The male Y chromosome is usually passed unchanged from father to son. However, every so often, there is a mutation in one of the alleles, or segments. Therefore, taking the average rate of mutation as the standard, an estimate can be made of how far back two individuals are related with a common ancestor. This estimate of Most Recent Common Ancestor (MRCA) is the heart of a DNA study.
Keep in mind that it is ONLY an estimate. Even the average rate of mutation is not completely agreed on; different data seem to show somewhat different rates. Also, the average may not be true for all sub populations.
With a perfect match between two males on a 25 marker test there is a better than 50% probability that the MRCA is less than 8 generations back, rising to about 80% at 12 generations or less. However, if two individuals show numerous differences, it is almost certain that they are unrelated within any meaningful number of generations.
There is another type of DNA, mitochondrial DNA (mtDNA), which tests DNA passed by the mother. Testing for mtDNA would not meet the objectives of this study. It cannot trace a surname as the female surname changes with marriage. To see how Y chromosome DNA and mtDNA work in practice click here for a chart.
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CLICK on the links below to read more about DNA and the Pace Society DNA Project
Understanding DNA research findings may be confusing at first reading these emails can help overcome residual confusion.