Genetic Disorders
What are genetic disorders?
A genetic disease disorder is a disease caused by a different form of a gene called a variation, or an alteration of a gene called a mutation. Many diseases have a genetic aspect. Some, including many cancers, are caused by a mutation in a gene or group of genes in a person’s cells. These mutations can occur randomly or because of an environmental exposure such as cigarette smoke.
Other common genetic disorders are inherited. A mutated gene is passed down through a family and each generation of children can inherit the gene that causes the disease. Still other genetic disorders are due to problems with the number of packages of genes called chromosomes. In Down syndrome, for example, there is an extra copy of chromosome 21. If you know that you have a genetic problem in your family, you can have genetic testing to see if your baby could be affected.
A human genetic disorder is a disease caused in whole or in part by a "variation" (a different form) or "mutation" (alteration) of a gene.
As we unlock the secrets of the human genome, we are learning that nearly all diseases have a genetic component. Some, including many cancers, are caused by a mutation in a gene or group of genes in the cells of an individual. Such mutations can occur randomly or due to some environmental exposure (such as cigarette smoke).
Other genetic disorders are hereditary - such as Huntington disease or Marfan's disease - where a mutated gene is passed down through a family and each generation of children can inherit the gene that causes the disease.
But most genetic disorders are "multifactorial inheritance disorders," meaning they are caused by a combination of small variations in genes, often in concert with environmental factors.
Through research on the human genome, we now know that many common diseases usually caused by genetic alterations in the genes of an individual's cells - such as breast cancer and colon cancer - also have rare hereditary forms. In these cases, gene variants that cause or strongly predispose a person to these cancers run in a family and significantly increase each member's risk of developing the disease.
Genes and Heredity
Heredity is the passing of genes from one generation to the next. You inherit your parents' genes. Heredity helps to make you the person you are today: short or tall, with black hair or blond, with green eyes or blue.
Can your genes determine whether you'll be a straight-A student or a great athlete? Heredity plays an important role, but your environment (including things like the foods you eat and the people you interact with) also influences your abilities and interests.
How Do Genes Work?
DNA contains four chemicals (adenine, thymine, cytosine, and guanine - called A, T, C, and G for short) that are strung in patterns on extremely thin, coiled strands in the cell. How thin? Cells are tiny - invisible to the naked eye - and each cell in your body contains about 6 feet of DNA thread, for a total of about 3 billion miles (if all your DNA threads were stretched out straight) of DNA inside you! The DNA patterns are the codes for manufacturing proteins, chemicals that enable the body to work and grow.
Genes hold the instructions for making protein products (like the enzymes to digest food or the pigment that gives your eyes their color). As your cells duplicate, they pass this genetic information to the new cells. Genes can be dominant or recessive. Dominant genes show their effect even if there is only one copy of that gene in the pair. For a person to have a recessive disease or characteristic, the person must have the gene on both chromosomes of the pair.
Changing Genes
Sometimes scientists alter genes on purpose. For many years, researchers have altered the genes in microbes and plants to produce offspring with special characteristics, such as an increased resistance to disease or pests, or the ability to grow in difficult environments. We call this genetic engineering.
Gene therapy is a promising new field of medical research. In gene therapy, researchers try to supply copies of healthy genes to cells with variant or missing genes so that the "good" genes will take over. Viruses are often used to carry the healthy genes into the targeted cells because many viruses can insert their own DNA into targeted cells.
But there are problems with gene therapy. Scientists haven't yet identified every gene in the human body or what each one does. Huge scientific efforts like The Human Genome (pronounced: jee-nome) Project and related projects have recently completed a map of the entire human genome (all of the genetic material on a living thing's chromosomes), but it will take many more years to find out what each gene does and how they interact with one another. For most diseases, scientists don't know if and how genes play a role. Plus, there are major difficulties inserting the normal genes into the proper cells without causing problems for the rest of the body.
There are also concerns that people might try changing genes for ethically troubling reasons, such as to make smarter or more athletic children. No one knows what the long-term effects of that kind of change would be.
Still, for many people who have genetic diseases, gene therapy holds the hope that they - or their children - will be able to live better, healthier lives.
Geneticists group genetic disorders into three categories:
- Single gene disorders are caused by a mutation in a single gene. The mutation may be present on one or both chromosomes (one chromosome inherited from each parent). Sickle cell disease, cystic fibrosis and Tay-Sachs disease are examples of single gene disorders.
- Chromosome disorders are caused by an excess or deficiency of the genes that are located on chromosomes, or structural changes within chromosomes. Down syndrome, for example, is caused by an extra copy of chromosome 21, but no individual gene on the chromosome is abnormal.
- Multifactorial inheritance disorders are caused by a combination of small variations in genes, often in concert with environmental factors. Heart disease and most cancers are examples of these disorders. Behaviors are multifactorial, complex traits involving multiple genes that are affected by a variety of other factors. Researchers are learning more about the genetic contribution to behavioral disorders such as alcoholism, obesity, mental illness and Alzheimer disease. There is currently no recommended genetic testing for behavioral disorders such as alcoholism or obesity.