Why the DNA 17 Multiplex was Needed

Question 1

What was the SGM+ profiling test?

Answer 1

• The SGM+ test was an improvement from the standard SGM test.
• As the SGM database grew larger, it became clear that more discrimination was needed, and so the number of loci increased from 6 to 10.
• This test also included all of the previous SGM loci and the sex test so the test was backwards-compatible with the SGM database.

Question 2

What are the three types of DNA match?

Answer 2

• Between a reference sample and undetected crime sample
• Crime to crime matches to provide an investigative link between the two and helpful in undetected crimes.
• Person-person matches can occur between reference samples. This identifies duplicates where people have given different names.

Question 3

What do we mean by concordance when talking about DNA 17 multiplex?

Answer 3

• If a crime stain is profile using one multiplex, and a reference sample uses a different one, they may not quite match for several reasons.
  
  • One reason may be due to an error in the entry of the data.
  • Another reason may be due to mutation. A crime stain profiled using one multiplex may produce 18-18 alleles at a locus. The reference using a different multiplex may produce 18, 19 alleles at a locus

Question 4

What is near-match reporting?

Answer 4

• Near-match reporting is used for non-concordant profiles.
• This process lists one-allele differences between the profiles.
• It can also be extended to investigate 2/3 allele differences between reference samples if needed.

Question 5

What are ESS Loci?

Answer 5

• ESS loci are the European Standard Set of Loci.
• These were specified as the basic set for comparison
• At least six of the 7 loci are needed before a search can be carried out.

Question 6

Why was a DNA 17 multiplex required?

Answer 6

• Samples obtained from crime stains were too poor to be searched because of damaged DNA.
  
  • Either due to degradation or inhibition.
• False positives and advantageous matches were becoming more frequent, as a result of the limited loci being tested.

Question 7

What were the ENFSI requirements for the DNA 17 profile?

Answer 7

1. To include more loci. 17 loci were specified as a minimum requirement for forensic providers.
2. To be more robust to inhibition.
3. Get better results from smaller and more degraded samples.
4. Continuity: The SGM+ loci were included in the DNA 17 multiplex to enable backward-compatibility between the two systems.

Question 8

What are the intepretational issues present in the DNA 17 multiplex?

Answer 8

• Rather than being a single test, companies have produced their own versions and consequently order and position of loci are not the same between multiplexes
• This means the DNA EPG for the same sample tested using 2 different multiplexes will not be directly comparable by eye.

Question 9

How does the sensitivity of the DNA 17 compare to SGM+?

Answer 9

• Starting threshold is 500 picograms of DNA.
• The SGM+ used a threshold which was double this.
• Therefore, DNA 17 produces a profile with half the amount of DNA previously required.
• With roughly 6 cells per picogram, as little as 80 cells are required for DNA 17
• Sensitivity has doubled.

Question 10

How has PCR improved the DNA 17 multiplex?

Answer 10

The number of PCR cycles has been increased from 28 cycles to between 29 and 31.

This makes the system more sensitive and able to give better profiles from smaller and more degraded samples.

Question 11

How has inhibition improved under the DNA 17 multiplex?

Answer 11

• The newer DNA multiplexes have fewer problems with the chemicals that inhibit samples