Matching the Pattern: DNA Fingerprinting Explained

If over 99% of the human genome is identical among all people, then scientists must focus on the remaining

If identical twins come from a single fertilized egg that splits, then they share 100% of their DNA sequence. If their DNA sequence is identical, then any test measuring that sequence will yield the same bands or peaks.

If the DNA must be broken into pieces to be sorted on a gel, then a chemical tool is needed. If that tool recognizes and cuts specific base sequences, then it is a restriction enzyme.

If a sample is to be used for DNA analysis, then it must contain a nucleus. If hair follicles, skin, white blood cells, and saliva contain nucleated cells, then they are viable; however, mature human red blood cells lack a nucleus and DNA.

If a child inherits 50% of their DNA from each biological parent, then the child's genetic markers are a composite of the parents. If a band appears in the child's lane, then it must have been passed down from one of the two biological sources.

If STR regions are short, then they can be easily amplified using PCR (Polymerase Chain Reaction). If PCR can create millions of copies from a single cell, then the STR method is far more sensitive than older, large-fragment methods.

If scientists are comparing specific repeating sequences across different individuals, then they must look at the same "address" on the DNA. If they are looking at a fixed position, then that position is a locus.

If the agarose gel acts as a physical mesh or sieve, then it provides resistance to movement. If larger molecules bump into the gel fibers more frequently than smaller ones, then they will be slowed down and remain closer to the starting point.

If the "Product Rule" is used to multiply the probabilities of independent markers, then more markers lead to a lower chance of a coincidental match. If the markers used are rare in the population, then the uniqueness of the profile increases.

If statistics are used to calculate the likelihood of a match, then the result is expressed as a probability (e.g., 1 in 1 quintillion). While the probability can be extremely high, in science, it is expressed as a statistical likelihood rather than absolute 100% "proof."

If an individual inherits a different number of repeats from their mother than from their father, then they possess two different alleles. If the alleles are different, then the genotype is heterozygous.

If the biological sample came from the suspect, then every single marker must match perfectly. If a mismatch exists at one or more locations, then the DNA cannot belong to that individual, leading to an exclusion.

If the goal is to analyze specific DNA segments, then the DNA must first be removed from the cells, then copied millions of times to be visible, and finally sorted by size to be read.

If DNA is broken into very tiny, random fragments, then the PCR primers may not have a long enough intact template to bind to. If some target regions are destroyed while others remain, then only some loci will appear on the final report.

If laser-induced fluorescence is used to detect DNA as it passes through a capillary, then the intensity of the light is recorded over time. If a fragment passes the sensor, then it appears as a vertical spike or peak on the chart.

If the computer needs to know if a peak represents 12 repeats or 13 repeats, then it needs a comparison. If the allelic ladder contains all common versions of a repeat for that locus, then it acts as a "molecular ruler" for sizing.

If a child must get one allele at every locus from the biological father, then the absence of a matching allele at any locus excludes the man. If they share no alleles at all, then they are not biologically related.

If the Federal Bureau of Investigation (FBI) standardized a set of 20 specific locations in the genome for all forensic labs to use, then any profile can be compared across state lines with high statistical confidence.

If both chromosomes contain the same number of repeats, then the amplified fragments will be the same size and move at the same speed. If they move together, then they will hit the sensor at the same time, creating a single, taller spike.

If DNA polymerase accidentally slips during PCR, then it creates a fragment that is one repeat shorter than the true allele. If this "stutter" is not recognized by the analyst, then it might be incorrectly labeled as a real genetic marker from a second person.