Every cell in the human body (with the exception of red blood cells) contains a copy of our DNA. At conception a person receives DNA from both their father and mother. Everyone has 23 pairs of chromosomes, and for each pair, one is received from the father and one from the mother. These 23 pairs of chromosomes are known as nuclear DNA as they reside in the nucleus of the human cell. The Y chromosome is the one that determines whether a child is male or not. It is only present in males and is passed from father to son and so on, down the generations. 

Scientists have identified a small portion of the Y chromosome which is transmitted from father to son virtually unchanged (this is known as non-recombinant or "junk" DNA). Testing of this portion of the Y chromosome provides information about the direct male line back in time. This part of the Y chromosome, because it remains unchanged over the generations, therefore can be used to evaluate whether a group of men who are tested have some common ancestor in the past.

The locations on the Y chromosome that are tested are called "markers" Occasionally a mutation or small harmless change may occur at one of the markers on the Y chromosome as it is passed from father to son, and these mutations can sometimes be valuable for genealogists in identifying a branch in a family tree.

Analysis of the markers on someone's Y chromosome will produce a numerical value for each marker tested. The pattern of numbers produced across a range of different markers represents effectively the DNA signature of that person, or "haplotype". If the Y chromosome haplotype results for a group of men are identical or very similar, and if these men have the same surname and are believed to have originated from the same geographical location - then the probability is high that they have a common ancestor.

The accuracy of the interpretation that can be made from such analysis depends on how many markers have been tested for each individual and how big the differences are between values. There are several different Y chromosome tests providing increasing levels of accuracy.

The most basic test available analyses 12 markers of the Y chromosome for each participant. Then if the test results for a number of participants are compared within a study and are found to be identical on all 12 markers, then it is extremely likely that they are all related. If however some of the results for two people are different on 1 out 12 of the markers by a small change in value, then these persons might be related or not. Performing an additional more accurate test on 25 markers for these two individuals will usually answer this question. If this second analysis is still not decisive, then a final more accurate analysis can be performed on 37 markers and this usually will resolve the issue to a high degree of probability.

Thus the Y chromosome test in its current state of development does not give a 100% definite answer about common ancestors - but when used together with additional genealogy research knowledge can be very effective in confirming or denying existing theories and providing new insights. This additional genealogy knowledge is available for the Creer family. More information on interpreting test results can be found here.