Our body's cells each contain a complete sample of our DNA. One cell is roughly equal in size to the cube described in the previous paragraph. 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. There are a few exceptions. For example, our red blood cells lack DNA. Blood itself can be typed because of the DNA contained in our white blood cells.

Not only does the human body rely on DNA but so do most living things including plants, animals and bacteria.

A strand of DNA is made up of tiny building-blocks. There are only four, different basic building-blocks. Scientists usually refer to these using four letters, A, T, G, and C. These four letters are short nicknames for more complicated building-block chemical names, but actually the letters (A,T, G and C) are used much more commonly than the chemical names so the latter will not be mentioned here. Another term for DNA's building blocks is the term, "bases." A, T, G and C are bases.

For example, to refer to a particular piece of DNA, we might write: AATTGCCTTTTAAAAA. This is a perfectly acceptable way of describing a piece of DNA. Someone with a machine called a DNA synthesizer could actually synthesize the same piece of DNA from the information AATTGCCTTTTAAAAA alone.

The sequence of bases (letters) can code for many properties of the body's cells. The cells can read this code. Some DNA sequences encode important information for the cell. Such DNA is called, not surprisingly, "coding DNA." Our cells also contain much DNA that doesn't encode anything that we know about. If the DNA doesn't encode anything, it is called non-coding DNA or sometimes, "junk DNA."[1]

The DNA code, or genetic code as it is called, is passed through the sperm and egg to the offspring. A single sperm cell contains about three billion bases consisting of A, T, G and C that follow each other in a well defined sequence along the strand of DNA. Each egg cell also contains three billion bases arranged in a well-defined sequence very similar, but not identical to the sperm.

Both coding and non-coding DNAs may vary from one individual to another. These DNA variations can be used to identify people or at least distinguish one person from another.

What is a Locus?
A locus (with a hard "c", LOW-KUS) is simply a location in the DNA. The plural of locus is, loci ( with a soft "c", pronounced LOW-S-EYE). Again, the DNA is a long string like object as illustrated below. A locus is simply a location in the DNA. Such locations, or loci, reside at specific places on chromosomes.

What is a Chromosome?
When a cell is getting ready to divide creating two daughter cells, it packs its DNA into bundles called chromosomes. Chromosomes are just bundles of DNA. For humans, there are consistently 23 pairs of chromosomes, each with a consistent size and shape. Chromosomes are numbered. Chromosome number 1 is the largest chromosome; chromosome number 2 a little smaller and so on. Among the 23 pairs of chromosomes there is a pair called the sex chromosomes. This is something of a misnomer, since there are many functions on the "sex" chromosomes that have nothing to do with sex. In females, the sex-chromosome pair consists of two similar size chromosomes called X chromosomes. Males have one X and one sma

What is DNA Profiling or DNA test?
DNA profiling (also called DNA testing, DNA typing, or genetic fingerprinting) is a technique employed by forensic scientists to assist in the identification of individuals on the basis of their respective DNA profiles. DNA testing compares genetic material from two or more people to determine if they are biologically related. The genetic material for testing is usually taken from a mouth swab or blood sample.

How the profiling is done?
DNA profiling commences with the extraction of an individual's DNA (typically called a "reference sample"). The most desirable method of collecting a reference sample is the use of a buccal swab, as this reduces the possibility of contamination. When this is not available (eg because a court order may be needed and not obtainable) other methods may need to be used to collect a sample of blood, saliva, semen, or other appropriate fluid or tissue from personal items (e.g. toothbrush, razor, etc) or from stored samples (e.g. banked sperm or biopsy tissue). Samples obtained from blood relatives (biological relative) can provide an indication of an individual's profile, as could human remains which had been previously profiled.

A reference sample is then analysed to create the individual's DNA profile using one of a number of techniques.

Why the Genealogical DNA test is done?
A genealogical DNA test examines the nucleotides at specific locations on a person's DNA for genetic genealogy purposes. The test results are meant to have no informative medical value and do not determine specific genetic diseases or disorders (see possible exceptions in Medical information below); they are intended only to give genealogical information. Genealogical DNA tests generally involve comparing the results of living individuals as opposed to obtaining samples from deceased people.

Courtesy from the follwing websites. Please visit further for more information on DNA Tests.

http://www.scientific.org/tutorials/articles/riley/riley.html

http://en.wikipedia.org/wiki/Genetic_fingerprinting

http://en.wikipedia.org/wiki/Genealogical_DNA_test