June 4, 2012 | A gene is the basic physical and functional unit of heredity. Genes, which are made up of DNA, act as instructions to make molecules called proteins. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases. The Human Genome Project estimates that humans have between 20,000 and 25,000 genes. Every person has two copies of most genes, one inherited from each parent. A genetic disorder is a disease caused by a different form of a gene called a variation, or an alteration of a gene called a mutation. Almost all diseases have a genetic aspect. Some, including many cancers, are caused by a mutation in a gene or a group of genes in a person’s cells. These mutations can occur randomly or because of an environmental exposure such as cigarette smoke. Other 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.

June 4, 2012 | Genetic disorders are grouped into one of three categories. Monogenetic disorders are caused by a mutation in a single gene, with the mutation present in one or both of the patient’s chromosomes. The mutated gene is passed down through a family, and each generation of children can inherit the gene that causes the disease. Such disorders include sickle cell disease and cystic fibrosis. Multifactorial inheritance disorders are caused by a combination of small inherited variations in genes, often acting together with environmental factors. Heart disease, diabetes, and most forms of cancer are examples of this kind of genetic disorder. Chromosome disorders, meanwhile, are caused by an excess or deficiency of the genes that are located on chromosomes, or by structural changes within chromosomes. Down syndrome, for example, is caused by an extra copy of chromosome 21, which is called trisomy 21, although no individual gene on the chromosome is abnormal.

June 4, 2012 | Researchers have identified more than 4,000 diseases that are caused by genetic variants. But having a genetic mutation that may cause a disease doesn’t always mean that a person will actually get that disease. Because you inherit a gene from each parent, having one disease gene usually does not cause any problems because the normal gene can allow your body to make the normal protein it needs. On the average, people probably carry from five to 10 disease causing genes in their cells. The only time problems arise is when the disease gene is dominant or when the same recessive disease gene is present on both chromosomes in a pair. Problems can also occur when several variant genes interact with each other, or with the environment, to increase susceptibility to diseases. If a person carries the dominant gene for a disease, he or she will usually have the disease and each of their children will have a 50 percent chance of inheriting the gene and getting the disease.

June 4, 2012 | Chromosomes are the structures that hold our genes. Genes are the individual instructions that tell our bodies how to develop and function. They govern our physical and medical characteristics, such as hair color and blood type. Some recessive genetic variants are carried only on the X chromosome, which means that usually only men can develop the genetic disorder since they have only one X chromosome. Women have two X chromosomes, so they would need to inherit two copies of the recessive gene to get the disease. In some cases, people who are concerned that they might carry certain variant genes can have genetic testing so they can learn their children’s risk of having a disease. Pregnant women can also have tests done to see if the fetus they are carrying might have certain genetic disorders. Genetic testing usually involves taking a sample of someone’s blood, skin, or amniotic fluid, and checking it for signs of genetic diseases or disorders.

June 4, 2012 | A genetic disorder can go undiagnosed for years when medical personnel are unable to find a reason for certain characteristics or symptoms. In fact, finding an underlying diagnosis for many conditions can be a very long and frustrating experience. That’s why it is important to have patients suffering from an undiagnosed illness evaluated by a specialist like Bradley Schaefer, director of UAMS’ Division of Genetics. The only medical geneticists in Arkansas, Schaefer and his team offer the kind of specialized care patients with genetic disorders like Down syndrome and certain forms of cancer need. Many genetic disorders require specialized evaluation and ongoing monitoring and care. Decades ago, many of these conditions were viewed as pediatric disorders because of the tragically short life expectancy of patients. Today, medical advances have vastly expanded those patients’ life expectancy, making specialized adult care a priority.