When genes in genetic testing are working properly, our bodies develop and function smoothly. But should a single gene - even a tiny segment of a single gene - go awry, the effect can be dramatic: deformities and disease, even death.



In the past 20 years, amazing new techniques have allowed scientists of genetic testing to learn a great deal about how genes work and how genes are linked to disease. Increasingly, researchers are able to identify mutations, changes within genes that can lead to specific disorders. In genetic testing tests for gene mutations make it possible not only to detect diseases already in progress but also, in certain situations, to foresee diseases yet to come. That is called Genetic Testing.

This new ability raises both high hopes and grave concerns. On the one hand, predictive genetic testing holds out the possibility of saving thousands of lives through prevention or early detection. On the other, the implications of test results are enormous, not only for the individual but also for relatives who share this genetic testing legacy, and for society as a whole.

This article of Genetic Testing presents key concepts and issues relevant to genetic testing and answers questions that are frequently asked about the techniques, potential risks, and possible benefits of attempts to link genetic markers with disease.

WHAT ARE GENES IN GENETIC TESTING?

In Genetic Testing Genes are working subunits of DNA. DNA, which carries the instructions that allow cells to make proteins, is made up of four chemical bases. Tightly coiled strands of DNA are packaged in units called chromosomes, housed in the cell's nucleus. Working subunits of DNA are known as genes. DNA is a vast chemical information database that carries the complete set of instructions for making all the proteins a cell will ever need. Each gene contains a particular set of instructions, usually coding for a particular protein.

DNA exists as two long, paired strands spiraled into the famous double helix. Each strand is made up of millions of chemical building blocks called bases. While there are only four different chemical bases in DNA (adenine, thymine, cytosine, and guanine), the order in which the bases occur determines the information available, much as specific letters of the alphabet combine to form words and sentences.

DNA resides in the core, or nucleus, of each of the body's trillions of cells. Every human cell (with the exception of mature red blood cells, which have no nucleus) contains the same DNA. Each cell has 46 molecules of double-stranded DNA. Each molecule is made up of 50 to 250 million bases housed in a chromosome.

The DNA in each chromosome constitutes many genes (as well as vast stretches of noncoding DNA, the function of which is unknown). A gene is any given segment along the DNA that encodes instructions that allow a cell to produce a specific product - typically, a protein such as an enzyme - that initiates one specific action. There are between 50,000 and 100,000 genes, and every gene is made up of thousands, even hundreds of thousands, of chemical bases.

Human cells contain two sets of chromosomes, one set inherited from the mother and one from the father. (Mature sperm and egg cells carry a single set of chromosomes.) Each set has 23 single chromosomes - 22 autosomes and an X or Y sex chromosome. (Females inherit an X from each parent, while males get an X from the mother and a Y from the father.)

For a cell to make protein, the information from a gene is copied, base by base, from DNA into new strands of messenger RNA (mRNA). Then mRNA travels out of the nucleus into the cytoplasm, to cell organelles called ribosomes. There, mRNA directs the assembly of amino acids that fold into completed protein molecule.

Each human cell contains 23 pairs of chromosomes, which can be distinguished by size and by unique banding patterns. This set is from a male, since it contains a Y chromosome. Females have two X chromosomes. Different genes are activated in different cells, creating the specific proteins that give a particular cell type its character.

HOW DO GENES WORK IN GENETIC TESTING?

Although each cell contains a full complement of DNA, cells use genes selectively. Some genes enable cells to make proteins needed for basic functions; dubbed housekeeping genes, they are active in many types of cells. Other genes, however, are inactive most of the time. Some genes play a role in early development of the embryo and are then shut down forever. Many genes encode proteins that are unique to a particular kind of cell and that give the cell its character - making a brain cell, say, different from a bone cell. A normal cell activates just the genes it needs at the moment and actively suppresses the rest. Genes, through the proteins they encode, determine all body processes, including how the body responds to challenges from the environment.