Impact Of Population Structure Corrections

The National Research Council (NRC II) report entitled The Evaluation of Forensic DNA Evidence discusses issues that surround population structure. The NRC II report made several recommendations for taking population structure into account (see Appendix VI). The impacts of these recommendations on homozygote and heterozygote frequency calculations are illustrated in Table 21.4. Again examples are given with a TH01 homozygote 6,6 and a D13S317 heterozygote 11,14.

Under HWE

Unconditional (NRCII Recommendation 4.1)

Conditional with Substructure Adjustment

(NRCII recommendation 4.10a)

(0.23)2+(0.23)(1 - 0.23)(0.01) [(0.23)(1 - 0.01)+2(0.01)][(0.23)(1 - 0.01)+3(0.01)]

(1 + 0.01)(1 + 2(0.01)) = (0.2477)(0.2577)/(1.01)(1.02) = 0.062

Heterozygote 2p,py

(NRCII recommendation 4.10b)

2[(0.34)(1 - 0.01) + 0.01][(0.05)(1 - 0.01) + 0.01] (1 + 0.01)(1 + 2(0.01)) = 2(0.3466)(0.0595)/(1.01)(1.02) = 0.0400

The NRCII recommendation 4.1 substructure adjustments replace p2 for homozygote calculations with p2 + p(1 — p)8, where 8 is an empirically determined measure of population subdivision (see Chapter 20). A conservative value for 8 is 0.01 for typical at-large populations and 0.03 with smaller, isolated, and more inbred groups of people. Budowle et al. (2001) have demonstrated that 8 = 0.01 is a reliable and conservative estimate of population substructure with extensive population data.

Further calculations with these population substructure adjustment equations are illustrated in Table 21.5. Note that NRC II recommendation 4.1 adjustments only change the overall profile frequency estimate from 1.20 X 10—15 to

I.35 X10—15 with 8 = 0.01 and to 1.70 X10—15 with 8 = 0.03. NRC II recommendation 4.10a and 4.10b generate further minor differences as well.

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