Polymorphisms

The study of polymorphism has many uses in medicine, biological research, and law enforcement. Genetic diseases may be caused by a specific polymorphism. Scientists can look for these polymorphisms to determine if a person will develop the disease, or risks passing it on to his or her children. Besides being useful in identifying people at risk for a genetically based disease, knowledge of polymorphisms that cause disease can provide valuable insight into how the disease develops. Polymorphisms located near a disease gene can be used to find the gene itself, through mapping. In this process, researchers look for polymorphisms that are co-inherited with the disease. By finding linked polymorphisms on smaller and smaller regions of the chromosome, the chromosome region implicated in the disease can be progressively narrowed, and the responsible gene ultimately can be located.

A related use of polymorphism is widely employed in agriculture. If a polymorphism can be identified that is associated with a desirable characteristic in an agriculturally important plant or animal, then this polymorphism can be used as a genetic flag to identify individuals that have the desirable characteristic. Using this technique, known as marker-assisted selection, breeding programs aimed at improving agriculturally important plants and animals can be made more efficient, since individuals that have the desired trait can be identified before the trait becomes apparent.

Polymorphisms can be used to illuminate fundamental biological patterns and processes. By studying polymorphisms in a group of wild animals, the familial relationships (brother, sister, mother, father, etc.) between them can be determined. Also, the amount of interbreeding between different groups of the same species (gene flow) can be estimated by studying the polymorphisms they contain. This information can be used to identify unique populations that may be important for survival of the species. Sometimes it is not immediately obvious if two different groups of organisms should be classified as different species. Comparing the genetic polymorphisms in the two groups aids in making a judgment as to whether they warrant classification as different species.

If enough polymorphisms are analyzed, it is possible to distinguish between individual humans with a high degree of confidence. This method is known as DNA profiling (or DNA fingerprinting) and provides an important tool in law enforcement. A person's genotype, or DNA profile, can be determined from very small samples, such as those that may be left at a crime scene (hair, blood, skin cells, etc.). The genotype of samples found at the crime scene can then be compared to a suspect's genotype. If they match, it is very likely that the suspect was present at the crime scene. Currently, the FBI uses thirteen different polymorphic loci for DNA fingerprinting. In a similar manner, analysis of polymorphisms can help prove or disprove fatherhood (paternity) in cases where responsibility for a child is disputed.