The association between traits can be caused by environmental factors that affect both traits similarly. For example, availability of food or exposure to disease organisms probably affect both of these traits similarly. Association resulting from environmental factors is referred to as environmental correlation. Association between two traits can also be caused by genes that affect both traits simultaneously. This effect results in a genetic correlation.
Genetic correlations are of greater importance to the animal breeder.
The genetic correlation can be thought of as the correlation between the breeding values of two traits. They indicate the strength of the genetic relationship between two traits. An example of a gene that causes association between two traits is the halothane sensitivity gene, which results in improved meatiness but a decline in carcass quality. A negative genetic correlation indicates that animals with breeding values that are negative for one trait are likely to have breeding values that are positive for the other and vice versa. A positive correlation indicates that breeding values tend to have the same sign.