Each son receives DNA on his Y chromosome from his father.  This DNA is not mixed with that of the mother, and it is identical to that of the father, unless a mutation occurs.  It has been estimated that a mutation occurs about once every 500 generations, or every 15,000 years, give or take a few millennia.  Because we look at several different sites on the Y chromosome, however, we do have to worry about mutations.  The more sites examined, the more likely mutations are present. 

Y chromosomal DNA (Y DNA for short) is passed on from father to son.  It represents a nearly unbroken chain that moves through all recorded history and into the cloudy prehistory of our Neolithic and Paleolithic ancestors.  The qualifier "nearly" is inserted to remind us that mutations are possible.  Starting with the living donor of Y DNA, the genetic information is inherited from the donor's father, from that man's father, and so on, up the male, or paternal, or Adam line.  It provides a fingerprint of this particular line.  Barring mutations, two brothers would carry this same Y DNA, as would two first cousins who were the sons of brothers, or a nephew and his paternal uncle.  In most Western cultures, these men would all have the same surname.  The Y DNA thus becomes a genetic label for the surname.  Comparison with other men with the same surname could confirm whether they had a common ancestor. 

Y DNA within such a family group may not match, because of what are delicately called "non-paternal events."  There are three causes for such events.  An adopted individual would carry the surname of his new family but the Y DNA of his birth father.  An individual who chooses to change his name would bequeath his new surname to a son along with the Y DNA of his father, who carried the original surname.  Finally, illegitimacy results in a man with the surname of his cuckolded father (or in some cases the surname of the unmarried mother) but the DNA of his genetic father.  Often the first two causes can be anticipated through research of historical or family records.  Genealogists have had to assume that no illegitimacy occurred. 

The Lambert DNA project uses FamilyTreeDNA for analysis of the samples.  The results of the analysis are the identification of numerical categories for either 12 or 25 pieces of Y DNA.  These pieces were chosen because they are among the fastest mutating parts of the DNA.  The faster the mutations, the more opportunity the genealogist has to distinguish different families or subgroups of families.  The pieces are referred to by a DYS label (for "DNA Y Segment").  Within each segment (also called a locus or a marker), certain molecular components are repeated, so that the segments have been called short tandem repeat (STR) units.  The components may be repeated say 10 or 15 or 27 times.  These repetition numbers are what we get from the DNA analysis.  Mutations have resulted in variations of the repeat number for a given marker within the population.  A particular STR such as #390 might occur in different individuals as different variants or alleles, possessing say 22 or 23 or 24 repeats.  The smallest commercial analysis uses 12 markers.  The list of results for a 12 marker analysis looks something like this.
 

This particular set of results, called a haplotype, is for the Lambert who is the coordinator for the Lambert DNA Project.  It is usually said that three unit deviations from these 12 numbers indicate little relationship between two individuals.  Even when two individuals have identical numbers for all 12 markers, they still may not be closely related.  For further information, more markers are needed, for which FamilyTreeDNA provides the 25 marker test, which includes the additional markers with DYS labels 458, 459a, 459b, 455, 454, 447, 437, 448, 449, 464a, 464b, 464c, and 474d.  At present, these are the only Y DNA tests available.  Because of the greater amount of information in the 25 marker test, we recommend it.  The 12 marker test, however, is less expensive.

The pattern of alleles often allows the individual to be placed into genetic categories, or haplogroups, that have been established.  The original set of groups was given HG numbers, probably named as they were established.  Thus HG1 is the most common European haplotype.  Seven of the above 12 DYS markers are commonly used to distinguish the different haplotypes.  Here is one such listing, from Bill Bailey of San Antonio, giving the characteristic alleles respectively for the markers 388...393...392...19...390...391 (the seventh haplogroup marker, 426, is not included in this compilation):

HG1...12...13....13...14...24...11
HG2...14....13....11...14...22...10
  (excludes HG6, HG9)
HG3...12...13....11...16...25...11
HG4................?
HG6...16...12...11...14....23...10
  (same as HG9)
HG7................?
HG8...12...15...11...17...21...10
HG9...15...12...11...14...23...10
  OR...16...12...11...14...23...10
  (same as HG6)
HG12...............?
HG16...12...14...14...15...23...11
HG21...12...13...11...13...24...11
HG22...?...13...13...14...24...11
HG26...12...13...12...13...23...10
HG25................?
HG28...12...11...13-16...14...22-23...10
HG35...16...12...11...15...22...10

The haplotype listed above for the Lambert coordinator thus is seen to fall into the HG2 category, sometimes called the Anglo-Saxon haplogroup.  It is very common in central, western, and northern Europe.  Since these seven markers tend to be constant for a given haplogroup, we have to look at the other five markers for distinctions within the haplogroup.  Since five is a small number, the 25 marker test is better, providing 18 useful markers beyond those identifying the haplogroup.  Sometimes there is no precise fit to a haplogroup, and one most be satisfied with close approach to one, say a difference by one or two units from the listed numbers.

Because there is no logic in the HG numbers of these haplogroups, and because some like HG2 proved to be unwieldy and heterogeneous, new classifications have been developed.  The haplogroups by one such system have letters of the alphabet, with A corresponding to the earliest group.  Steady mutations created new groups, which have been given letters progressively deeper in the alphabet as mutations occurred.  At present (2003) there are haplogroups from A to R.  HG1 and HG3 were combined as R (respectively R1b and R1a), and HG2 was split into F, G, I, and J.  Each of these in turn is further subdivided, according to specific mutations.  The part of HG2 containing the above Lambert haplotype is group I, specifically (possibly) group I1a1.  The system is called the Y chromosome consortium, or YCC for short.  Numbers also have been given to the subgroups, so that I1a1 is the same as YCC63, I believe.  It still represents an Anglo-Saxon group, or loosely Germanic Scandinavian.  It is thought to have come from the Gravettian culture that arrived in Europe about 25,000 years ago from Southwest Asia.  The culture was known for Venus figurines, shell jewelry, and houses constructed of mammoth bones.  The part of HG2 that become HG6 and is now called J originated from the Neolithic farmers of Southwest Asia who were the first to practice agriculture about 8000 years ago.

The results of Y DNA testing on the one hand can provide identification of the haplogroup, with interesting information about the deep history of your paternal line.  On the other hand, comparisons of one person's haplotype with that of others with the same surname (or even with different surnames) can lead to conclusions about the relationships among these individuals.  Drastically different haplotypes imply little or no relationship.  Identical or nearly identical haplotypes, particularly at the 25 marker level, allow statements to be made about how recently their common ancestor lived.