Y Chromosomal STR Testing and the Future of Rape Investigations

There have been many advances over the past several years in DNA evidence for rape. New ways of collecting, preserving, and analyzing evidence have been developed. Y-Chromosomal STR testing is one of the best new methods in solving rape cases that would have previously been unsolved.

At conception every person receives an X chromosome from their mother and either an X or a Y chromosome from their father to determine their sex. The Y chromosome that is inherited from the father in males is the same chromosome that is shared by the other males on the paternal side of the family (Sibille et al. 212). Mutations to this chromosome happen only after several generations (212). This can be one way to determine paternity of a male child

There are many uses for finding specific types of short tandem repeat (STR) segments that are found on the Y-chromosome for determining a suspect in rape cases (Prinz et al. 1). This technique seeks out only the male DNA that would be found in the female's mouth, vagina, or anus (1). This would automatically exclude any of the DNA from the victim since she would have no X chromosomes leaving only evidence from the suspect.

There are certain criteria that evidence must meet before a lab will run STR analysis on the samples. It can be helpful if they are looking to find out how many suspects there are and if there isn't a large amount of semen available you can use these short segments to represent a larger DNA code (Prinz et al. 1). This can be helpful if only a small amount of sample is available because the victim waited to report the attack, the suspect has low sperm counts, or only a small amount was left behind if the attacker did not fully ejaculate (Betz et al. 126). By being able to confirm that the victim gave the right number of rapists that attacked them they will be able to make the story more credible for court proceedings (Prinz et al. 2).

Since these are new scientific tests that are being used admissibility in court is always an issue (Harmon 1). The Commonwealth of Massachusetts v. Rosier, the State of Nebraska v. Jackson, and Watts v. the State of Mississippi all deal with using this type of evidence in a conviction (1). All three cases allowed STR evidence since it was a form of DNA evidence that was able to repeat itself with solid results (1). The product rule profile frequency was the main factor in determining it use and with repeated testing the results were still the same (1).

By using Y chromosomal STR testing it can detect one male DNA for every 2000 female DNA segments (Sibille et al. 214). The old fashioned DNA testing systems that search for individual autosomal DNA can only detect one male in 50 female cells (214). Sometimes the autosomal DNA tests will also only show the victims DNA type since it is the only one found in abundance (Prinz et al. 2). This gives an advantage for the victims that do not report their rape until after a couple of days, after they have taken a shower, if they were menstruating, or if the attacker used a condom (Sibille et al. 214). If the woman becomes pregnant during the attack, which there is a 5-30% chance of, this can also be used to find out if the rapist is the fetus's father (Shulman et al. 205).

One drawback to this method is that it can not be used in saliva samples (Prinz et al. 2). The saliva quickly dissolves any of the DNA that would have been found in the mouth (2). Bacteria in the anus also break down the proteins that hold the DNA chains together making it harder to get an adequate sample if the victim waits to report the crime (2). This makes vaginal samples the best candidate for STR testing. Another potential drawback would be if this is the only test results that are available. This only would narrow down the suspects to a family where there could be more than one suspect (2).

Many older crimes and new crimes can be solved using the profile that is created by this testing (Harmon 2). Databases such as the CODIS system created by the FBI give a place for DNA profiles from solved and unsolved crimes to be stored to help solve other crimes (Saferstein 389). This gives a place for the DNA of convicted sexual predators to be stored and used as comparison for unsolved rape cases (389).

Y-Chromosomal testing may only be able to identify males, but with males being the largest group of offenders it gives a valuable edge to investigators. By being able to isolate the chromosomes that a man would leave behind at a rape you have a better chance at finding the DNA you need to convict the offender. This test will serve many great purposes in the future and not just to rape investigations.

ReferencesBetz, Andrea, Gerhard Babler, Gabriele Dietl, Xenia Steil, Gerda Weyermann, and Werner Pflug. "DYS STR Analysis with Epithelial Cells in a Rape Case." Forensic Science International. 2001: 118. 126-130. Science Direct. 21 November 2005.

Harmon, Rockne. "Legal Experiences with PCR-STR Typing." AlamedaCounty (Calif.) District Attorneys Office (nd):1-2. JSTOR. 22 Nov 2005.

Prinz, Mechtchild, Asako Ishii, Maribel Sansone, Howard Baum, and Robert Shaler. "Experiences with Using Y Chromosome Specific STRs in Forensic Casework." Department of Forensic Biology, Office of Chief Medical Examiner, New York City, NY (nd) JSTOR. 22 Nov 2005.

Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 8th ed. Upper Saddle River: Pearson Education, Inc., 2004.

Sibille, I., C. Duverneuil, G. Lorin de la Grandmaison, K. Guerrouache, F. Teissie`re, M. Durigon, and P. de Mazancourt. "Y-STR DNA Amplification as Biological Evidence." Forensic Science International 2002: 125. :212-216. Science Direct. 21 Nov 2005.

Shulman, Lee, David Muram, and Patricia Speck. "Counseling Sexual Assault Victims Who Became Pregnant After the Assault." Journal of Interpersonal Violence. 1992. 7(2). 205-210. SAGE Full-Text Collections: Criminology. 23 November 2005.