Using Y-STR DNA Profiling in Sexual Offence Cases

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

Use of Y-STR DNA profiling techniques has noticeably increased in the last year or two. Scientific evidence to demonstrate that penile or digital penetration has taken place during a sexual offence is often difficult to obtain.  Despite the sensitivity of standard ‘autosomal’ DNA profiling techniques, such as DNA17,  the few male cells potentially deposited can be overwhelmed by the large amount of female DNA present on intimate swabs taken from a complainant. However Y-STR profiling is another highly sensitive form of DNA analysis which 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 is also useful when there are multiple male contributors to a sample and for paternity relationship testing since Y-STR profiles are inherited from father to son and all male descendants have essentially the same Y-STR profile

The Y23 STR technique is most commonly used in forensic applications in the UK.  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.


Statistical evaluation of Y-STR Results

The statistical evaluation of the results is currently limited to comparing a Y-STR profile against a worldwide database (which currently holds approximately 90 000 Y23 profiles) and reporting whether the profile has been seen before, and if so how many times. The size of the database can be an issue when a specific sub population needs to be considered.

The Forensic Science Regulator has recently issued guidance on Y-STR profiling analysis which includes guidance for statistical evaluation.


Sensitivity of the technique

The sensitivity of Y-STR typing in a male/female mixed sample makes it very useful in cases where there has been penile or digital penetration without the deposition of semen, or where the semen is sperm-free (due to the male having been vasectomised or some medical conditions).  Studies have shown in 50% of digital penetration cases a Y-STR profile was generated with some information and in 24% a high level of information was provided.  Y-STR profiles may be obtained from samples taken up to 48 hours after an alleged incident in penile penetration cases. 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 penetration cases where no spermatozoa have been found.

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 intimate or oral swabs where standard DNA profiling techniques have given only a female profile.


Exclusions using Y-STR profiling

From a defence perspective, Y-STR profiling offers the potential for the exclusion of the defendant if a non-matching profile is obtained. Whilst a Y-STR profile can offer substan­tial power to discriminate between individuals, this is less than with autosomal STR profiles. It should also be noted that 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.

Careful expert consideration would be necessary regarding the potential significance of the absence of a profile in the case context, in particular the time interval between the alleged offence and swabs being recovered.

Overall, Y-STR profiling is a useful tool in the investigation of sexual offences and it is likely that it’s use will continue to grow.  Forensic Context’s experts can review Y-STR profiling findings (and findings from other DNA profiling techniques) and, where appropriate, advise if testing may be of use – contact us if you need to know more.