Selection

1. The mean performance of the progeny generation minus the mean performance of the parent generation.
2. Depends on many things:
   1. Heritability (h2)
   2. The superiority of the selected animals

      Nota:

      • Measured by the selection differential (s). S = mean of selected animals - mean of the population.
3. Calculation = R = h2 x s.

1. Selection differential is determined by two things:
   1. The proportion selected (p)
   2. The standard deviation (SD)

      Nota:

      • The measure of the spread of the population.
      1. As the standard deviation increases, the selection differential increases.
2. Often we want to predict response of a population to a selection from a knowlege of the proportion of animals selected (p)
   1. S = i x SD
3. Selection intensity (i) is the number of standard deviations that the animals are superior to the mean.

   Nota:

   • Value of i therefore decreases as p increases.

1. A low proportion of selected animals.
   1. So that selection intensity is high.
   2. So that selection differential (s) is high
2. A high phenotypic variation

   Nota:

   • So that the animals we select are a lot better than the average.
3. A high heritability

   Nota:

   • So the superiority of the parents is passed on to the progeny.

1. R = S x h2
2. R = i x h2 x SD
3. Often it is more useful to predict the response per year
   1. To do this we need to know the generation interval (L) in years

      Nota:

      • Response per year =  Response per generation / L = S x h2 / L or h2 x i x SD / L
4. To maximise response per year we need:
   1. High heritability
   2. High selection intensity
   3. High phenotypic variation
   4. Low generation interval

Nota:

• Humans = 20-30 Sheep = 3-4 Pig = 1-3 Cattle = 4-6

1. The average age of the parents when the progeny are born.
2. Calculated from the average of the parents in a hed.

   Nota:

   • For example if we had cows 2, 3, 4 and 5 at calving it would be: 2 + 3 + 4 +5 / 4 = 3.5.
3. The progeny get half their genes from their sire and half from their dam.
   1. Average age of cows + average age of bulls / 2