Three different types of traits can be observed in the phenotype. Respectively the qualitative, the quantitative and the threshold traits.
A qualitative trait is expressed qualitatively, which means that the phenotype falls into different categories. These categories do not necessarily have a certain order. The pattern of inheritance for a qualitative trait is typically monogenetic, which means that the trait is only influenced by a single gene. Inherited diseases caused by single mutations are good examples of qualitative traits. Another is blood type. The environment has very little influence on the phenotype of these traits.
A quantitative trait shows continued variation. This is because the trait is the sum of several small effects caused by the gene. An example of this is an animal’s metabolism, which is under the influence of many different genes. The final products of the metabolism, as for instance milk yield or growth rate, are good examples of quantitative traits. If several small gene effects are present, the phenotype values for a population will typically have a normal distribution.
In some cases the phenotype values are not distributed normally, even though the trait has a polygenetic inheritance. Traits, which only show a few classes, are called threshold traits.
Consider a continued genotypic distribution. When a threshold is crossed, the phenotype will jump onto another level. It passes from one category to another, or from one phenotype to another. An example of a threshold trait is mastitis in dairy cows. The inheritance is polygenetic, but the only thing that can be recorded is whether a cow contains the disease or not – not at which point on the continued normal distribution it should be recorded.