selection
High = choosing only the very best individuals based on
selection criteria
No = choosing animals purely at random
Equation for selection intensity
mean selection criteria for individuals selected as parents minus average selection criteria of all potential parents
divided by standard deviation of the selection criteria
What is selection differential
Numerator of selection intensity equation
size of the difference in performance between the parents and the total population
What is selection intensity with truncate selection
Selection on the basis of a distinct division in the selection criterion (point of truncation) above which individuals are kept and below which they are rejected
What is proportion saved (p)
Number of individuals chosen to be parents of the next generation in selection intensity w truncate selection
Why can’t proportion of saved animals be unlimitedly small?
- Selected animals need to be able to produce sufficient number of offspring to replace the entire generation of animals
- Large inbreeding rate
What is genetic variation
Variability of BVs within a population for a trait under selection
Difference in genetic change for high vs low genetic variation
High/large: rate of genetic change will be fast
Low/little: rate of genetic change will be slow
What is low genetic variation
Small range of BV and the best individuals are only slightly better than the worse/above average, so will their progeny, rate of change is slow
What is high genetic variation
High range of BV and the best animals are far superior to the worst
Selected individuals and their offspring will be far better than average, rate of change is fast
What is generation interval (L)
Amount of time required to replace one generation with the next
Average age of the parents at birth of their offspring
Shorter the generation interval, the…
faster the rat of genetic change
What is realized response to selection (R)
difference in average performance or average breeding value between two generation
Selection differential vs selection response
Differential = superiority of the selected parents in comparison to their generation
Response = superiority of the offspring in comparison to their parents
What is a genetic trend
Change in mean breeding value of a population over time
Existence of genetic trends suggests…
Accuracy of selection, selection intensity, genetic variation and generation interval are not constant; change over time
Trade-offs between selection intensity and generation interval
Increase in selection intensity is associated with an increase in generation interval and vice versa
Trade-off between accuracy and generation interval
Decrease in generation interval usually causes a decrease in accuracy of selection (fewer records available for genetic prediction)
Trade-offs between accuracy and selection intensity
Increase in accuracy of selection is often accompanied by a decrease in selection intensity and vice versa (fewer bulls, more records on each bull = greater accuracy but lower selection intensity)
