Quantitative and qualitative traits
Only very few of the traits that are exhibited by organisms are qualitative, i.e. have the nature of being “all or nothing”. The presence or absence of a certain mark on the surface of the body could be an example of such a trait. The size, intensity or colour of the mark, similarly to the size of the body and its parts, however, are quantitative traits, i.e. we can measure the intensity, size or, in relation to behaviour, the probability or frequency of its occurrence. While qualitative traits can be determined by the presence or absence of certain (frequently dysfunctional) alleles of one gene, quantitative traits are usually dependent on, or are at least affected by, a large number of genes located in various parts of the genome.