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Under the infinitesimal model we assume that selection has no influence of the size of the genetic variance from one generation to the next. Therefore we assume that var(Asire) = var(Adam) = var(A). This means that var(MS) must be equal to ½ var(A). This is quite a large component! That explains why they say that breeding is genetic gambling… Fortunately there are tools to decrease the chance factor in breeding. More about that in the chapter about ranking the animals.