March 04 2025
Y-STR DNA profiling follows the same basic principles as conventional DNA profiling with the only notable difference being that all of the DNA areas under investigation are confined to the male-specific Y sex chromosome. The Y chromosome is passed down from father to son and although mutations do occur from time to time, closely related males in a paternal lineage are likely to share the same Y-STR profile. In effect what this means is that a male’s father, son, brother, paternal uncle and paternal grandfather etc. are likely to share the same Y-STR profile. This is in contrast to autosomal STRs (analysed using conventional DNA profiling techniques) which are inherited independently of each other thus have a high degree of variation even in those who are biologically related.
One of the main applications of Y-STR profiling is in sexual assault cases where no semen has been detected, or where there is no expectation of semen being present (for example where digital penetration is alleged). In such cases, it can be difficult to isolate and analyse the male DNA using traditional methods because of the very high level of female DNA present. As female DNA does not produce results with Y-STR DNA profiling, the swamping effect caused by the female DNA is removed. Using this specialist DNA profiling technique it is possible to specifically target and analyse the male DNA, even if it is present in small amounts.
Y-STR profiling can also be used in cases where there is a need to identify paternal relationships, for example to identify missing persons or to establish paternity in child support cases (assuming alternative fathers are not related to the person under test).
Although comparison of a Y-STR profile with other results is undertaken in the same way as with conventional analysis, assessment of the significance of a ‘match’ is significantly more complicated because of the way in which the Y chromosome is inherited. In an effort to place evidential significance on matching profiles convention in the UK is to consult a worldwide database containing several thousand Y-STR profiles (found at www.yhrd.org) to gain some indication of how common the profile might be in a generalised population. It is however important to consider the correct reference population and whilst typically the Western European dataset is used, on occasion it can be more appropriate to consider a dataset that corresponds more closely with the suspect’s ethnic appearance.
As with all DNA profiling evidence, it is always crucial that consideration is given to the significance of the Y-STR profiling results in the context of the case as a whole. Limitations in respect of ageing DNA deposits, attributing them to a particular biological material and determining the specific action(s) that resulted in DNA deposition can have a significant impact on the probative value of DNA evidence. These must therefore always be fully appreciated before assigning evidential value to a Y-STR profile match.
Y-STR profiling is a powerful forensic tool, offering unique advantages in cases involving male DNA. It does, however, have limitations which must be appreciated to present this form of evidence in a robust and reliable way.
If you have a case involving Y-STR profiling, or that you think might benefit from Y-STR profiling, any one of our highly experienced forensic biologists would be happy to help. Call us on 0191 332 4999 or email kbc@keithborer.co.uk.
