Gene technology is rapidly improving. The polymerase chain reaction (PCR) is a laboratory technique that can produce many copies of a gene or segments of a gene, which makes studying the gene much easier. A specific segment of deoxyribonucleic acid (DNA), such as a specific gene, can be copied (amplified) in a laboratory. Starting with one DNA molecule, at the end of 30 doublings (only a few hours later) about a billion copies are produced.
Various methods may be used to find (probe) changes in genes. A gene probe can be used to locate a specific part of a gene (a segment of the gene's DNA) or a whole gene in a particular chromosome. Probes can be used to find normal or mutated segments of DNA. A DNA segment that has been cloned or copied becomes a labeled probe when a radioactive atom or fluorescent dye is added to it. The probe will seek out its mirror-image segment of DNA and bind to it. The labeled probe can then be detected by sophisticated microscopic and photographic techniques. With gene probes, a number of disorders can be diagnosed before and after birth. In the future, gene probes will probably be used to test people for many major genetic disorders simultaneously.
An oligonucleotide is a chain of bases (nucleotides). Sometimes these chains are missing or have duplicate segments of DNA. An oligonucleotide array is used to identify deleted or duplicated segments of DNA in specific chromosomes. In an oligonucleotide array, DNA from a person is compared to a reference genotype using many oligonucleotide probes. Like some gene probes, a fluorescent dye is added to the oligonucleotide probes. If a segment is missing, the probes detect a decreased amount of the fluorescent dye. If a segment is duplicated or tripled, the probes detect an increased amount of the fluorescent dye. These probes can be used to test the entire genotype.
Microchips are powerful new tools that can be used to identify DNA mutations, pieces of ribonucleic acid (RNA), or proteins. A single chip can test for millions of different DNA changes by using only one sample.
Newer technologies can detect even smaller parts of genes and DNA by breaking the entire genotype into small segments and then analyzing the DNA sequence of some or all of the segments. The results are then analyzed by a powerful computer. Single variations in bases may be identified as well as very short segments of bases. Some of these variations can help doctors diagnose genetic disorders. Some of the newer technologies called next-generation sequencing are so sensitive that doctors can detect DNA from the fetus in a sample of blood drawn from the mother and analyze it to determine whether the fetus has Down syndrome. However, the error rates of such testing are still being determined.
Last full review/revision August 2013 by David N. Finegold, MD