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How DNA Profiling (or DNA Fingerprinting) Works

Using DNA Profiling to Identify Individuals

Each individual inherits two alleles of each STR marker, one from their mother and one from their father. If the two alleles are the same, the individual is homozygous for that particular marker and only one band will appear on the gel. If two different bands appear on the gel, then the individual has inherited two different alleles for that marker and is heterozygous. The gel electrophoresis images you have been examining represent four genetic markers, which could result in a maximum of eight bands (at most two for each marker) on a gel. The pattern of bands is the individual's genetic profile, or DNA fingerprint.

The more markers used to build the profile, the less likely it is that two individuals will have the same profile by chance. (You will learn why in the "Frequency Primer.") This means that if two samples have exactly the same profile, it's almost certain that they are from the same individual! Typically, to positively identify humans, scientists use 13 markers; to identify elephants, 16 markers. At a crime scene, investigators look for biological material, such as blood or hair, from which they can extract DNA. They then look for an exact match between the genetic profile of the sample and that of a suspected criminal or of the victim. Here, you will look for a match between a sample of the ivory found on the truck and one of the dead elephants.

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