How Accurate is DNA Testing? Really?

https://daily.jstor.org/forensic-dna-evidence-can-lead-wrongful-convictions/

DNA testing is not without controversy and unforeseeable consequences. The article shown above highlights some of the serious issues facing the forensic use of DNA evidence. With the popularity of genealogical DNA testing, it is important to understand the differences and similarities between forensic DNA testing and that done by the large genealogically motivated testing companies.

To begin to understand those differences and the inherent limitations of DNA testing, here is a quote from the above JSTOR Daily article:

DNA (deoxyribonucleic acid) is a code that programs how we will develop, grow, and function. Humans are thought to have DNA that is 99.9% identical, but the remaining 0.1% makes us individuals, marking us out as unique. The fact that humans and chimpanzees have just a 1% difference in their DNA further highlights how meaningful a small difference can be. Generally, the more closely related we are to someone, the more similar our DNA will be to theirs.

Continuing with a discussion of the limits of DNA testing in a criminal investigation, the article states:

Realistically, then, DNA profiles should only be thought of as being likely to have come from a specific individual. Statistical approaches such as “match probability,” which is based on comparisons between crime scene DNA and a hypothetical “random” person, often are misunderstood. A more rigorous statistical approach is likelihood ratio, which directly compares two hypotheses: the likelihood of the DNA coming from the suspect vs. the likelihood of the DNA coming from someone else. If the likelihood ratio is less than one, the defense position (the DNA is not the suspect’s) is better supported; if it is greater than one, there is more support for the prosecution case. Still, the ratio at most provides scientific support for a theory, not a yes-or-no answer.

One issue that has been ignorantly raised in several online articles is that the genealogical DNA testing being done, could ultimately be used in a court of law for a criminal prosecution. I wrote about this issue previously in two posts entitled, "Is genealogically submitted DNA discoverable in a criminal investigation?" and "A Little More About DNA and Criminal Investigations."

In reality, the standards for criminal justice in the United States would be extremely unlikely to admit genealogical data as evidence in a criminal trial. Here is one of the most limiting of the standards for DNA Evidence from the American Bar Association:

Standard 2.4 Collecting DNA samples from Persons in a group by consent 

A law enforcement officer should be permitted to obtain a DNA sample from a person by consent, except that: 

(a) consent should not be sought from persons based primarily upon their membership in a constitutionally protected class;
(b) consent should not be sought from a large number of persons based on grounds other than individualized suspicion that each committed the crime under investigation unless seeking such consent has been authorized by the head of a law enforcement agency or the chief prosecutor in that jurisdiction; and
(c) when consent is sought as provided in subdivision (b) of this standard, each person should be informed of the reason for the request and of the right to refuse it, and the consent should be obtained in writing.

Note that when DNA testing is directed at a group, those tested have a right to refuse testing based on the proposed forensic use. So, if you have your DNA test done by one of the genealogy companies, the results are not useable in court unless your consent was obtained in writing prior to the test. Standard 3.1 goes on to state the standards applied to forensic DNA testing laboratories.

Standard 3.1 Testing laboratories 

(a) A laboratory testing DNA evidence should:
(i) be accredited every two years under rigorous accreditation standards by a nonprofit professional association actively involved in forensic science and nationally recognized;
(ii) be governed by written policies and procedures, including protocols for testing and interpreting test results, and permit deviation from protocols only by a technical leader or other appropriate supervisor;
(iii) use quality assurance and quality control procedures, including audits, proficiency testing, and corrective action protocols, that are consistent with generally accepted practices and in writing;
(iv) use protocols for testing and interpreting DNA evidence that are scientifically validated through studies that are described in writing;
(v) follow procedures designed to minimize bias when interpreting test results;
(vi) timely report credible evidence of laboratory misconduct or serious negligence to the accrediting body; and
(vii) make available to the public the written material required by this standard.
(b) A laboratory testing DNA evidence should make available to the prosecution the information and material that the prosecutor must disclose to the defense pursuant to Standard 4.1, and to defense counsel the information and material that the defense must disclose to the prosecutor pursuant to that standard.
(c) When an accrediting body receives notice of credible evidence of laboratory misconduct or serious negligence concerning DNA evidence at the testing laboratory, either as provided in subdivision (a) (vi) of this standard or through other means, it should audit laboratory procedures and cases that may have been affected by the misconduct or serious negligence and issue a written report.

I could probably give many more examples of the limitations imposed on forensic DNA testing but here are a few links to get you started if you are interested:

• Putting crime scene DNA analysis on trial
• Evolution of DNA Evidence for Crime Solving - A Judicial and Legislative History
• Using DNA to Solve Crimes

For genealogists, the issue is the accuracy of the DNA data supplied by any testing company. Coupled with the use of online family trees, DNA testing can be quite accurate for near realatives of no less than three or four levels of separation, i.e. 2nd or perhaps 3rd cousins. Every level of "removal" or separation decreases the accuracy and thereby the reliability of the results. However, when reliable, traditional genealogical research is coupled with reliable DNA testing, the results may be extended further. 

Presently, testing done by different genealogically oriented testing companies will differ because of the fact that their testing procedures and databases are, in some cases, significantly different. These differences are presently unresolvable. 

From a legal standpoint, as a retired trial attorney, I would not feel it possible to use genealogically obtained DNA testing for any type of court proceeding.