DNA profiling can be performed on any biological substance. Analysis must be performed on a stain to determine that it is blood, since the appearance of blood varies greatly depending on the age, weather, and other factors. If the sample is blood, the species origin must then be determined. This can be done, but, in most cases, only to the level of the taxonomic family used in animal classification. If the blood is human, further classification will be achieved by isoenzyme and DNA analysis.
Different individuals have different types due to the genes they received from their parents.
Each type has a certain percentage of occurrence in the population. By multiplying together the frequency of occurrence of each type found, the number of people who could have been the source of that bloodstain can be determined. If one blood type is different, a person is eliminated as the possible source. DNA profiling of bloodstains is now regularly performed when necessary. Additional information can be obtained from the size, shape, and distribution of blood spatters at the scene.