Associating the Incest and Paternity Index to discriminate sexual aggression among close blood relatives

Abstract

Sexual violence in Brazil, as well as being a heinous crime, is a public health issue. In the majority of cases of sexually abused children, close relatives are the most common aggressors. When a crime of a sexual nature results in pregnancy, a paternity test is the most reliable form of evidence. The purpose of this test is to verify the sharing of alleles between the child and alleged father, in order to assign probabilistic terms related to the transmission of alleles from the suspect in relation to an unrelated man in population—this is called the Paternity Index. However, in cases where the alleged father has a biological relationship with the victim, some mathematical parameters should be considered, in order to avoid overestimation of the values assigned to the indices of paternity. It becomes necessary to distinguish alleles identical by descent from those identical by state. From a comparative analysis of 3 incestuous cases, calculations of the Paternity Index were performed in the usual way, both considering the suspect as being unrelated to the victim, and considering the suspect as being related to the victim, and this calculation was named the Incest Index. The Combined Paternity Index was higher than the combined ratio of Incest by up to 103 times, showing that the usual analysis may overestimate the results in the exams of genetic link. Thus, the use of this mathematical methodology is proposed for both incestuous scenarios, in order to avoid issuing overrated or erroneous statistical reports of incest.