at the institute of forensic medicine in the swiss city of bonn around 4 and 5 of the analyses that geneticist silvio but securities out in her lab involves samples left by casual contact so-called touch d.n.a. that a contact. with touch d.n.a. you can get a range of results from a good profile to an unusable one but we've reached the point where we can create a genetic profile from just 10 to 20 cells. since the early days in the development of d.n.a. profiling 35 years ago the amounts of genetic material needed to get a result have fallen steadily. every cell nucleus in the body contains a person's entire genome packed into 23 pairs of chromosomes. half of your d.n.a. comes from your mother and the other half is from your father forensic investigators compare stretches of d.n.a. that can vary widely from individual to individual segments that don't encode for genes the trick is to 1st make millions of copies of the d.n.a. under examination that's what enables can even tiny traces of d.n.a. to be analyzed. at a crime scene there may be thousands of touch d.n.a. samples. like on the wine glass