Question

Here is an example using 3 CODIS STR results. DNA evidence was found at a crime scene. Due to the poor quality of the DNA sample, only 3 of the 13 CODIS STRs could be determined. The table below lists the analytical results for the three STRs in the second and third columns (Allele A and Allele B).
Here is an example using 3 CODIS STR results. DNA evidence was found at a crime scene. Due to the poor quality of the DNA sample, only 3 of the 13 CODIS STRS could be determined. The table below lists the analytical results for the three STRs in the second and third columns (Allele A and Allele B). STR Locus Allele A Allele B P(Allele A) P(Allele B) D3S1358 11 12 0.5 0.2 TH01 4 4 | 0.5 0.5 D21911 7 10 0.1 0.5 Which STR is homozygous for the DNA evidence? Recall that homozygosity means that for a given locus, an individual has 2 identical alleles. Select one: O a. TH01 O b. D3S1358 O c. D21511

Answer 1

1. The correct answer is option A, TH01.

As it has two identical alleles i.e. 4.

2. The correct answer is option A, allele 7 in D21S11.

As the probability of this particular allele is lowest, p=0.1.

3. The correct answer is option A, allele 11 at D3S1358, allele 4 at TH01, allele 10 at D21S11

As all these alleles have the same probabillity, p=0.5

4. Genotype frequency for D3S1358 = 2pq= 2(0.5x0.2)= 0.2.

5. Genotype frequency for TH01= $p^2$ = 0.5x0.5 = 0.25.

6. Genotype frequency for D21S11= 2pq = 2(0.1x0.5)= 0.1.

7. Probability of a random individual to have these 3 genotypes for STR loci = (0.2x0.25x0.1)= 0.005