Genetic Diagnostic Technologies

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.

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.

DNA chips are powerful tools that can be used to identify DNA mutations. A single microarray can test for millions of different DNA changes by using only one sample. DNA microarrays are used in genome-wide association studies (GWAS) to identity variants that can contribute to disease risk by comparing DNA from large numbers of people and normal populations.

aCGH is a type of microarray now routinely used to identify deleted or duplicated segments of DNA in specific chromosomes. In this array, DNA from a person is compared to a reference genotype (a person’s unique combination of genes or genetic makeup) using many probes. Different coloured fluorescent dyes are added to the person's DNA and to the reference sample. If a segment is missing, the probes detect a decreased amount of the fluorescent dye in the person's DNA sample relative to the reference sample. If a segment is duplicated or tripled, the probes detect an increased amount of the patient fluorescent dye relative to the reference sample. These probes can be used to test the entire genotype.

Next-generation sequencing technologies can detect even smaller parts of genes and DNA by breaking the entire genotype (or genome) 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 or multiple variations in bases may be identified as well as areas where bases are missing or have been inserted in the wrong place. The costs of this technology have dramatically fallen and continue to fall. The equipment and computational methods also continue to improve.

Some of these variations can help doctors diagnose genetic disorders. Next-generation sequencing technologies 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 sheer volume of information generated by analyzing the genotype results in a variety of problems that sometimes make it difficult for doctors to understand and interpret the results (for example, to distinguish important differences from random or trivial ones). Despite these issues, these techniques have become the mainstay of genetic testing.