Zusammenfassung der Ressource
Selection
- Aim is to breed from animals which will have the best progeny.
- Response to selection
- The mean performance of the progeny generation minus the mean performance of the parent generation.
- Depends on many things:
- Heritability (h2)
- The superiority of the
selected animals
Anmerkungen:
- Measured by the selection differential (s).
S = mean of selected animals - mean of the population.
- Calculation = R = h2 x s.
- Selection Intensity
- Selection differential is determined by two things:
- The proportion selected (p)
- The standard deviation (SD)
Anmerkungen:
- The measure of the spread of the population.
- As the standard deviation increases, the
selection differential increases.
- Often we want to predict response of a population to a selection
from a knowlege of the proportion of animals selected (p)
- S = i x SD
- Selection intensity (i) is the number of
standard deviations that the animals are
superior to the mean.
Anmerkungen:
- Value of i therefore decreases as p increases.
- Increasing the rate of genetic gain
- A low proportion of selected animals.
- So that selection intensity is high.
- So that selection differential (s) is high
- A high phenotypic variation
Anmerkungen:
- So that the animals we select are a lot better than the average.
- A high heritability
Anmerkungen:
- So the superiority of the parents is passed on to the progeny.
- Response per generation and response per year
- R = S x h2
- R = i x h2 x SD
- Often it is more useful to predict the response per year
- To do this we need to know the generation interval (L) in years
Anmerkungen:
- Response per year =
Response per generation / L
=
S x h2 / L or h2 x i x SD / L
- To maximise response per year we need:
- High heritability
- High selection intensity
- High phenotypic variation
- Low generation interval
- Generation Interval
Anmerkungen:
- Humans = 20-30
Sheep = 3-4
Pig = 1-3
Cattle = 4-6
- The average age of the parents when the progeny are born.
- Calculated from the average of the parents in a hed.
Anmerkungen:
- For example if we had cows 2, 3, 4 and 5 at calving it would be:
2 + 3 + 4 +5 / 4 = 3.5.
- The progeny get half their genes from their sire and half from their dam.
- Average age of cows + average age of bulls / 2