Gene

In the early 20th century following the rediscovery of the important work of Gregor Mendel that provided the foundation for the future field of genetics, scientists developed the classical concept of the gene as the entity responsible for many human traits and diseases. It was soon learned that genes are located on the chromosomes and each person has two copies of a particular gene, one on each of the paired chromosomes that are inherited from one's parents, one of maternal origin and the other from the father. The alternate forms of a gene that result from the process of mutation are called alleles.

The 21st century likewise began with significant discoveries in the world of genetics. The Human Genome Project (HGP) begun in the mid-1980s by an international team of researchers led by the U.S. Department of Energy and National Institutes of Health (NIH) announced in 2003 that a map of the human genome was completed. The announcement of the human genome map coincided with the 50th anniversary of the discovery of the DNA helical structure by Watson and Crick in 1953. This latter discovery gave us the basis for the structure of the gene and how it replicates and marked the beginning of the molecular revolution in genetics and in biology and medicine.

Growing knowledge from the HGP and other genomic research will make it possible to diagnose and treat disease and to identify individuals at risk for a certain disease in ways that, until recently, were inconceivable. The HGP discoveries add to our understanding of how disease mechanisms occur so that treatment can focus on the primary dysfunction or disease progressions rather than treating only the secondary manifestations or outcomes. Genetics also plays a role in the prevention of disease and in health promotion. Most diseases and our health are now thought to be the result of the interplay between multiple genes (called polygenes) and the multitude of environmental exposures to which an individual is subjected during development and over the course of life. Some geneticists now think that even single gene (monogenic) diseases and traits are really multifactorial (the result of polygenes and environmental influences) and are complex due to interactions between genes and between genes and environmental factors.

To understand the new knowledge from the map of the human genome and how this knowledge affects society, it is important to have a clear understanding of what a gene is, as we understand it today. A gene is defined as the fundamental physical and functional unit of heredity. A gene is made up of deoxyribonucleic acid, called DNA. If this DNA were stretched out, it would look like a very long circular staircase with millions of rungs. Small sections of rungs are analogous to genes. Each of these sections of DNA is called a gene—a piece of genetic information that does one particular job. That job is the encoding of a chain of [Page 406]amino acids to form a polypeptide, the major components of the essential proteins of the body. Each gene is a different packet of information necessary for our bodies to grow and function. Our genes also contain the information governing physical traits such as the color of our eyes, how tall we are, and the shape of our ears and nose, and some of the traits that affect our vulnerability to diseases. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases. The HGP has estimated that humans have between 20,000 and 25,000 genes.

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