New mathematical model gives courts valuable DNA tool

When police and prosecutors investigate cases of sexual assault of women, traces of DNA left by the perpetrator may be an important clue. But in cases such as rape, there is often so much DNA from the victim that it may be difficult to isolate a sample that came from the perpetrator with certainty.

So forensic geneticists in this situation often focus on the DNA found on Y chromosomes because this can only come from a male.

Unlike an ordinary DNA profile that is normally created from all 22 autosome pairs (the pairs that are not sex chromosomes), there has been a great deal of uncertainty about how much evidential value to give to a Y profile match between the profile from the crime scene and a suspect. This is due to the fact that the Y chromosome in males who have common paternal ancestry will often lack any distinct variation. So Mikkel Meyer Andersen, along with colleague David Balding of the University of Melbourne in Australia, produced a computer model that can help understand how many men in a population have the same profile. This can have a decisive impact on the judgment in a criminal case.

"With this new method, we can help the court by narrowing down the number of possible perpetrators from perhaps several hundred thousand to under a hundred -- and often even far fewer," explains Mikkel Meyer Andersen.

Knowledge about the uncertainty

The computer model produced by the two researchers can simulate populations with a wide range of variables such as the average number of baby boys, the size of population growth, etc. Along with existing knowledge on how Y chromosomes are inherited and how often they mutate, the model can provide a fairly accurate picture of Y profiles' uniqueness. This gives courts a valuable tool for assessing how much weight to give a DNA match from the Y chromosome in a criminal case.

Unfortunately, thus far, a profile that is only based on DNA from Y chromosomes has been more difficult to interpret than a "regular" DNA profile that as a rule is created from all 22 autosome pairs (the pairs that are not sex chromosomes).

Apart from some individual mutations, Y chromosomes are inherited directly from father to son without any special variation. This means that men who are related to each other through common paternal ancestry have a high probability of having the same DNA on the Y chromosome, which means they cannot be distinguished from each other in this way.

All in the family

Deciding the evidential value of a DNA profile relies on the concept of random match probability, i.e., how likely it is that a random man on the street has the same DNA profile as that found at the crime scene, and thus how much weight to give the profile in terms of meeting the burden of proof. With normal DNA profiles, the number is negligible, but with Y profiles, it is harder to figure out.

"Brothers, fathers, grandfathers and male cousins on the father's side will often have the same Y profile. This can extend to up to fifty father-son pairs through so there may well be match between men who don't know they are related to each other, explains Mikkel Meyer Andersen. -However, they are much more closely related through their fathers than two random men," he says.

Mikkel Meyer Andersen also emphasizes that DNA matching is not only useful as a method to identify the perpetrator of a crime. It can also be an effective way to rule out a suspect.

"If you have a suspect that does not match on the Y-DNA, you can often exclude him as the perpetrator," says Mikkel Meyer Andersen. -- And that's a really, really good thing