Archived | STR Data Analysis and Interpretation for Forensic Analysts | Two Person Mixture with Three Alleles at a Locus

Consider the following information:

**D3S1358**
Allele	Peak Height
14	1080
16*	320
18	690
* Visual minor alleles

Assuming these results came from the same case as the FGA data in the first example above, the FGA data can be used to estimate the mixture percentages. This information can aid in determining the most likely genotype(s) of the major and minor contributors for a locus with three alleles. There are numerous scenarios that may be possible. However, in many instances the analyst can easily dismiss some based on the available data.

**Genotypes per Scenario**
Scenario	Genotype #1(Major)	Genotype #2(Minor)
1	14,18	14,16
2	14,18	16,16
3	14,18	16,18
4	14,14	16, 18
5	16, 16	14, 18
6	18, 18	14,16
7	14, 16	14, 18
8	16, 18	14, 18
9	14,16	16,18
10	14,16	18,18
11	16,18	14,16
12	16,18	14,14

If it is assumed that the major contributor is approximately 75%, the data only support scenarios 1-3. In this example, the assignment of a genotype for the major contributor is straightforward. When alleles are shared or possibly masked, assigning a possible genotype of the minor contributor is more complex. Use of the above example to assess these three scenarios will assume that there is a 1:1 RFU ratio for heterozygous alleles.

**Scenario #1**
Major 14, 18; Minor 14, 16
In this scenario, 14 is a shared allele and it is assumed that the contribution is equal to that of the 16 allele, which is 320 RFUs (Genotype #2). This would leave Genotype #1 contributing 760 RFUs for the shared 14. 1080-320 = 760 The estimated peak height percentage for Genotype #1 would be 91% 690/760 = 91% (note that no estimated peak height percentage is calculated for Genotype #2 since the assumption was a 1:1 contribution from each allele--320 RFU) The percent contributions would be: Major = 690 + 760/ 1080 + 320 + 690 = .69 x 100 = 69% Minor = 320 + 320/ 1080 + 320 + 690 = .31 x 100 = 31%

**Scenario #2**
Major 14, 18; Minor 16, 16
If the minor contributor is a homozygous 16, the estimated peak height percentage for Genotype #1 would be 64% 690/1080 = 64% The percent contributions would be: Major = 690 + 1080/ 1080 + 320 + 690 = .85 x 100 = 85% Minor = 320/ 1080 + 320 + 690 = .69 x 100 = 15%

**Scenario #3**
Major 14, 18; Minor 16, 18
In this scenario, 18 is a shared allele, and it is assumed that the contribution is equal to that of the 16 allele which is 320 RFUs (Genotype #2). This would leave Genotype #1 contributing 370 RFUs to the 18. 690-320 = 370 The estimated peak height percentage for Genotype #1 would be 34% 370/1080 = 34% The percent contributions would be: Major = 1080 + 370/ 1080 + 320 + 690 = .69 x 100 = 69% Minor = 320 + 320/ 1080 + 320 + 690 = .31 x 100 = 31%

Based on these estimations, Scenario #1 is the best fit, but Scenario #2 should be considered. Scenario #3 has an estimated peak height percentage for Genotype #1 of 34%, making this combination unlikely.

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Date Created: July 11, 2023