DNA forensic analysis relies on one key characteristic of DNA: the configuration is the same in all cells of an individual. Altogether each person carries around 200,000 genes in each cell, comprising three billion 'base pairs' -the chemical building blocks of DNA. Analyzing all these base pairs and genes in order to identify an individual would be impossible, with current technology. Instead, forensic scientists focus on certain genetic sequences called 'markers'. Here, the arrangement of genetic information is highly variable and particular to each person.
DNA is material that governs inheritance of eye color, hair color, stature, bone density and many other human and animal traits. DNA is a long, but narrow string-like object. A one foot long string or strand of DNA is normally packed into a space roughly equal to a cube 1/millionth of an inch on a side. This is possible only because DNA is a very thin string.
Our body's cells each contain a complete sample of our DNA. There are muscle cells, brain cells, liver cells, blood cells, sperm cells and others. Basically, every part of the body is made up of these tiny cells and each contains a sample or complement of DNA identical to that of every other cell within a given person.
STRs are the type of DNA used in most of the currently popular forensic DNA tests. STR is a generic term that describes any short, repeating DNA sequence. It turns out that our DNA has a variety of STRs scattered among DNA sequences that encode cellular functions.