Population Genetics. Matthew B. Hamilton

D21S11 D18S51 Genotype 17, 18 29, 30 18, 18

To determine the expected frequency of a one‐locus genotype, we employ the Hardy–Weinberg Eq. (2.1). In doing so, we are implicitly accepting that all of the assumptions of Hardy–Weinberg are approximately met. If these assumptions were not met, then the Hardy–Weinberg equation would not provide an accurate expectation for the genotype frequencies! To determine the frequency of the three‐locus genotype in Table 2.2, we need allele frequencies for those loci, which are found in Table 2.3. Starting with the locus D3S1358, we see in Table 2.3 that the 17‐repeat allele has a frequency of 0.2118 and the 18‐repeat allele a frequency of 0.1626. Then, using Hardy–Weinberg, the 17, 18 genotype has an expected frequency of 2(0.2118)(0.1626) = 0.0689 or 6.89%. For the two other loci in the DNA profile of Table 2.2, we carry out the same steps.

D21S11	29‐Repeat allele frequency = 0.1811
	30‐Repeat allele frequency = 0.2321
	Genotype frequency = 2(0.1811)(0.2321) = 0.0841 or 8.41%
D18S51	18‐Repeat allele frequency = 0.0918
	Genotype frequency = (0.0918)2 = 0.0084 or 0.84%

      The genotype for each locus has a relatively large chance of being observed in a population. For example, a little less than 1% of Caucasian U.S. citizens (or about 1 in 119) are expected to be homozygous for the 18‐repeat allele at locus D18S51. Therefore, a match between evidence and suspect DNA profiles homozygous for the 18 repeat at that locus would not be strong evidence that the samples came from the same individual.

Table 2.3 Allele frequencies for nine STR loci commonly used in forensic cases estimated from 196 US Caucasians sampled randomly with respect to geographic location. The allele states are the numbers of repeats at that locus (see Box 2.1). Allele frequencies (Freq) are as reported in Budowle et al. (2001). Table 1 from FBI sample population.

D3S1358	vWA	D21S11	D18S51	D13S317
Allele	Freq	Allele	Freq	Allele	Freq	Allele	Freq	Allele	Freq
12	0.0000	13	0.0051	27	0.0459	<11	0.0128	8	0.0995
13	0.0025	14	0.1020	28	0.1658	11	0.0128	9	0.0765
14	0.1404	15	0.1122	29	0.1811	12	0.1276	10	0.0510
15	0.2463	16	0.2015	30	0.2321	13	0.1224	11	0.3189
16	0.2315	17	0.2628	30.2	0.0383	14	0.1735	12	0.3087
17	0.2118	18	0.2219	31	0.0714	15	0.1276	13	0.1097
18	0.1626	19	0.0842	31.2	0.0995	16	0.1071	14	0.0357
19	0.0049	20	0.0102	32	0.0153	17	0.1556
				32.2	0.1122	18	0.0918
				33.2	0.0306	19	0.0357
				35.2	0.0026	20	0.0255