Unless there was some repair mechanism to correct errors in the Y, or male, chromosome, the errors would accumulate until the gene became nonfunctional. Then there would be no more male humans (and many other mammals). Other chromosomes occur in pairs, one from the mother and one from the father. They preserve genetic integrity by comparing matching genes on the homologous chromosome, a process called crossing over. The Y chromosome lacks that option, being an unpaired chromosome. Only recently, it was discovered that the Y chromosome has an internal or self-correcting mechanism. There are repeated sequences along the chromosome that are mirror images of each other. The Y chromosome carries two copies of each gene sequence. This is used as a corrective mechanism. The base pairs that carry genetic information are arranged as palindromes.
These extra base pairs are not "junk" DNA. The extensive use of gene conversion or comparison plays a role in the ability of the Y chromosome to edit out genetic mistakes and maintain the integrity of the relatively few genes that it carries. This mechanism is so ancient it is found also in chimpanzees, bonobos, and gorillas. This would carry us back at least five million years before humans and the non-human primates diverged from each other.
Errors in the Y chromosome do occur, but not all of these errors are life threatening. Once a benign [non-harmful] change in the Y chromosome occurs, it is passed on from the man where it first occurs to his sons, and to that son's sons, ad infinitum. These errors can be identified and labeled. If two men today have the same error, then the probability they descend from the same male is extremely high. This originating male may have lived centuries or thousands of years ago. It could have been so long ago that additional mutations or changes in the Y chromosome could have occurred. Thus, two males today may share a mutation (showing a descent from a common individual) but they may have an additional mutation showing a divergence in the family tree at some later time.
These mutations are relatively rare, occurring only infrequently. Their value is in showing the larger, broader genetic path that occurs over the long term.
The Germanna Thomases share a common Y chromosome mutation. This mutation has been plotted over several continents. It is never found in Wales, for example. Thus, the researchers who believed that the Germanna Thomas family was Welsh, could not be correct. Today, we have male descendants of Johann Thoma of the Germanna family who inherited their Y chromosome from Johann Thomas. They have a Y chromosome mutation that is never found today in Wales.
(17 Apr 06)
We gratefully acknowledge the work of John Blankenbaker who published over 2,500 Germanna History Notes via the Germanna-L@rootsweb.com email list from 1997 to 2008. We are equally thankful to George Durman (Sgt. George) for hosting the list and republishing the notes via rootsweb.com.