Practical application further: If the questioned sample is contaminated with the defendants known sample at the lab, every time you test the questioned sample, it will show the defendants profile – even if he was in Topeka at the time of the crime.
Finally, when they want your clients sample – and they will... The American Bar Association recently released the ABA Criminal Justice Standards on DNA Evidence.
2.2 (b)(i)(C) (i) If the person from whom the sample is to be collected is suspected of committing a crime, an order should issue only upon an application demonstrating: (A) probable cause that a serious crime has been committed, and (B) if the sample is to be collected from a person is: (1) a sample collected by a physically noninvasive means, reasonable suspicion that the person committed the crime charged; and (2) a sample collected by physically invasive means, probable cause that the person committed the crime charged; and (C)that the sample will assist in determining whether the person committed the crime.
Controls! The idea is that if there is a contamination, it will show up in the control. Using the previous example, if the defendants profile contaminates the questioned, it ideally will also contaminate the control.
Blind Control The Blind Control also has a right answer. However, this right answer is not immediately known. It is the DNA profile of one of the scientists in the lab. When they do DNA testing, they check out a DNA profile from the lab supervisor, who keeps track of whos sample was given. The scientist must compare the answer known to the supervisor with the answer they got during testing to confirm the test was done correctly. Failure to match SHOULD result in a retest.
Controls generally The idea behind controls, of course, is to have a way to see if you contaminated your sample. Controls arent the exclusive or only way to tell, however.
Other ways to detect contamination Known samples, such as from a victim or a suspect, should generally contain no more than two alleles (remember that word?) at each locus.
A few words about alleles in single source samples: Heterozygote In the first blue locus, the individual has a 16 and a 17 (one from mom and one from dad, remember?). Homozygote At the next locus, we only see a 15. Freak of nature?? No!! This person got a 15 from mom AND a 15 from dad.