Page 84 - Genetics_From_Genes_to_Genomes_6th_FULL_Part3

378 Chapter 11 Analyzing Genomic Variation

CODIS database. All 50 states mandate the collection of conclusive evidence in paternity suits, and DNA finger-
DNA fingerprint data for CODIS from felons convicted of prints can be used to identify human remains, as was the
certain crimes, such as sexual offenders; the database also case for the victims of the World Trade Center disaster
includes profiles from missing persons. on September 11, 2001. The benefits of this technology
As of 2016, CODIS has assisted more than 300,000 are not restricted to DNA fingerprints of humans. Wildlife
criminal investigations. Typically, forensic investigators biologists study populations of endangered species by
use the database to match the DNA profile of evidence fingerprinting individual animals to increase the chance
left at crime scenes with that of a felon. But DNA can for success of captive breeding programs or to identify
also establish innocence: Suspects can be excluded if no illegally poached animals. Owners of valuable domesti-
match with crime scene evidence exists. In fact, a public cated animals such as show dogs, thoroughbred horses,
policy organization called the Innocence Project has used or cattle can in some cases establish lineage through
DNA fingerprint evidence to help exonerate more than DNA fingerprints. In one fascinating if bizarre case from
300 people convicted of capital crimes, including several Argentina, scientists were enlisted to help apprehend a
who were awaiting execution. butcher who moonlighted as a cattle rustler. Meat hang-
The power of DNA fingerprinting technology raises ing in the butcher’s shop had the same DNA profile as a
many concerns about privacy and possible discrimination in tissue sample that a rancher had taken from one of his
the collection of data. Just to give one interesting example, cows before it was stolen.
consider that siblings will share 50% of all SSR alleles; the
same is true of parents and children. As a result, it is possi-
ble to identify the perpetrator of a crime not by a match to DNA Microarrays Genotype
his or her own DNA, but instead by a partial match to the Millions of SNPs
DNA of a close relative. This kind of familial DNA search
was critical for the apprehension in 2010 of the major sus- Nucleic acid hybridization, the ability of complemen-
pect for the “Grim Sleeper” serial killer in Los Angeles. The tary single strands of DNA or RNA to come together to
suspect’s son had recently been convicted on a felony weap- form double-stranded molecules, is the basis for many
ons charge, so his DNA was analyzed. The son’s DNA pro- techniques in molecular biology. We have already dis-
file partially matched DNA fingerprints from semen and cussed the importance of hybridizing oligonucleotide
saliva found on the Grim Sleeper’s victims. Policemen fol- primers to DNA templates in Sanger DNA sequencing
lowed the father, and one detective posing as a waiter ob- and in PCR. Both DNA sequencing and PCR assume that
tained a partially eaten slice of pizza with the father’s DNA. a perfect complementary match exists between all the
Stunningly, this DNA was a perfect match with the crime nucleotides in the primers and templates. But what will
scene evidence. (The trial has just begun as of this writing happen if a mismatch exists between two single strands
in early 2016.) Should criminal investigators be allowed to of nucleic acid?
conduct such familial searches given that family members Consider a 21-base oligonucleotide that hybridizes to a
of felons who have not committed any crime are in effect target strand that differs at a single base in the middle of the
under lifelong genetic surveillance? sequence (Fig. 11.16). The resulting double-stranded
DNA fingerprints have many important uses beyond hybrid is significantly less stable than a similar hybrid in
forensics for capital crimes. They now provide the most which all the nucleotides match. The reason is that the

Figure 11.16 Short hybridization probes can distinguish single-base mismatches. Researchers allow hybridization between a
short 21-base probe and two different target sequences. (a) A perfect match between probe and target extends across all 21 bases. When
the temperature rises, this hybrid has enough hydrogen bonds to remain intact. (b) With a single-base mismatch in the middle of the probe,
the effective length of the probe-target hybrid is only 10 bases. When the temperature rises, this hybrid falls apart.
(a) 21-base probe/target hybrid with no mismatches
Probe Raise
temperature
Completely complementary
target strand
(b) 21-base probe/target hybrid with middle mismatch
Raise
temperature

Mismatch at base 11