Breeding is full of equations. The foremost is simply P = G + E. But because genetics are never that simple, the equation is actually
P = µ + GA + GD + GI + Ek + Es
where P = Phenotype, everything about the animal which can be seen or measured
µ = the mean value of the trait in the population (for example, milk yield)
G = genes, where A denotes additive genetic traits, D dominance and I epistasis
E = environment, which may affect either positively, negatively or not at all to a given trait (for example, illnesses and quality of feed). Ek stands for random errors, and Es for systematic errors.
As the equation suggests, what we see in an animal is created and modified by it's genes, the mean of the population and the environment. Genetically identical animals (clones) can be very different if they are raised in different environments. Then again a genetically superior animal can produce well even if the environment is less than desirable.
Comparing animalsIf we're about to select animals and then pair them, we should be able to compare animals. The selection is based on measurable information, which can be gathered and combined. For a single trait each animal can be assigned a calculated breeding value, which changes each time more information is gathered from the animal. The more information is available, the higher the accuracy of the breeding value. The formulae for calculating the breeding value (BV) and the accuracy may vary in each country.
Since the environment cannot be calculated into the value, it must be evaluated separately for systematic or random errors. That's why the formula above has two E's (Ek and Es). Systematic errors affect the BV results systematically, i.e. always the same way. Age, gender and permanent environmental conditions are systematic errors. Young animals are smaller than adults, males cannot produce milk, females produce less offspring than males etc. Systematic errors can be predicted and must be corrected when calculating the BV to make the results for all animals comparable. Random errors are errors which cannot be corrected. They just happen, like misspelling the results or a measurement error. Factors affecting the BV are listed below.
|Strengthen BV or accuracy||Weaken BV or accuracy|