Genetic markers are sequences of DNA which have been traced to specific locations on the chromosomes and associated with particular traits. They demonstrate polymorphism, which means that the markers in different organisms of the same species are different. A classic example of this type of marker is the area of the DNA which codes for blood type in humans: all humans have and need blood, but the blood of individual humans can be very different as a result of polymorphism in the area of the genome which codes for blood.
There are a number of applications for genetic markers. One of the most obvious is in tracing the inheritance and history of traits, from eye color to genetic diseases. Understanding which areas of the genome are involved in inheritance of particular traits can help researchers understand these traits, and this can sometimes be applied to the diagnosis and treatment of disease. For example, several genetic markers are associated with an increased risk of breast cancer, and women with these markers can opt for more aggressive preventative care to address this risk.
Genetic markers are also used in paternity testing and the investigation of crimes. By looking for specific markers, known samples of DNA can be compared to unknown samples or samples in question to determine whether or not the samples are related or identical. DNA testing usually includes comparisons of several alleles, not just one, to confirm that the similarities between the samples are actually meaningful.
Some genetic markers are whole genes, while others are snippets of DNA which do not necessarily serve a function. They can appear at one point in the genome, or in multiple areas, sometimes on different chromosomes. They can also consist of strands of repeating DNA; polymorphism these repetitious strands are of immense interest to scientists studying inheritance of numerous traits.
The variations between genetic markers in two people can sometimes be very subtle, but these small variations can make a huge difference. Once the genome was sequenced, researchers began to focus on identifying genetic markers so that they could learn more about how the genome was arranged. Identifying new markers is a painstaking process which includes processing DNA from related individuals or people with the same genetic condition to find commonalities which could be used to pinpoint particular traits. Especially when markers appear at multiple points in the genome and interact with each other, tracking them down can be very challenging.