Understanding the DNA tested.

Adapted from http://www.dnaheritage.com/tutorial1.asp

The DNA tested in DNA studies is  in the Y-chromosome within males. This is one of the sex chromosomes and is responsible for maleness. All males have one in each cell and copies are passed down (virtually) unchanged from father to son every generation. Therefore, the Y-chromosome test is solely for males, but it can be a cousin, brother or other male relative.

DNA is often called the 'molecule of life'. Within it are the code words that help build proteins and thus help our bodies to function. However, most of it doesn't encode for anything at all - we call these regions 'junk DNA' and they contain no medical information whatsoever.

The DNA tested is examined for special markers along the DNA strands in these junk' regions to read into the genealogy of the person. At these markers, the sequence of bases  repeats itself many times.

As an example, at a particular marker, the sequence may read as:

where TCTA is repeated 9 times.

Because the marker is given a name e.g.  'DYS391', we record: DYS391 = 9

The Y-chromosome only comes from the father so the number of repeats in a baby boy will be the same as his father. Very occasionally these repeats increase or decrease, usually one at a time.

That is, a father may have DYS391 = 9 and his son DYS391 = 10.

We have gained a TCTA somewhere! This is a mutation.

These mutations are very useful because otherwise every male would have exactly the same Y-chromosome. The fact that related males that share the same surname will have identical Y-chromosomes (or at least a very close match), is why Y-chromosomes are so useful to genealogists. It is also very easy to tell if two males aren't related at all.

Also, because we know roughly how often these changes take place, we can also estimate when the most recent common ancestor (MRCA) lived.