In genetic genealogy, a unique-event polymorphism (UEP) is a genetic marker that corresponds to a mutation that is likely to occur so infrequently that it is believed overwhelmingly probable that all the individuals who share the marker, worldwide, will have inherited it from the same common ancestor, and the same single mutation event.
[citation needed] But UEPs may also be large-scale additions, such as the YAP insertion that defines Y-DNA haplogroup DE, inversions or deletions.
[2] The discovery and widespread testing of new UEPs has been the key to the increasingly detailed analysis of the patrilineal and matrilineal ancestry of mankind into more distinct family trees of Y-DNA and mtDNA haplogroups.
[3] The properties of UEPs can be contrasted with those of short tandem repeat sequences (STRs), the other main type of genetic variation used in genealogical DNA testing.
This is the case of the occurrence of a large-scale deletion event, which caused a sudden big change in the Y-STR repeat number, rather than the usual single increment or decrement.