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Q: WHAT IS DNA? PART 2: HUMAN DNA & HOW IT IDENTIFIES US
A: From one person to the next, our DNA is almost identical. What makes us ‘unique’ at a genetic level is that our genes differ slightly from individual to individual. These variations in a person’s genes are called polymorphisms. In human Identification, we are interested in a specific type of polymorphism known as Short Tandem Repeats (STRs).
STRs are short sequences of 2-5 base-pairs that repeat themselves in a single locus. For example, at locus of gene D7S280, the repeat sequence is “GATA” and it may occur six times. In another individual, the repeat sequence may be seen 10 times. This alternative form of a gene is known as an allele and it is expressed as the number of times the repeat sequence occurs.
Continuing our example with gene D7S280, alleles of this gene are found on human chromosome 7 and have between 6-15 tandem repeats of the “GATA” sequence. An example of a sequence with 15 tandem repeats can be seen in the table below. Note that the table below shows the base sequence for only one chromosome. Because we have a pair of chromosomes, there will be another sequence generated for the other gene.
If the genes from both chromosomes have identical alleles (homozygous), the sequences will have an identical number of tandem repeats. Therefore, both alleles are expressed as a single number.
If the genes from both chromosomes are different alleles (heterozygous), the sequences will have different tandem repeats. Therefore, the alleles will be expressed as two numbers.
D7S280 6 15
The FBI has developed a standard set of 13 STR loci to be used for human identification, known as CODIS STRs. At PTC Laboratories, we use up to 26 STR loci for testing, including all 13 CODIS STRs, to generate a DNA profile for every person tested. These loci have the highest degree of polymorphisms and are always used in forensic cases and relationship/paternity testing worldwide.
An example DNA profile in a DNA Profile report for a single individual would look like this:
An example of DNA profiles in a paternity test report would look like this: