Psychopath minds, give it up!

Even the vasectomy or lack of sperm cells won't save your butt from jail now ...

A new DNA analysis technique will allow individual detection even in semen samples with no sperm cells, in cases of sexual assault. In fact, the main problem of forensics trying to fingerprint the DNA of rapists is that the quantity of male DNA in samples taken from the woman is often extremely low compared to the amount of her DNA. "The female DNA profile is so strong in the analyzed sample that the male DNA is swamped," says Andy Hopwood of the Forensic Science Service in Birmingham, UK.

The conventional methods of DNA replication (which greatly increase DNA amounts) are ineffective, as the female DNA would be replicated also. "We're looking for ways to isolate out the male component," Hopwood says.

One method is preferential lysis, in which enzymes are employed to dissolve the membranes of ordinary cells, leaving intact only the more robust sperm cells. But in many cases, the rapist may not leave any sperm, because of a disease or a vasectomy. "The semen may have come from a vasectomised male in around 10 to 15 % of the cases we deal with. We got to thinking: 'What else is there in this sample that we could look for?'"said Hopwood.

Semen can harbor, beside sperm cells, immune and epithelial cells. But these cell types have been impossible to differentiate from female cells present in a sample.

Now, this has been solved by the British team by mixing a technique called laser microdissection (LMD), which extracts single cells from a microscope slide, with fluorescence in-situ hybridization (FISH), which marks by fluorescence chromosomes bearing a particular DNA sequence.

The fluorescent tags were specifically developed for the repetitive DNA areas on the X and Y chromosomes; these tags turned the X chromosomes glow red and the Y chromosomes green, thus XY male cells were easy to distinguish from XX female cells.

LMD was employed to cut out male cells from microscopic slides.

Their DNA was replicated and the researchers could compare it with a database of DNA profiles in order to search for the criminal.

The new technique delivered full male DNA profiles from vaginal samples, even those lacking sperm cells, taken up to 24 hours after sexual intercourse. "The FSS gets around 90 cases a year in which the new technique could be useful", says Keith Elliott, one of Hopwood's team. "These are really difficult cases where you have a sample that's semen positive, but sperm negative," he said.

This technique already detected in January 2007 a suspected rapist, whose DNA profile was found to match the male DNA recovered from the victim.