An example would be average daily gain. If average daily gain records from weaning to market weight of every pig in a group, were plotted the points would form a continuous line. An exception of this are some of the reproductive traits. These factors make it difficult to identify individuals that have superior genotypes for quantitative traits. Since qualitative traits are usually not affected by the environment, the phenotype of a qualitative trait is a good indicator of the genotype.
Environmental effects do influence the phenotypic expression of a quantitative trait.
An individual with an inferior genotype can rank higher phenotypically than individuals with superior genotypes because of favorable environmental effects. Usually an individual's phenotype, compared to the herd average, is a good indicator or estimate of his genotype, or genotypic value. Ranking individuals by their EBV's for specific traits is a means of identifying individuals that are expected to have superior genotypes. Genetic evaluation programs often estimate the transmitting ability of an individual. The estimated transmitting ability is equal to one-half of an individual's estimated breeding value.