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We can’t read an animal’s genes or a plant’s genes by looking at them. So to get the best chance of seeing their potential, we have to raise them under similar conditions and hope that this will give us the chance to see that potential expressed
Animals or plants raised under different conditions to their peers will not have the same advantages and disadvantages and thus will develop differently
The old saying, compare apples with apples is at play here. To be sure that you are assessing seedstock evenly, you need to make the conditions as even as possible so that each animal or plant has the capacity to reach full expression of its genetic potential.
If a plant is disadvantaged by being on poorer soil than others you are comparing it with, it will not be an even assessment. So ensure that growing conditions are as close to identical as you can achieve.
Similarly for animals, the ages and conditions need to be as close as possible. This may mean ensuring that the lambing or calving season is short so all animals are assessed at a similar age. And they need to be raised on the same pasture etc.