Suppose that a crime scene DNA sample was recovered and a suspect was later arre
ID: 3301349 • Letter: S
Question
Suppose that a crime scene DNA sample was recovered and a suspect was later arrested. The DNA profiles of the crime strain and the suspect were typed and both had the same DNA profiles at loci THO1, TPOX and CSF1P0. If both had allele 7 for THO1, alleles 8 and 10 for TPOX, and alleles 10 and 12 for CSF1P0, compute the match probability and likelihood ratio using the chart below. State the likelihood ratio in words.
THO1
TPOX
CSF1P0
Allele
Frequency
Allele
Frequency
Allele
Frequency
6
0.100
8
0.545
7
0.009
7
0.316
9
0.100
8
0.002
8
0.053
10
0.022
9
0.037
9
0.440
11
0.313
10
0.239
9.3
0.029
12
0.020
11
0.261
10
0.060
12
0.362
11
0.002
13
0.082
14
0.006
15
0.002
The match probability rounded to four decimal places is:
The Likelihood Ratio as a whole number is:
THO1
TPOX
CSF1P0
Allele
Frequency
Allele
Frequency
Allele
Frequency
6
0.100
8
0.545
7
0.009
7
0.316
9
0.100
8
0.002
8
0.053
10
0.022
9
0.037
9
0.440
11
0.313
10
0.239
9.3
0.029
12
0.020
11
0.261
10
0.060
12
0.362
11
0.002
13
0.082
14
0.006
15
0.002
Explanation / Answer
For locus THO1, allele 7 matches, whose frequency is p = 0.316. The genotype frequency for locus THO1 is thus p2 (allele 7 coming from both parents, hence p2) = (0.316)2 = 0.099856.
For locus TPOX, alleles 8 and 10 match, whose frequencies are p = 0.545 and q = 0.022. The genotype frequency for locus TPOX is thus 2pq (allele 8 coming from Mother and allele 10 coming from Father OR allele 8 coming from Father and allele 10 coming from Mother, hence 2pq) = 2*0.545*0.022 = 0.02398.
For locus CSF1P0, alleles 10 and 12 match, whose frequencies are p = 0.239 and q = 0.362. The genotype frequency for locus CSF1P0 is thus 2pq (allele 10 coming from Mother and allele 12 coming from Father OR allele 10 coming from Father and allele 12 coming from Mother, henc 2pq) = 2*0.239*0.362 = 0.173036.
Thus, the profile frequency (match probability) using the product rule is = 0.0004143428139 = 4.143 * 10-4.
The likelihood ratio (LR) is inverse of genotype frequency = 1/4.143 * 10-4 = 2413.46 = 2413 (as a whole number).
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