All living things, including humans, are made up of cells. Humans are made up of many different kind of cells, including skin cells, blood cells, buccal cells (inside the mouth), muscle cells, fat cells, and many more.

Most of the cells in our bodies (with the exception of red blood cells) have a nucleus. The nucleus of all of our cells, doesn't matter which cell type, contains chromosomes, and chromosomes are responsible for storing our hereditary information. Chromosomes are made up of DNA (stands for deoxyribonucleic acid). DNA is like a blueprint because it holds the informational code for all of the genetic information for that person. The DNA for each individual is unique to that person.

With the exception of the egg and sperm cell, all of the cells in our body contain 23 pairs of chromosomes, 46 in total. One chromosome from the pair is inherited from our mother and the other one is passed down from our father. This is a picture of all of the chromosomes in a cell (this type of picture is called a karyotype).

Both males and females have 23 pairs of chromosomes. However, in males, the 23rd pair consists of an X-Chromosome and a Y-Chromosome, whereas females have two X-Chromosomes. The Y-Chromosome is special because it carries ancestral information regarding a male's paternal line.

DNA looks like a twisted ladder and is often referred to as a "double helix". The double helix consists of two complementary chains of DNA twisted together.

If we were to hypothetically untwist the DNA strand and lay it flat, it would look like a ladder. The two sides of the ladder are called the DNA's "backbone". The steps inside the ladder represent "bases". There are 4 types of bases in DNA: A (for adenine), C (for cytosine), T (for thymine), and G (for guanosine). In the DNA strand, A always pairs with a T, and C always pairs with a G. The unique sequence of the A, C, T, and G in DNA forms codes which carry genetic information.

When DNA is deciphered by genetic testing, the DNA code can be written in the following manner:

A G C T G G G A C A A T G G G C G C T A G G C C C C C C...

No two individuals (except for identical twins) have exactly the same genetic code and that is what makes everyone unique. However, all males with the same surname who are originated from a common paternal lineage will share the same or very similar genetic code in their Y-Chromosome. Unrelated males from a different family line will have a different Y-Chromosome code.

A male inherits his Y-Chromosome directly from his father. The Y-Chromosome that a male receives from his father is very special because it holds a lot of valuable information about his ancestry. This is because the Y-Chromosome is passed down along the male line, relatively unchanged from generation to generation. A forefather will pass his Y-Chromosome down to all of his sons, and they will then pass it down to all of their sons, and so on throughout the generations along the male line. Thus, males who are descendents of the same line will have the same or nearly identical Y-Chromosomes.

When a Y-Chromosome genealogy test is performed, the laboratory examines specific regions (markers) along the Y-Chromosome called "hypervariable" regions. Hypervariable regions are areas within the Y-Chromosome that may differ greatly between different family lines. The type of hypervariable region which is studied in Y-Chromosome testing is called STR markers (stands for "Short Tandem Repeat" markers). STR markers are regions of the Y-Chromosome where small chunks of the DNA are repeated over and over again. The number of times that these small chunks of DNA repeat themselves in the Y-Chromosome is variable amongst different family lines.

To follow is an example of a Y-DNA STR marker called DYS19. The section of DNA which repeats itself is TAGA. Thus, someone with a DYS19 marker of 6 will have TAGA repeated 6 times. The DNA test will indicate that the DYS19 marker is 6 for this individual. DYS19 = 6.

Someone with a DYS19 marker of 4 will have TAGA is repeated 4 times. In this case, the DNA test will indicate that the DYS19 marker is 4 for this individual. DYS19 = 4.

By testing your Y-DNA STR markers, a DNA laboratory can provide you with your Y DNA STR marker profile which is specific for your paternal ancestry. Because all males with the same ancestors will have the same or similar Y DNA STR marker profile, you can compare your markers against other males to solve questions about your paternal ancestry, to conclusively link family lines and to discover you distant relatives who share a common paternal ancestor with yourself. DNA testing has become the most exciting and fastest growing branch of genealogy.

When a Y-Chromosome test is performed, 20, 67 or 101 Y-DNA STR markers are analyzed to generate a unique "profile" for that individual. Two males with the same male lineage with the same forefathers will have the same or similar profiles. The closer the match in profiles, the more recently two individuals shared the same forefather. Obviously, the more markers that are tested, the more powerful your test becomes and the more stringent your searches are when searching for matches.