Crime Scene Investigator PCR Basics™ Kit

% total population
45
40
35
30
25
20
15
10
5
0
6 7 8 9 9.3 10
TH01 alleles
Caucasians (N=427)
African American (N=414)
Hispanics (N=414)
Fig. 4. TH01 allele frequencies among different ethnic groups.
According to the data at the TH01 locus, Caucasians are more likely to have either
allele 6 or allele 9.3, but hardly ever have allele10. Similarly, most African Americans have
allele 7 at this locus, but almost never have allele 10.
Because of how chromosomes are inherited (described by Mendel's Law of
Independent Assortment), the frequency of each allele can be multiplied together to give
you a genotype frequency. First, let's look at the D3S1358 locus, another locus used in
DNA profiling in addition to TH01. Let's say that at the D3S1358 locus suspect A has a
copy of allele 16 (allele frequency = 0.253 for Caucasians) and a copy of allele 17 (allele
frequency = 0.215 in Caucasians). At this locus, this particular combination of alleles is
seen fairly frequently in Caucasians, and the allele frequency of this combination of alleles
works out to 2 x (0.253) x (0.215) = 0.109, or about 1 out of every 10 Caucasians. If you
were investigating this crime, and these particular genotypes were found at the crime
scene, that's not very good news for your case! That means that many Caucasians could
have been involved in the crime. The essential job of investigating a crime is ruling out, or
excluding, suspects. How does DNA profiling make that happen?
It does so by examining many different loci at the same time. Now, in addition to
D3S1358, let's examine the TH01 locus. At the TH01 locus, suspect A has two copies of
TH01 allele 10. For Caucasians, the allele frequency of a single copy of allele 10 is 0.008.
For a Caucasian person with two copies of allele 10, the total allele frequency at the TH01
locus is then (0.008) x (0.008) = 0.000064, which works out to 1 out of every approximately
15,600 Caucasians. If you combine the TH01 allele frequency for this person with that of
the D3S1358 allele frequency, that works out to a combine allele frequency of (0.109) x
(0.000064) = 0.000007, or 1 out of every 150,000 or so Caucasians. So, the more loci you
add to your analysis, the better the power of discrimination becomes. Or, put another way,
the smaller your suspect pool becomes.
To think of this another way, examine figure 5, where you have a pool of 13 potential
suspects (stars) implicated in a crime. Using STR analysis, you discover that 6 of the
potential suspects have the THO1 genotype of 6-3, which is a genotype found at the scene
of the crime. The other 7 samples have a different genotype at the TH01 locus. So, in the
10