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A genetic disease is due to a faulty gene or group of genes. While not all gene defects cause disease, many do. New genetic diseases are discovered every month; as of 2001, there are estimated to be approximately 1,100 genetic diseases.
How Gene Defects Cause Disease
A gene is a recipe for making a protein. Proteins control cell functions, and defects in the instructions for making a protein can prevent the cell from functioning properly. Genes are made of deoxyribonucleic acid (DNA), a chemical composed of units called nucleotides (../Mo-Nu/Nucleotides.html) , and are carried on chromosomes within the cell nucleus (../Mo-Nu/Nucleus.html). Most genes are present in pairs (corresponding to the two sets of chromosomes inherited from one's parents). As well as coding for proteins, genes are the hereditary material. Therefore, genetic diseases can be inherited.
Genetic defects cause diseases in a variety of ways. The simplest way is through a "loss-of-function" mutation. In this type of defect, a change in the DNA nucleotides prevents the gene from making protein, or prevents the protein from functioning once it is made. Genetic diseases due to loss-of-function mutations are very common, and include cystic fibrosis (which affects the lungs and pancreas), Duchenne muscular dystrophy, and the hemophilias, a group of blood-clotting
Duchenne muscular dystrophy is a genetic disease due to the loss-of-function mutation. The bottom diagram shows a typical pedigree for inheritance of an x- linked trait such as Duchenne muscular dystrophy.
disorders.
A second mechanism for causing disease is called a "toxic-gain-of-function" mutation. In this type of defect, the gene takes on a new function that is harmful to the organism—the protein produced may interfere with cell functions, or may no longer be controllable by its normal regulatory partners, for instance. Many degenerative diseases of the brain are due to this type of mutation, including Huntington disease.
More complex mechanisms are possible. Most traits are multifactorial, meaning they are determined by many different genes. In the human population, there are several variants (alleles) of most genes, each form of which
is functional and does not cause disease by itself. However, some alleles may predispose a person to a certain disease, especially in combination with other alleles or environmental factors that influence the same trait. Such susceptibility alleles have been found in breast cancer and colon cancer, for instance. Carriers of these alleles have an increased likelihood of developing that disease, a risk that can be increased or decreased by such factors as diet, exposure to environmental toxins, or presence of particular alleles for other genes. As more is learned about the human genome (../Fo-Gr/Genome.html) , a large number of susceptibility genes are likely to be discovered for a wide variety of conditions.
Disease can also be caused by chromosome abnormalities rather than gene defects. Down syndrome is due to having three copies of chromosome 21, instead of the normal two copies. It is likely the extra protein from the extra gene copies lead directly to the disease symptoms, but this is not yet clear.
Condition Chromosome Locati
Gaucher Disease 1, recessive Achondroplasia 4, dominant Huntington's Disease 4, dominant Juvenile Onset Diabetes 6,11,7, others Hemochromatosis 6, recessive Cystic Fibrosis 7, recessive Friedreich's Ataxia 9, recessive Best Disease 11, dominant Sickle Cell Disease 11, recessive Phenylketonuria 12, recessive Marfan Syndrome 15, dominant
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