Selection - created from Mind Map

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: Calculation = R = h2 x s. Heritability (h2) The superiority of the selected animals

Selection Intensity Selection differential is determined by two things: Often we want to predict response of a population to a selection from a knowlege of the proportion of animals selected (p) Selection intensity (i) is the number of standard deviations that the animals are superior to the mean. The proportion selected (p) The standard deviation (SD) As the standard deviation increases, the selection differential increases. S = i x SD

Increasing the rate of genetic gain A low proportion of selected animals. A high phenotypic variation A high heritability So that selection intensity is high. So that selection differential (s) is high

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 maximise response per year we need: To do this we need to know the generation interval (L) in years High heritability High selection intensity High phenotypic variation Low generation interval

Generation Interval The average age of the parents when the progeny are born. Calculated from the average of the parents in a hed. The progeny get half their genes from their sire and half from their dam. Average age of cows + average age of bulls / 2

Measured by the selection differential (s). S = mean of selected animals - mean of the population.

The measure of the spread of the population.

Value of i therefore decreases as p increases.

So that the animals we select are a lot better than the average.

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

Response per year =  Response per generation / L = S x h2 / L or h2 x i x SD / L

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

For example if we had cows 2, 3, 4 and 5 at calving it would be: 2 + 3 + 4 +5 / 4 = 3.5.

Selection

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