Mutations
As DNA passes from one generation to the next, it acquires small changes, known as mutations. The most common is a change to a single base, for example a change from a T to a C. Other possible changes include the loss or addition of one or more bases. The effect of a mutation depends upon the type of changes and their location in the sequence. Just like one single letter can change a word or even a sentence, a mutation can change the instructions in a gene. Most mutations are considered to be neutral, having little to no impact. Serious mutations can actually cause a protein to stop functioning properly.Mutations in the DNA can be inherited or acquired. When a mutation is inherited from a parent it is present in almost all of the body’s cells. Acquired mutations are changes in the DNA that develop throughout a person’s life. They arise in the DNA of individual cells, either spontaneously, or in response to environmental factors such as radiation or viruses. Spontaneous mutations are the most common, caused by copying mistakes in the DNA code as cells form and divide. Most of the time the cell recognizes the mistake and repairs it, but sometimes it passes the mutation on as it divides and creates new cells.
DNA doesn’t have long-term memory, so any mutations which develop in a gene are reproduced and passed down to future generations. By comparing the mutations of two individuals, it is possible to calculate how closely they’re related. By calculating the mutation rate, researchers can deduce how far back in time different groups split apart.
Markers
A marker is a segment of DNA with known genetic characteristics. These markers, which can be found at specific locations, or loci, on the chromosome, are essentially places where the same pattern repeats a number of times – sort of a "stutter" in the DNA. The number of repeats in a marker is known as an allele, basically a variant form of a specific gene. Since the number of repeats within these sequences is inherited, they make useful mileposts for genetic testing.A special type of marker known as a Short Tandem Repeat (STR) is the one most often used for hereditary and forensic testing. STRs are short sequences of DNA (usually 2-5 base pairs) that are repeated as many as 100 times along the DNA strand. For example, the four-base pattern CAGT might be repeated four times: CAGTCAGTCAGTCAGT. STRs are chosen for their tendencies to display variations, caused by mutations, among different people, allowing scientists to differentiate between individuals.
To determine a connection between two individuals, specific markers on the DNA strand are analyzed for the number of repetitions at each marker. Because mutations happen randomly, however, a mutation which appears at a specific marker may have begun with the current generation, or it may have been handed down through five generations. This is why a number of different markers are tested and compared. The number of markers examined varies from test to test and company to company, but most ancestry DNA tests are typically in the 12-40 marker range. The DNA test results provide you with the number of repetitions at each of the specific markers tested. The more locations that match, the more likely it is that the two individuals are related.
