Page 83 - Genetics_From_Genes_to_Genomes_6th_FULL_Part3

11.3 Sampling DNA Variation in a Genome 377

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evolution and history. Of additional practical significance, genome is (0.1) , or one chance in 10 trillion. (This
anonymous loci serve as molecular markers for specific calculation is simplified but gives you a rough idea of
regions of the genome. Even if these DNA markers do not these probabilities; Chapter 21 on population genetics
themselves cause disease, scientists can follow their inheri- will show you how to make such calculations precisely
tance to locate hard-to-find genes responsible for genetic from actual data.) Given that the earth currently has only
diseases and other phenotypes. about 7 billion human inhabitants, you can see that a gen-
otype for 13 unlinked, polymorphic SSR loci would serve
as a DNA fingerprint unique to any one person, except-
Forensic DNA Fingerprinting ing identical twins.
Examines Multiple SSR Loci A simple extension of a method we have already dis-
cussed to genotype a single SSR locus allows the simulta-
SSR loci are highly polymorphic: Many alleles that differ neous genotyping of multiple SSRs. Figure 11.12 showed
in the number of repeating units exist in the population, that the PCR products amplified from an SSR locus will
although any one person carries only two of these alleles have different sizes reflecting the number of repeating units
for any given locus. The polymorphism of SSR loci makes in each allele. To examine 13 SSRs at the same time, you
them a powerful resource in identifying a person from his would label the 13 pairs of PCR primers with dyes that
or her DNA. fluoresce in different colors, and then combine all the
The power comes from the possibility of examining primer pairs in the same PCR reaction tube. After gel elec-
multiple polymorphic SSR loci simultaneously. Suppose trophoresis, you could then identify the allelic variants for
the likelihood that any two random people share exactly each SSR locus based on the fluorescent colors and sizes of
the same combination of two alleles of a particular SSR the PCR products (Fig. 11.15).
locus is 10% (0.1), and that the same is true for a second, In the United States, the Federal Bureau of Investiga-
independently segregating SSR locus. Using the product tion (FBI) maintains a database called CODIS (Combined
rule for independent events, the probability that two ran- DNA Index System) that allows forensic laboratories
domly chosen people will have the same alleles at the two throughout the country to share and compare DNA pro-
SSR loci is (0.1) × (0.1) = 0.01 or 1 in 100. Now consider files. All these laboratories use the same 13 primer pairs to
13 such SSR loci. The chance that two people will have amplify the 13 SSR loci. The laboratories carefully catalog
the same combinations of alleles at all 13 positions in the the sizes of the PCR products and submit the results to the

Figure 11.15 DNA fingerprinting. (a) Basis of DNA fingerprinting by PCR multiplexing (simultaneous analysis of PCR products from
multiple loci). PCR primer pairs (ovals) amplify separate SSR loci, usually from nonhomologous chromosomes. The primer pairs are labeled
with different fluorescent molecules (blue and green in this example for two loci). (b) Gel electrophoresis of multiplexed PCR products. This
example shows the analysis in three people (A–C) of six loci, two of each color (blue, green, and yellow). The alleles for the two different loci
of the same color (i.e., locus 1 and locus 2) differ sufficiently in their sizes that it is clear which allele belongs with which locus. Red bands in
each lane are size standards.
a: © Scott Camazine & Sue Trainor/Science Source b: © Alila Medical Images/Shutterstock RF
(a) (b)
DNA Fingerprinting
AATG A B C
Locus 1 1 2 3 4 5 6 7 8 9
Allele 1
Locus 1
Allele 2
1 2 3 4 5 6 7 8
Locus 3
Locus 4

CCG
Locus 5
4
1
3
2
5
6
Locus 2 Allele 1
Allele 2 Locus 2
1 2 3 4 5
Locus 6