Medical Genetics Index	Glossary

	DNA Polymorphisms: The Basis of DNA Typing

All DNA testing is based on the observation that the genome of each person or animal is unique (except of course identical twins). The myriad of small and large differences in nucleotide sequence among individuals are known as DNA polymorphisms. Two fundamentally different types of polymorphisms have been widely exploited for DNA typing: tandem repeats and retriction fragment length polymorphisms. Tandemly Repeated DNA The eukaryotic genome is densely populated with islands of short sequences that are repeated over and over in small to large arrays called minisatellites and microsatellites. Another term commonly used to describe these sequences is variable number tandem repeats or VNTRs. For a given repetitive locus, the number of repeats is highly variable among individuals and heterozygosity is high (i.e. the number of repeats at the locus is usally different on the two pairs of chromosomes of one individual). Analyzing the number of repeats at one or more such loci provides a highly sensitive measure of individual identity and is the technique most often used for forensic DNA typing. The figure above depicts a VNTR locus with 8 versus 3 repeats. Digestion with the restriction enzyme Hinf1 will yield fragments of two lengths that will hybridize to the (red) probe. Variability in Restriction Sites Single base changes in DNA often introduce or obliterate a restriction enzyme site. For example, a mutation that changes the sequence AGATCC to GGATCC will introduce a BamH1 site into that segment of DNA. Such sequence variability is exceedingly common, particularly in non-coding regions of DNA, and determining whether or not a particular group of restriction sites exists in DNA is a very sensitive means of differentiation one individual from many others. Because polymorphisms in a restriction sites translates into variability in the length of fragments after digestion of DNA with that restriction enzyme, these DNA markers are called restriction fragment length polymorphisms or RFLPs. The figure depicts a BamH1 RFLP in which the top strand of DNA has only two GGATCC sites while the lower has three. Digestion followed by hybridization with the (red) probe will reveal two fragments of differing length.

	Next Topic for DNA Testing: Techniques for DNA Testing
	Advanced and Supplemental Topics Tandemly Repeated (Satellite) DNA

Last updated on March 18, 1996

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