Vinny the Mackem
Winger
The scientist who gave the talk was quite technical (as he had to be) but he said that the bottom line is that the try to "conclusively" prove something from a very limited analysis. Basically, the amount of DNA tested is like picking out individual words from a single page in a book from one shelf in a library the size of the British Library.Until insurance, mortgage companies or workplaces get hold of the information and use that to descriminate against certain hereditary conditions.
Or about a hundred other reasons but let's start with that one
This, you can know for a fact if it's not a suspects DNA. But you can never conclusively prove it is a suspects. Of course it can be supported with other evidence