Forensic Services

Y-STR analysis

Conventional forensic DNA analysis targets specific areas of chromosomes within the cell nucleus that show great variation between different individuals. These specific areas contain a short, repeating pattern of DNA and are called Short Tandem Repeats (STRs). There is no discrimination between DNA from males and females as everyone contains these repeating patterns of DNA.

Frequently DNA profiles showing more than one individual ("mixtures") are encountered in forensic casework and difficulties can occur when either one contributor is present to a very much greater extent than another contributor with the risk that the minor contributor's DNA is swamped out by the major contributor, or that there is masking of one contributor's DNA profile by another contributor simply because the two individuals happen to share the same DNA components in their profiles.

Y-STR analysis is a technique that can be used to assist in such circumstances because it only targets DNA on the Y chromosome (i.e. is specific to male individuals). Even if there is a large amount of DNA from a female individual in the mixture, Y-STR analysis has the ability to analyse only the male DNA that is present.

Y-STR analysis is generally used as a secondary technique once conventional DNA analysis has been attempted and is often used to assist in the interpretation of complex, mixed DNA profiles. Examples of where Y-STR analysis may be helpful are:

• Interrogation of mixtures where there is a very large amount of female DNA present to determine more information about any male contributor.
• As a means of assisting in determining how many males may have contributed to a mixture.
• As a technique to assist in determining whether two male individuals might be related by their paternal line.

A typical casework example where Y-STR analysis might be considered is in a case of sexual assault where intimate samples from a female victim have been taken but only traces of sperm detected. In such a case, techniques are employed to try and separate any sperm from epithelial material from the victim but this can still often result in female DNA being detected in the sperm fraction as it is not possible to ensure that all epithelial cells are removed. There may be some indication in the resulting DNA profile that there is a male contributor present. Y-STR analysis can be used to generate more information about the DNA of the male contributor and can be particularly useful when there is a suspect to compare against.

Determining how many people may have contributed to a mixed DNA profile can often be important in a case and Y-STR analysis can be used to assist with resolving this issue. There may be a DNA profile obtained from an item in a case and it may be necessary to try and determine how many males may have contributed to it. Addressing this issue using conventional DNA analysis may be problematic if there is female DNA present. Y-STR analysis can be used to interrogate a mixed DNA profile and the results obtained can significantly assist in the interpretation of the mixed DNA profile generated using conventional DNA analysis.

Y-STRs can also be a very useful technique is in relationship analysis. An individual's Y-STR profile is inherited down the paternal line (i.e. from father to son) so that individuals within the same family will have the same Y-STR profile. In cases of disputed relationships, Y-STR analysis can be a very powerful technique.

There is no National DNA Database of Y-STRs available to search against for possible suspects and this technique is most suited to those cases where there are specific males to compare against.