My friend and recent DNA Interview Series guest, Anjali B. Dooley (, and I have been discussing the intersection of attorneys and experts.  We have decided to do a collaboratorive posting about understanding DNA reports  in criminal cases.  Based on the material I have posted here, she will be offering additional commentary and questions from the attorney perspective (see comments section).

Basic Technologies   



 Autosomal STR DNA testing is the most common method used in criminal cases.  It assesses DNA variation present on non-sex determining chromosomes.  In plain English, it detects both male and female DNA.  Autosomal STR DNA testing is the garden variety method which can produce DNA profiles with sufficient information to provide an impressive discrimination power between individuals.  It is not uncommon for expected population frequencies of DNA profiles to boast numbers such as ‘1 in a quadrillion’ (1 in 1,000,000,000,000,000).

Y STR DNA testing is becoming more prevalent in forensic DNA laboratories.  It exclusively detects male DNA by focusing on areas on the Y chromosome. It is particularly useful for samples that contain trace amounts of male DNA mixed with large concentrations of female DNA which can effectively swamp out the male DNA present during autosomal STR DNA testing.

Examples of situations in which Y STR testing is helpful: 

-male digital penetration of vagina 

The amount of DNA transferred from skin cells and perspiration on the finger is relatively low compared to the cellular-rich environment of the vagina.

 –semen from a vasectomized male in a vagina 

A vast majority of the DNA content in semen is found in the sperm cells.  The semen of vasectomized males will have some DNA, but in very low concentration compared to the DNA present in a vagina.

In situations such as these, Y STR DNA testing is helpful since it ignores the female DNA present and provides information on the male DNA present.  However, there is a price to be paid.  Y STR DNA profiles are nowhere near as discriminating as autosomal STR DNA profiles and are consistent within a paternal lineage.  A Y STR DNA profile said to be consistent with a suspect might be expected to occur with a frequency of 1 in a few thousand individuals and is indistinguishable from the biological father or brother of the suspect.

DNA Discovery-Getting the Full Picture

The forensic DNA reports generated by crime laboratories are seldom user friendly for defense attorneys.  Forensic DNA testing is highly complex and sensitive and requires rigorous interpretation.  It is not clinical-type testing in which the testing provides simple “positive” and “negative” results.

Tip of the Iceberg


When encountering reports from either autosomal STR DNA testing or Y STR DNA testing, perhaps the most important point to bear in mind is that the DNA report itself is only the tip of the iceberg. 

Thorough DNA discovery is necessary to determine whether:  protocols were adhered to, the actual raw results support the conclusions, there are any quality issues with the testing or analysts, sufficient documentation of the process exists, and statistical corrections are appropriate.

Examples of information that may not be included in DNA reports: 

-whether or not contamination or other problems occurred during testing 

-names of all analysts involved in the testing process 

-appearance and size of samples and stains 

-sampling technique used 

-exact levels of DNA detected 

-complete chain of custody information 

-reasoning behind any assumptions made in the interpretations 

-specific interpretation guidelines

An effective DNA discovery request needs to be highly specific and void of boiler plate ‘fluff’ that merely creates busy work for the laboratory. Essential components of a DNA discovery request include:  laboratory bench notes, CV and proficiency test results for analysts, QC reports (e.g. contamination instances, corrective actions, and unexpected results), raw electronic STR DNA data, protocols for all analyses, interpretation guidelines, the Quality Assurance Manual, and complete chain of custody information.

Whether or not you have a DNA expert on your case, simply obtaining the final DNA report is not sufficient due diligence. Further discovery is essential to understanding the full impact of the DNA testing on your case. Unfortunately, both prosecutors and defense attorneys do not always realize this.