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Understanding mutations

Mutations are changes in DNA which happen naturally and are the basis for evolution. A mutation occurs when DNA polymerase makes a mistake when copying the DNA strand during cell division, resulting in a slight change in the DNA of the newly created cell. When a cell divides (during a process called meiosis), the DNA in the original cell must make a copy of itself for the new cell. It does this through a process known as DNA replication. During DNA replication, an enzyme known as DNA polymerase makes a copy of the DNA using the original DNA strand as a template. Although this process is usually very accurate and results in an exact replica of the original DNA, on occasion, the DNA polymerase may miscopy a section of the DNA, resulting in a slight change in the newly formed DNA strand. This is called a mutation and although mutations do not occur frequently, they are an important part of evolution and occur naturally.

During a Y-Chromosome test, we study regions of the DNA called STR (short tandem repeat) markers which consist of repeating blocks of DNA. The mutation rate of Y-Chromosome markers is roughly once every 500 generations. When a mutation occurs at a STR marker, the number of repeats usually increases by one or decreases by one. Although mutations which occur in the Y-Chromosome Y-STR have no apparent effect on the individual, it does provide useful information for a genealogist who is trying to decide how long ago two family lines shared a common ancestor.

The estimated mutation rate of Y-STRs of once every 500 generations is a rough estimate and some markers may have a higher mutation rate and others may have a lower mutation rate. Currently, there is not yet any scientific data which can pinpoint the exact mutation rate for each specific marker, so for all practical purposes, scientists use the average mutation rate of 500 when studying and analyzing mutation rates in Y-STR markers. Thus, when you are conducting a 44 marker test, the chances that you will see a mutation is once every 11.36 generations (500 / 44 markers). When we say generations, we mean for every event of a father having a son (a transmission event).

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