Over time, the changes that occur to our DNA are called mutations. These changes can happen at the active part of the DNA (about 10% of the DNA), or at the inactive segments (some call the variable part). The changes are actually substitutions of one base pair (the bucket chair that you put out the window) for another. In essence, you change one letter for another. This can happen in three ways. First, one base pair can have a "substitution" made for it. The letter A can be substituted for the letter G. This would mean that the opposite side would also be changed since G would only go with C whereas, A only goes with T. Secondly, there can be an "insertion" of a nucleotide base between two existing bases. This would change the codon sequence thus changing the code for this segment. Thirdly, there can a "deletion" of a nucleotide base thus changing the sequent of the code for this segment of DNA. Once a mutation is established, it is past on down the line to each new generation.
Now here is where all the terminology comes. When only a single nucleotide base is substituted, it is called a "Single Nucleotide Polymorphisms" or SNP (pronounced snip). You can now visualize this in your 3-D dinning room since you would only be replacing one bucket truck. The inactive segments are much more likely to undergo mutations which can occur up and down these segments at multiple locations. [Remember that this is also about 90% of your DNA!] The changes are grouped together (in tandem) and occur at variable places along the codon. They can be of variable lengths. The short length changes are called "short tandem repeats" (STRs). The longer series of changes have been called "variable number tandem repeats"(VNTRs). These are the terms which are thrown about in all the literature regarding DNA and genetics. They are also used in describing the test which can be ordered for your own DNA.
It might now be helpful to go back and reread the previous blogs that present the DNA strutcture. More to come.