Munich

Forensic scientists say that methods that make use of the Y chromosome will soon provide a valuable new generation of tools for investigating sex crimes and settling paternity suits.

Its male specifity and pronounced variation make the Y chromosome a useful tool for forensic geneticists. This is particularly true in difficult cases of sexual assault where the DNA samples from offenders and victims may have been mixed in a way that makes them difficult to separate for conventional DNA analysis.

At the biannual meeting of the International Society for Forensic Genetics, held in Münster, Germany, late last month, scientists announced plans to enlarge and improve existing reference databases of Y-chromosome distributions in European and US male populations. Data from Asia and South America are also to be added to the database, they said.

Unlike the gender-independent chromosomes normally used by forensic geneticists, the Y chromosome carries mutations over many generations, providing unambiguous male lineages. To differentiate chromosomes, geneticists use haplotypes, which are chunks of DNA containing closely linked genetic variations that were inherited as a unit.

Humboldt University's Institute of Legal Medicine, the origin of the European Y database. Credit: CHARITÉ, HUMBOLDT UNIVERSITY

A standardized European reference database was created in 1995 by geneticists at the Institute of Legal Medicine of the Humboldt University in Berlin. It is based on a set of nine highly variable markers on the Y chromosome, and allows forensic researchers to extrapolate the frequency of a given haplotype in a sufficiently homogeneous population, such as that in much of Europe.

The database currently contains around 7,000 haplotypes from 51 European population samples, gathered from 40 forensic institutes across the continent.

A Y-chromosome haplotype can help to identify an offender or determine fatherhood with a probability of 99.9% or more, says Lutz Roewer, one of the database's founders. The results can be used in court as additional evidence, although they are less conclusive than conventional DNA analysis of markers on non-sex-specific chromosomes.

Y-chromosome analysis could also help to determine, or exclude, the ethnological background of male suspects — raising serious ethical and political questions about its use. As Roewer points out: “Even if forensic geneticists were at a stage to tell the police not to search for a rapist among, for example, the white European population, it would be ethically problematic.”

Apart from forensic applications, scientists believe that a well-maintained and continuously growing Y-chromosome database could be valuable in many areas, such as population history, migration research and evolutionary genetics.

“The Y chromosome is extremely interesting for us because of its variability and high frequency of tolerated mutations,” says Werner Schempp, an anthropologist and human geneticist at the Albert Ludwigs University in Freiburg, Germany.

→ http://www.ystr.org/europe

→ http://www.ystr.org/usa