GENETIC GENEALOGY, page 2
Markers. There are a number of different kinds of mutations (changes in the genetic code) that can occur when DNA is copied within a cell and passed on to the next generation. Short-tandem repeats (STR's), also known as microsatellites, are the markers tested in y-chromosome studies. These occur when short segments of DNA sequences get repeated over and over along a portion of a chromosome. Once an STR is present, it may change by adding or subtracting a repeat or two during the replication process. Estimates of the frequency of changes range from 2 mutations per 1000 generations to nearly 4 per 1000 generations for each STR. Thus over a long period of time, individuals will tend to have at least some differences in the values (number of repeats) on the various STR markers on their y-chromosome. If you compare 25 markers, there is about a 50% chance you will find at least 1 mutation in 9-10 generations (or, counting both up and down from the common ancestor, between yourself and a 4th cousin). Markers are given names such as DYS390, and their values are reported as the number of repeats. DYS390 may have values ranging from 17 to 28 repeats, with 22 to 25 being common in populations with European ancestry.
Haplogroups and "clans." A change to a given base of DNA (single-nucleotide polymorphism or SNP) is extremely rare compared to changes in the number of repeats on STR's, and specific substitutions are believed to have occurred only once each in human history. Thus, these SNP's can be used for broad anthropological studies of our ancestry, and have been used to create a "family tree" of the paternal heritage of all humankind. Large haplogroups or "clans" that originated with a single ancestor who had a specific SNP mutation have been given names beginning with capital letters. The most common haplogroup in Europe is labeled R1b. It is especially common along the Atlantic seaboard (over 80% of some populations), but is also frequent throughout Europe. Other common European haplogroups include R1a and I, common in northern or central Europe. In order to know your haplogroup with 100% certainty, you would need to pay for a separate SNP test, which is offered by at least three testing companies. But certain combinations of STR values are so commonly associated with specific haplogroups, that most people's haplogroups can be accurately guessed from their STR values. This is because even after thousands of years, the STR values (haplotype) of the original fathers of the various haplogroups are still reflected in the STR values found among their descendants. For example, most of the Hammans who have been tested so far have STR haplotypes that are clearly R1b. My Ritter y-chromosome has typical haplogroup I1a values, and my haplogroup has been confirmed to be I1a1 by SNP tests.
Knowing one's haplogroup does not tell you much about your more recent genealogy, but it is of interest to many to know if their ancient patrilineal ancestor was one of the Cro-Magnon people who first resettled western Europe after the ice ages (R1b), one of the Gravetians who came into Europe from the east a bit later (I), or one of the early agriculturalists who came from the Middle-east thousands of years later (J or G, among others).
One consequence of haplogroups is that there is a greater danger of "false" positive matches among individuals in the more common haplogroups. The Hammans who have over 80 12/12 matches to random (probably) unrelated individuals, are all R1b. Another group of Hammans is also R1b and matched the first group on 10 of 12 markers. But when we compared 25 markers, the numbers matching dropped to only 18 of 25, making a common ancestor within thousands of years very unlikely. Thus, if you find that you appear to belong to a common haplogroup, you would probably want to pay to be tested on more markers to reduce the likelihood of "false" matches.
Although specific SNP mutations may be very rare (on the order of about 1 every 50 million generations), it should be noted that because the y-chromosome contains around sixty million bases, it is likely that each male has an average of slightly more than one "personal" y-chromosome SNP compared to his father. Someday, if sequencing large sections of the y-chromosome becomes feasible and inexpensive, we may be able to use SNP's to establish detailed patrilineages for any male who wishes to be tested. That day is likely far in the future.
Last Updated: Feb 2005; Copyright 2004-2005, Philip Ritter