Solving a rape crime oftentimes has a textbook solution. We would expect DNA from the rapist to be found on the victim. Usually inside the vagina if no condom was used, or even under the nails if she fought against it. The DNA is collected with what is commonly called a rape kit. Essentially, it is a box of swabs with a pre-paid shipping envelope thrown in that is addressed to the local forensic lab. Later, the police would find and arrest the suspect, collects his DNA and compares it to that obtained from the rape kit.
So far so good.
But what happens if the suspect says: “I have an identical twin and he is the one who did it, not me!”. The investigator checks the claim and finds that it is true; he does have an identical twin. Furthermore, neither of the twins have an alibi.
In the past, each of the twins' lawyer would argue that it is the other twin who did it. The jury will be unable to meet the standard of beyond a reasonable doubt and both would be let go.
This is exactly what happened 16 years ago in downtown Grand Rapids, Michigan when a student was raped by a twin. Traditional DNA tests have identified a match to either of the twins. They each had a history of sexual assault and neither had an alibi. Today, they are both free, having denied the attack.
Eurofins Scientific based in Ebersberg Germany, has developed a DNA test to distinguish between identical twins but the cost is $160,000. The police department of Grand Rapids would like to re-open the case, but does not have the funding for it and is considering applying for a federal grant.
How does the test works?
Well, because of mutations. When embryonic cells divide, they replicate their DNA almost perfectly, but not quite. Mutations occur. In fact, about one DNA site per 10 million will be different between identical twins. The company performs a full genome scan which detects all of the these mutations. As a result, the test matches to only one of the twins.
In comparison, traditional DNA testing looks at about 16 to 24 genetic sites so the chances of finding a mutation are slim to none and the price is about $100 to $150.