Sometimes two traits are correlated in an unfavourable way. For example, milk production in cows (but also in other species) is negatively correlated with fertility. High producing animals often have more difficulties in becoming pregnant again. However, there are animals that combine high production with good fertility. If those are selected for breeding, milk production can be improved without decreasing fertility. The fact that two traits are undesirably correlated does not automatically mean that they cannot both be improved in the population. Unless the correlation between both traits is 1 or -1, there will always be some animals that have the desired genotype for both traits. Obviously, the genetic gain for each of the traits will be lower than if their correlation would have been desirable. Because some of the best animals for that trait will not be very good in the other and those should not be selected. So the selection intensity decreases.
As we have seen in the chapter on ranking the animals, selection on multiple traits simultaneously can be applied for traits that are both in the breeding goal. In chapter 2 on defining the production system and the breeding goal, you have learned how traits can be weighed into an index (single value). The weights can be the economic value of the trait: how much profit can be made from 1 trait unit genetic improvement? But sometimes economy is not the best way to weigh the traits under selection and desired gains would be a better option. For example, if changes in the market or legislation are expected in the future, then selection weights can be defined based on how fast traits need to change to meet these expectations. Similarly, sometimes societal pressure is large to change the result of the breeding program. For example, broilers should grow less fast, or certain dog breeds should have be able to breath freely instead of having certain looks, or calves should be born without caesarean section. Even though from direct economic perspective selection weight should be small, the society demands otherwise. Apart from serving the consumers, so your market, taking the societal demand serious is also wise to maintain a good reputation. And a good reputation is very important to keep and increase market share.
Thus:
Even if a genetic correlation between two traits is undesirable, selection for both traits is still possible. Though genetic response will be affected by a lower selection intensity |