Demonstrating that penetration has occurred when no semen is found: the whys and wherefores of Y-STRs

Human chromosomes - Y-STR tests areas of the Y chromosome

Evidence to demonstrate that penile or digital penetration of the vagina has taken place can be difficult to find in some cases.  Traditional DNA profiling techniques can’t really help as the few male cells potentially deposited are overwhelmed by the large amount of female DNA present in the vaginal cells.  Y-STR typing is a form of DNA analysis that targets just the DNA present on the male Y chromosome allowing production of a male Y-STR profile even in the presence of seemingly overwhelming amounts of female DNA.  It has been around for some time, but the use of this technique in forensic applications is relatively low.  The sensitivity of Y-STR typing in a male/female mixed sample is reported to be 1 male cell in 2000 whereas traditional DNA profiling has a 1 in 50 limit for male DNA detection.   This makes Y-STR analysis potentially useful in cases where there has been penile or digital penetration but without the deposition of semen, or where the semen is sperm-free (due to the male having been vasectomised or some medical conditions).

A recent paper studied swabs from such cases and found that in 30% of these cases a Y-STR profile was generated with some information and in 21% a high level of information was provided.  The data presented provides support for the use of Y-STR profiling to provide scientific evidence to investigate whether alleged sexual activity had occurred as well as to obtain probative evidence in spermatozoa negative penetration cases.  The findings backed up data from a previous Australian study.  The new study also indicated that Y-STR profiles could be obtained from samples taken up to 48 hours after an alleged incident in penile penetration cases, and it is reasonable to expect a similar persistence time in digital penetration cases.  This has resulted in changes to the Faculty of Forensic and Legal Medicine’s sampling guidelines (previously a limit of 12 hours was recommended).

So why isn’t this technique being used more?  Cost is clearly an issue.  The test isn’t routinely used in the UK and as it is more expensive that standard DNA profiling tests.  And whilst the results can provide information of evidential significance where a suspect is already identified, Y-STR profiling results are not searchable against the National DNA Database and therefore cannot be used in the investigative stages of a case other than to indicate that male DNA is present and to link cases.  The statistical evaluation of the results is also limited to comparing a Y-STR profile against a worldwide database which holds approximately 136,000 profiles (last update November 2014) and reporting whether the profile has been seen before, and if so how many times.

However Y-STR profiling can provide information to assist where there is little or no hope of standard DNA profiling techniques progressing the case.  It could be considered for vaginal, anal or oral swabs where standard DNA profiling techniques have given only a female profile.  From a defence perspective, Y-STR profiling offers the potential for the exclusion of the defendant if a non-matching profile is obtained. However the absence of a Y-STR profile could be because insufficient male DNA was present and would not necessarily mean that penetration had not occurred.  The recent study was small and more data would be helpful, but it seems that application of the Y-STR profiling option should be considered more frequently for sexual offence cases.