There is a greater likeness between relatives for a highly heritable trait than for a lowly heritable trait. For example, a group of sibs is more likely to be similar for backfat thickness than for number born in their first litter. Genes, as compared to management, have a greater effect on performance for a highly heritable than for a lowly heritable trait. Differences among animals can have both genetic and environmental causes. The heritability of a trait is the ratio of variation due to genetic causes to the variation due to genetic plus plus environmental causes.
Producers differ in their ability to control the production environment.
Poor control of the production environment can disguise genetic differences among animals. For that reason heritabilities can differ between herds. Genetic Correlation Extremes in performance for certain traits tend to be associated. An animal that is shorter than average at a given age is also likely to be lighter than average. These associations are known as phenotypic correlations.